Essential roles of ferric reductase-like proteins in growth, development, stress response, and virulence of the filamentous entomopathogenic fungus Beauveria bassiana

In yeasts, ferric reductase catalyzes reduction of ferric ion to ferrous form, which is essential for the reductive iron assimilation system. However, the physiological roles of ferric reductases remain largely unknown in the filamentous fungi. In this study, genome-wide annotation revealed thirteen ferric reductase-like (Fre) proteins in the filamentous insect pathogenic fungus Beauveria bassiana, and all their functions were genetically characterized. Ferric reductase family proteins exhibit different sub-cellular distributions (e.g., cell periphery and vacuole), which was due to divergent domain architectures. Fre proteins had a synergistic effect on fungal virulence, which was ascribed to their distinct functions in different physiologies. Ten Fre proteins were not involved in reduction of ferric ion in submerged mycelia, but most proteins contributed to blastospore development. Only two Fre proteins significantly contributed to B. bassiana vegetative growth under the chemical-induced iron starvation, but most Fre proteins were involved in resistance to osmotic and oxidative stresses. Notably, a bZIP-type transcription factor HapX bound to the promoter regions of all FRE genes in B. bassiana, and displayed varying roles in the transcription activation of these genes. This study reveals the important role of BbFre family proteins in development, stress response, and insect pathogenicity, as well as their distinctive role in the absorption of ferric iron from the environment.

Entomophthoralean and hypocrealean fungal pathogens of the sugarcane aphid, Melanaphis sacchari (Hemiptera: Aphididae), on sorghum in Georgia

The sugarcane aphid, Melanaphis sacchari, is a widely distributed insect that attacks grasses in different genera including Miscanthus, Saccharum, and Sorghum. The invasive aphid superclone was first discovered in the U.S. attacking grain sorghum in Texas in 2013. Since then, it has been found in at least 25 states including Georgia. We conducted a survey of naturally occurring fungal pathogens of sugarcane aphids on five farms in Georgia, and identified a hypocrealean fungus, Akanthomyces dipterigenus, and two entomophthoralean fungi, Neoconidiobolus spp. From 2018 to 2020, fungal activity differed across farms but at one farm both major fungal species, A. dipterigenus and N. thromboides, were found each of the 3 years infecting sugarcane aphids, attacking adults, both alatae and apterae, and nymphs.

Adverse effects of the fungal biopesticide Beauveria bassiana on a predatory social wasp

Biopesticides are considered eco-friendly alternatives to synthetic agrochemicals. However, their impact on non-target organisms is still poorly understood. Social wasps, in particular, are a largely neglected group when it comes to risk assessment of plant protection products, despite the relevant ecological and economic services provided by these insects. In the present study, we evaluated the impact of a common biopesticide, the entomopathogenic fungus Beauveria bassiana, on the paper wasp Polistes dominula. We adopted a holistic approach in ecotoxicology by focusing not only on the detrimental effects on isolated individuals, but also on the whole colony. Both adult wasps belonging to different castes and immature larvae were topically exposed to a field-realistic concentration of fungal spores from the commercial strain of B. bassiana ATCC 74040 to assess the impact of the biopesticide on their survival, behavior and physiology. Our results showed that the fungus causes a number of adverse effects on P. dominula, that include increased mortality, altered locomotion and feeding rate, selective ejection of exposed larvae from nests, reduced oviposition rate and ovary development in foundresses, and colony failure. Our findings provide new insights on the often-neglected sublethal effects of pollutants that can jeopardize not only individual beneficial insects, but also the delicate social balance of their colonies and their valuable ecosystem services, highlighting that the natural origin of plant-protection products does not always guarantee environmental safety.

Functional insights of three RING-finger peroxins in the life cycle of the insect pathogenic fungus Beauveria bassiana

Peroxisomes play important roles in fungal physiological processes. The RING-finger complex consists of peroxins Pex2, Pex10, and Pex12 and is essential for recycling of receptors responsible for peroxisomal targeting of matrix proteins. In this study, these three peroxins were functionally characterized in the entomopathogenic fungus Beauveria bassiana (Bb). These three peroxins are associated with peroxisomes, in which BbPex2 interacted with BbPex10 and BbPex12. Ablation of these peroxins did not completely block the peroxisome biogenesis, but abolish peroxisomal targeting of matrix proteins via both PTS1 and PTS2 pathways. Three disruptants displayed different phenotypic defects in growth on nutrients and under stress conditions, but have similar defects in acetyl-CoA biosynthesis, development, and virulence. Strikingly, BbPex10 played a less important role in fungal growth on tested nutrients than other two peroxins; whereas, BbPex2 performed a less important contribution to fungal growth under stresses. This investigation reinforces the peroxisomal roles in the lifecycle of entomopathogenic fungi and highlights the unequal functions of different peroxins in peroxisomal biology.

Auto-dissemination of Cordyceps fumosorosea amongst adult females of the two-spotted spider mite

Tetranychus urticae is an important pest worldwide. The auto-dissemination of spores of entomopathogenic fungi from an infected individual to conspecifics may be important for controlling pests that can build high populations. The current study was carried out to determine the auto-dissemination of the entomopathogenic fungus Cordyceps fumosorosea strain PFs-1 (Priority®) between T. urticae females. The study consisted of four experiments. First, the efficacy of entomopathogenic fungus bioassays was assessed in Petri dishes (experiment 1) and on potted bean plants (experiment 2). In the auto-dissemination trials (experiments 3 and 4, in Petri dishes and on potted plants, respectively), contaminated adult females (1-5) were released among uncontaminated females (10 individuals). All experiments were carried out separately, and observations were made on days 3, 5, and 7. In exp. 1, the control was different from Priority on all observation days. In exp. 2, the average number of surviving individuals in the control was significantly higher than in the Priority treatment. In the auto-dissemination experiments, as the number of contaminated individuals increased, the mortality rate of uncontaminated individuals also increased, in exp. 3 (Petri dishes) on all observation days, and in exp. 4 (potted plants) only on days 5 and 7. The median lethal time (LT50) decreased as the number of individuals contaminated with Priority increased in both Petri dish and pot trials. Consequently, the effectiveness of biological control may increase with the occurrence of indirect contamination from infected to uncontaminated individuals.

Dual biocontrol potential of the entomopathogenic fungus Akanthomyces muscarius against Thaumetopoea pityocampa and plant pathogenic fungi

Akanthomyces spp. species are known for their capacity to biocontrol of certain insects and plant pathogens; however, their ability to biocontrol the pine processionary (Thaumetopoea pityocampa) and certain phytopathogenic fungi belonging to the genera Fusarium and Curvularia have not been studied before. In this study, a strain from Akanthomyces muscarius was isolated from wheat grains and then identified by morphological and molecular tests. The strain was further studied for its capacity to control Thaumetopoea pityocampa larvae through dose-mortality tests, and its ability to control some phytopathogenic fungi strains of the genera Fusarium and Curvularia was studied through direct confrontation tests. Dose-mortality tests at three concentrations of Akanthomyces muscarius against the first instar larvae revealed a mortality of 92.15% after 11 days for the concentration of 2.3 × 10⁶conidia.ml^-1, with a median lethal concentration of 7.6 x10³ conidia.ml^1. Our isolate also showed antifungal activity against these phytopathogenic fungi with inhibition rates ranging from 39.61% to 52.94%. Akanthomyces muscarius proved to be a promising biocontrol agent for plant pests and diseases.

Double-strand RNAs targeting MaltRelish and MaltSpz reveals potential targets for pest management of Monochamus alternatus

Overcoming the innate immunity of insects is a key process to improve the efficiency of biological control. Antimicrobial peptides (AMPs) are important effectors in insect innate immunity, usually mediating resistance to pathogenic microorganisms through Toll and IMD signaling pathways. This study investigated the effect of key genes on upstream immune recognition receptor (GNBP3) and downstream effectors (AMPs) by RNAi technology. The transcriptome KEGG enrichment analysis and differential gene annotation results showed that the immune response genes MaltSpz and MaltRelish are important regulators of Toll and IMD signaling pathways, respectively. Both dsSpz and dsRelish could affect AMP gene expression and increase the expression of the immune recognition receptor MaltGNBP3. Moreover, they significantly reduce the survival rate of Monochamus alternatus and promote hyphal growth after Beauveria bassiana infection. This helps to improve the biological control effect of B. bassiana, control the population of vector insects and cut off the transmission route of pine wood nematode. The combined MaltSpz and MaltRelish knockdown increased the infection rate of M. alternatus larvae from 20.69% to 83.93%, achieving the best efficiency in synergistic B. bassiana infection. Our results showed important roles of MaltRelish- and MaltSpz-mediated regulation of AMP genes function in insect entomopathogenic fungi tolerance and induced significant mortality in larvae. Based on this study, MaltSpz and MaltRelish could represent candidate gene targets for the biological control of M. alternatus by RNAi.

Impact of osmotic stress on production, morphology, and fitness of Beauveria bassiana blastospores

Beauveria bassiana is a cosmopolitan entomopathogenic fungus that can infect over 1000 insect species. During growth inside the host, B. bassiana transitions from hyphal to yeast-like unicellular growth as blastospores. Blastospores are well suited as an active ingredient in biopesticides due to their ease of production by liquid fermentation. Herein, we investigated the impact of hyperosmotic growth environments mediated by ionic and non-ionic osmolytes on two strains of B. bassiana (ESALQ1432 and GHA) relevant to growth morphology, blastospore production, desiccation tolerance, and insecticidal activity. Polyethylene glycol (PEG200) increased osmotic pressure in submerged cultures leading to decreased blastospore size but higher blastospore yields for one strain. Morphologically, decreased blastospore size was linked to increased osmotic pressure. However, smaller blastospores from PEG200 supplemented cultures after air-drying exhibited delayed germination. Ionic osmolytes (NaCl and KCl) generated the same osmotic pressure (2.5-2.7 MPa) as 20% glucose and boosted blastospore yields (> 2.0 × 10⁹ blastospores mL^-1). Fermentation performed in a bench-scale bioreactor consistently promoted high blastospore yields when using NaCl (2.5 MPa) amended media within 3 days. Mealworm larvae (Tenebrio molitor) were similarly susceptible to NaCl-grown blastospores and aerial conidia in a dose-time-dependent manner. Collectively, these results demonstrate the use of hyperosmotic liquid culture media in triggering enhanced yeast-like growth by B. bassiana. Understanding the role of osmotic pressure on blastospore formation and fitness will hasten the development of viable commercial fungal biopesticides. KEY POINTS: • Osmotic pressure plays a critical role in submerged fermentation of B. bassiana. • Ionic/non-ionic osmolytes greatly impact blastospore morphology, fitness, and yield. • Desiccation tolerance and bioefficacy of blastospores are affected by the osmolyte.

An entomopathogenic fungus exploits its host humoral antibacterial immunity to minimize bacterial competition in the hemolymph

BACKGROUND

The insect hemolymph (blood-equivalent fluid), composed of a large number of hemocytes (blood cells) and a variety of soluble immune effectors, is hostile for pathogens including fungi. In order to survive in the insect hemocoel (body cavity), the entomopathogenic fungus (EPF) has evolved two classical coping strategies, namely evasion and suppression of the host immune reactions. However, it remains unclear whether EPF has other ways of coping with host immunity.

RESULTS

In this study, we demonstrated that Metarhizium rileyi (an EPF) infection by injection of blastospores into the hemocoel enhanced the plasma antibacterial activity of cotton bollworm (Helicoverpa armigera), which was partially due to the enhanced expression of antimicrobial peptides (AMPs). The early stage of M. rileyi infection induced the translocation of gut bacteria into the hemocoel, where they were subsequently cleared due to the enhanced plasma antibacterial activity. Further, we showed that the enhanced plasma antibacterial activity and AMP expression were attributable to M. rileyi but not the invasive gut bacteria (opportunistic bacteria). Elevated ecdysone (major steroid hormone in insects) levels in the hemolymph at 48 h post-M. rileyi infection might contribute to the enhanced expression of AMPs. The fungus-elicited AMPs, such as cecropin 3 or lebocin, exhibited potent inhibitory activity against the opportunistic bacteria but not against hyphal bodies. In addition, the opportunistic bacteria competed with hyphal bodies for amino acid nutrients.

CONCLUSIONS

M. rileyi infection induced the translocation of gut bacteria, and then the fungi activated and exploited its host humoral antibacterial immunity to eliminate opportunistic bacteria, preventing them from competing for nutrients in the hemolymph. Unlike the classical strategies, EPF utilizes to evade or suppress host immunity, our findings reveal a novel strategy of interaction between EPF and host immunity. Video Abstract.

Effect of ultraviolet radiation on Beauveria bassiana virulence and development of protective formulations

Locusta migratoria is a serious agricultural pest in China. Beauveria bassiana is one of the most important pathogens of grasshoppers and locusts. The effects of ultraviolet light were evaluated on the B. bassiana strain BbZJ1. The results showed that 253.7 and 360 nm wavelength UV (Ultra Violet) did not affect the germination of B. bassiana after its recovery from UV treatments. Nevertheless, the virulence of B. bassiana BbZJ1 after its recovery from radiation of UV (253.7 nm) increased. The mortality rates were 85.00% for the BbZJ1 control, was 96.67% for BbZJ1 recovered from radiation of UV (253.7 nm) for 60 min. After treatment with 253.7 nm UV radiation for 60 min, the expression levels of stress-resistant genes BbAlg9 and Bbadh2 in BbZJ1 strain were 2.68 and 2.29 times higher than those in the control group, respectively. Meanwhile, the B. bassiana prepared in 5% groundnut oil showed highest tolerance levels to the ultraviolet radiation. The 5% groundnut oil was the most suitable potential UV-protectant for B. bassiana in terms of cost and availability.

Two are better than one: the combinations of Beauveria bassiana, diatomaceous earth, and indoxacarb as effective wheat protectants

The current study evaluates the efficacy of the entomopathogenic fungus Beauveria bassiana (Balsamo-Crivelli) Vuillemin (Hypocreales: Cordycipitaceae), diatomaceous earth (DE) (Protect-It), and the oxadiazine indoxacarb, at single or combined applications on wheat kernels, for the management of the rusty grain beetle, Cryptolestes ferrugineus (Stephens) (Coleoptera: Laemophloeidae), the red flour beetle, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), the khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), and the lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae). The study was conducted between November 2020 and August 2021 in Faisalabad under a complete randomized block design. The combination of DE + indoxacarb was the most efficient as it caused higher overall mortalities ranging between 59.34 and 100%, and lower overall progeny production ranging between 8.35 and 33.70 individuals per vial, than all other treatments. Beauveria bassiana alone exhibited the lowest mortality rates ranging between 22.33 and 47.76%, and the highest offspring emergence, ranging between 51.33 and 78.55 individuals per vial. Similar pattern was observed when persistence bioassays were conducted. For a period of 120 days, the DE + indoxacarb was the most powerful combination against all tested species, providing overall mortality rates between 17.06 and 63.80%. The overall progeny production was lower for the insect individuals exposed on wheat treated with the DE + indoxacarb combination, ranging between 13.66 and 52.23 individuals per vial, and higher for those exposed to B. bassiana alone, ranging between 44.03 and 107.67 individuals per vial, for the entire duration of storage. However, the efficacy of all treatments decreased gradually during the course of storage. The findings of the current study indicate that the combinations of entomopathogenic fungi, DE, and indoxacarb can be used for the prolonged protection of stored wheat from the tested noxious insect species of stored products. Further research, which will include other inert dusts in combination with entomopathogenic fungi and indoxacarb, may provide additional knowledge towards an effective management of noxious species occurring in storages.

Exposure of newly deposited Aedes aegypti eggs to Metarhizium humberi and fungal development on the eggs

Aedes aegypti, an important vector of viral diseases affecting humans in the tropics, generally oviposits just above the water line of small artificial bodies of water. Within the first hours after being deposited eggs are highly susceptible to desiccation, and the chorion undergoes profound processes of sclerotization. Most uneclosed eggs remain viable for months, and their susceptibility to entomopathogenic fungi turns them into reasonable targets for focal control strategies. This study explored the sensitivity of newly deposited eggs to Metarhizium humberi IP 46 conidia. Immediate exposure of eggs oviposited onto a wet, conidium-treated substrate or application of conidia onto eggs within the first 72h after deposition revealed no clearly higher ovicidal effect caused by pre-germinating or germinating conidia or by further fungal development during this initial phase of chorionic sclerotization and embryogenesis than occurs on fully sclerotized eggs. Fungal application techniques, whether direct or indirect, seemed to matter little at the low concentrations applied here; using higher conidial concentrations of the entomopathogen might yield greater mortality of eggs regardless of their physiological age. Quite apart from the data on the biocontrol potential of M. humberi against A. aegypti eggs, these studies demonstrate that the bleaching of highly melanized egg chorions allows detailed visualization of early events of pathogenic fungal attachment, germination, penetration, and initial development inside a target insect.

Study on the insecticidal activity of entomopathogenic fungi for the control of the fruit fly (Anastrepha obliqua), the main pest in mango crop in Colombia

The aim of this study was to evaluate and select entomopathogenic fungi that produces insecticidal compounds for the control of adults of Anastrepha obliqua Macquart (Diptera: tephritidae) that are the main pest of mango (Mangifera indica L. Bark) in Colombia. Nine entomopathogenic fungi isolates were evaluated, five belonging to the genus Metarhizium and four belonging to the genus Beauveria. One strain of the species Metarhizium robertsii with insecticidal activity was selected. By column fractionation, an active fraction was obtained, which caused mortalities higher than 90% after 48 h of exposure. Through HPLC it was determined that the active fraction is composed of more than 22 metabolites. Identification of the metabolites by UHPLC MS/MS revealed the presence of destruxin in E, D, A and B groups (destruxin E-diol, destruxin D, destruxin D1, destruxin D2, destruxin A2, destruxin A, destruxin A3, dihydrodestruxin A, desmB, destruxin B2, destruxin B and destruxin B1). The evaluation of the insecticidal capacity of the organic fractions obtained by HPLC indicated that the extract obtained from the isolate M. robertsii had a compound with high activity on adults of A. obliqua (destruxin A) causing massive mortality of up to 100%, after 48 h of the treatment administration. Furthermore, two other compounds with medium activity were found (destruxin A2 and destruxin B), showing mortalities between 60.0 and 81.3%, respectively. The extract of the isolate MT008 of M. robertsii showed higher insecticidal activity and a potential source for the control of A. obliqua.

Arginine metabolism governs microcycle conidiation by changing nitric oxide content in Metarhizium acridum

Microcycle conidiation commonly exists in filamentous fungi and has great potential for mass production of mycoinsecticides. L-Arginine metabolism is essential for conidiation and conditional growth and virulence, but its role in microcycle conidiation has not been explored. Here, a unique putative arginase (MaAGA) was characterized in the entomopathogenic fungus Metarhizium acridum. Conidial germination and thermotolerance were facilitated by the disruption of MaAGA. Despite little impact on fungal growth and virulence, the disruption resulted in normal conidiation after a 60-h incubation on microcycle conidiation medium (SYA) under normal culture conditions. In the MaAGA-disruption mutant (ΔMaAGA), intracellular arginine accumulation was sharply increased. Replenishment of the direct metabolites of arginase, namely ornithine and/or urea, was unable to restore the disruption mutant's microcycle conidiation on SYA. Interestingly, nitric oxide synthase (NOS) activity and nitric oxide (NO) levels of the ΔMaAGA strain were markedly decreased in the 60-h-old SYA cultures. Finally, adding N^ω-nitro-L-arginine, an inhibitor of NOS, into the SYA converted the microcycle conidiation of the wild-type strain to normal conidiation. In contrast, adding sodium nitroprusside, an NO donor, into the SYA recovered the mutant's microcycle conidiation. The results indicate that arginine metabolism controls microcycle conidiation by changing the content of NO. KEY POINTS: • The MaAGA-disruption led to normal conidiation on microcycle conidiation medium SYA. • Nitric oxide (NO) level of the ΔMaAGA strain was markedly decreased. • Adding an NO donor into the SYA recovered the microcycle conidiation of ΔMaAGA.

Genome-wide identification of BCS1 domain-containing proteins reveals the mitochondrial bcs1 essential for growth, stress response, and virulence of the filamentous entomopathogenic fungus Beauveria bassiana

In yeasts, bcs1 is a mitochondrial AAA protein (ATPase associated with diverse cellular activities) and required for biogenesis of the complex III in mitochondrial electron transfer chain. However, the presence and biological roles of bcs1 remain largely unknown in the filamentous fungi. In present study, genome-wide identification revealed that there were six BCS1-domain containing proteins (Bbbcs1a through f) in the filamentous insect pathogenic fungus Beauveria bassiana, five of which (except for Bbbcs1f) were functionally analyzed. Phenotypic evaluation revealed that only Bbbcs1b and Bbbcs1c contributed to fungal physiologies, and they localized to nuclei and mitochondria, respectively. Hence, Bbbcs1c is considered as the ortholog of yeast bcs1 in B. bassiana. Ablation of Bbbcs1c did not affect biogenesis of mitochondria, but its loss significantly attenuated mitochondrial functionality (e.g., ATP synthesis and mitochondrial targeting of proteins) significantly. ΔBbbcs1c mutant displayed the impaired phenotypes in vegetative growth, stress response, development, and virulence. Notably, ΔBbbcs1c mutant displayed the increased sensitivity to linoleic acid (LA) stress and lost the intracellular fatty acid homeostasis. The Bbbcs1c loss compromised the mitochondrial membrane potential, and LA stress exacerbated this damage. These findings indicate that Bbbcs1c is a functional homolog of yeast bcs1 in B. bassiana and links mitochondrial functionality to unique lifestyle in the entomopathogenic fungi.

Natural infection of Chiromyzinae larvae (Diptera: Stratiomyidae) in southern Chile by Tolypocladium valdiviae sp. nov

A new species from the fungal genus Tolypocladium (Hypocreales: Ophiocordycipitaceae) that infects Stratiomyidae larva from the genus Hylorops is described: Tolypocladium valdiviae Gallardo-Pillancari, Montalva & González. The description is based on both genomic data and morphological characteristics. The sexual stage of T. valdiviae presents fleshy and visible stromata; unlike Tolypocladium ophioglossoides, it is smaller and emerges directly from its host and resembles Tolypocladium longisegmentis and Tolypocladium capitatum, both of which are parasites of deer truffle fungi of the genus Elaphomyces (Ascomycota: Eurotiales). In the anamorphic state, T. valdiviae presents conidiogenous cells similar in shape and arrangement to those of Tolypocladium inflatum, however T. valdiviae produces larger conidiogenous cells and, occasionally, produces chlamydospores. Phylogenetic evidence suggested that T. valdiviae is in a clade close to T. longisegmentis, T. inflatum and T. ophioglossoides, species also recognized to be parasites of fungi of the genus Elaphomyces. The new species is known so far only from Valdivia, southern Chile.

Bt protein hasten entomopathogenic fungi-induced death of nontarget pest whitefly by suppressing protective symbionts

The risk assessment of Bacillus thuringiensis (Bt) crops on nontarget pests has received much attention. Despite the knowledge of various beneficial bacterial symbionts in pests, whether Bt proteins affect these symbionts and subsequently alter the pest's ecology remains largely unknown. The whitefly Bemisia tabaci is one of the most serious nontarget pests in Bt cotton. Here, we explored the Bt Cry1Ac protein-induced changes in whitefly symbiont abundance and the subsequent effects on whitefly response against a naturally prevalent entomopathogenic fungus Cordyceps javanica. The obligate symbiont 'Candidatus Portiera aleyrodidarum' (hereafter P. aleyrodidarum) as well as facultative symbionts 'Candidatus Hamiltonella defensa' (hereafter H. defensa), 'Candidatus Cardinium hertigii' (hereafter C. hertigii) and 'Candidatus Rickettsia bellii' (hereafter R. bellii) dominate the microbial community of whiteflies. The Bt exposure had no effects on H. defensa infected (H) and H. defensa-C. hertigii doubly infected (HC) whiteflies, but decreased the total copy number of symbionts as well as the R. bellii proportion in H. defensa-C. hertigii- R. bellii triply infected whiteflies (HCR). C. javanica caused whitefly adults 100 % mortality within 8 days. Without Bt protein exposure, HCR whiteflies survived significantly longer than H and HC whiteflies sprayed by C. javanica, suggesting that R. bellii confers protection. However, in Bt-exposed groups, C. javanica generated synchronous death of H, HC and HCR whiteflies. Specifically, in H and HC whiteflies, Bt protein-exposure showed no significant difference in progress of death caused by C. javanica. But in HCR whiteflies, Bt exposure hastened death induced by C. javanica, suppressing the R. bellii-conferred protection. This is the first report revealing that Bt protein altered symbiont community conferred adverse effects on nontarget pests, providing a new perspective for Bt risk assessment and biocontrol strategies of nontarget pests.

Functional characterization of two homologs of yeast acetyl-coenzyme A synthetase in the entomopathogenic fungus Beauveria bassiana

Acetyl-coenzyme A (CoA) synthetase (Acs) links cellular metabolism and physiology by catalyzing acetate and CoA into acetyl-CoA. However, the biological roles of Acs are not well studied in entomopathogenic fungi. In this study, two Acs proteins (BbAcs1 and BbAcs2) was functionally characterized in the filamentous insect pathogenic fungus Beauveria bassiana. BbAcs1 and BbAcs2 localize in cytoplasm and peroxisome, respectively. BbAcs1 contributes to vegetative growth on fatty acids as carbon source, and BbAcs2 did not. Both genes did not contribute to fungal response to stresses. The BbAcs1 loss conferred a slight influence on conidiation, and did not result in the defects in blastospore formation. On the contrary, BbAcs2 significantly contributes to lipid metabolism in germlings, blastospore formation, and virulence. The results indicated that Acs2 played a more predominant role than Acs1 in B. bassiana, which links the acetyl-CoA metabolism with the lifestyle of entomopathogenic fungi.

Clonostachys spp., natural mosquito antagonists, and their prospects for biological control of Aedes aegypti

Aedes aegypti (Linnaeus, 1762) is an important vector of arboviruses in the tropics and subtropics. New control strategies based on natural enemies such as entomopathogenic fungi are of utmost importance, and the present study reports the first isolation of Clonostachys spp. (Hypocreales: Bionectriaceae) from mosquitoes and their activity against A. aegypti. Entomopathogenic fungi were surveyed in central Brazil using A. aegypti larvae as sentinels and, also, a CDC light trap. Clonostachys eriocamporesii R.H. Perera & K.D. Hyde, 2020 (IP 440) and Clonostachys byssicola Schroers, 2001 (IP 461) were identified by sequence analysis of the nuclear ribosomal internal transcribed spacer gene, and tested against eggs, larvae, and adults. Both strains were highly active against A. aegypti third instar larvae, with mortalities ≥ 80% at 10⁷ conidia/mL after 5 days but distinctly less active against eggs and adults. This is the first report of both C. eriocamporesii and C. byssicola as naturally occurring pathogens affecting mosquitoes, and IP 440 appears to be a promising control agent against aquatic stages of A. aegypti.

Occurrence of entomopathogenic hypocrealean fungi in mosquitoes and their larval habitats in Central Brazil, and activity against Aedes aegypti

Collecting entomopathogenic fungi associated with mosquitoes and studies on their activity against mosquito developmental stages will improve the understanding of their potential as agents to control important mosquito vectors. Twenty one strains of entomopathogenic fungi affecting mosquitoes in Central Brazil were studied: 7 of Beauveria bassiana, 7 of Metarhizium humberi, 3 of M. anisopliae, 2 of Cordyceps sp. and one each of Akanthomyces saksenae and Simplicillium lamellicola. These fungi were isolated from field-collected mosquito adults (3 strains) or larvae (a single strain); the other 17 strains were isolated from laboratory-reared Aedes aegypti sentinel larvae set out in partially immersed cages placed in diverse small- to middle-sized aquatic mosquito habitats in or close to areas with secondary tropical forest. The frequent recovery of normally soil-borne Metarhizium spp. and B. bassiana from aquatic habitats is notable. Our laboratory findings indicated that M. anisopliae IP 429 and IP 438 and M. humberi IP 421 and IP 478 were highly active against immature stages and, together with M. anisopliae IP 432, also against adults. These strains appear to be the most promising candidates to develop effective control strategies targeting the different developmental stages of A. aegypti, the most important vector of viral diseases in humans in the tropics.

Laboratory and field studies on the combined application of Beauveria bassiana and fipronil against four major stored-product coleopteran insect pests

In the current study we have tested the application of Beauveria bassiana (Hypocreales: Cordycipitaceae) alone and in combination with fipronil at two doses against Tribolium castaneum (Coleoptera: Tenebrionidae), Rhyzopertha dominica (Coleoptera: Bostrychidae), Sitophilus granarius (Coleoptera: Curculionidae), and Trogoderma granarium (Coleoptera: Dermestidae) under laboratory and field conditions. At laboratory conditions, the combination of B. bassiana with the highest dose of fipronil produced the highest mortality. At different temperatures, mortality was increased with the increase in temperature. Maximum mortality was observed at 30 °C, followed by 25 °C and 20 °C for all tested species. Different treatments significantly reduced the progeny number in comparison to control groups for all tested species at all temperatures. In the persistence trial, all treatments that included the combinations of B. bassiana with fipronil produced significantly higher mortalities than the single treatments for all tested species over a period of 6 months. Furthermore, all treatments significantly reduced the number of progenies of all insect species in comparison with the control groups over the same storage period. In field trials, mortalities of all tested insect species were significantly higher on wheat treated with B. bassiana, fipronil, or their combinations than on controls for an entire storage period of 180 days. Overall, R. dominica was found the most susceptible species followed by S. granarius, T. castaneum, and T. granarium. The findings of the current study suggest that the use of B. bassiana and fipronil as grain protectants may provide elevated control against major stored-grain insect species during a prolonged period of storage.

Spatial distancing by fungus-exposed Myrmica ants is prompted by sickness rather than contagiousness

The ecological success of ants relies on their high level of sociality and cooperation between genetically related nestmates. However, these group-living insects suffer from elevated risks of disease outbreak in the whole nest. To face this sanitary challenge, social and spatial distancing of pathogen-exposed individuals from susceptible nestmates appear to be simple, although efficient, ways to limit the propagation of contact-transmitted pathogens. Here we question whether spatial distancing in Myrmica rubra ants is an active response of diseased individuals that correlates with their level of infectiousness. We contaminated foragers with spores of Metarhizium brunneum entomopathogenic fungus. We daily tracked the location of these pathogen-exposed individuals and we analyzed their movement patterns until their death on the 5th day post-contamination. Quite unexpectedly, we found that contagious individuals, whose body was covered with infectious spores, did not reduce their mobility nor stayed far away from larvae in order to limit pathogen transmission to healthy nestmates. Spatial distancing occurred later when diseased individuals were no longer contagious because spores had penetrated their body. These sick ants mainly stayed outside the nest, were less mobile and showed a shift from a superdiffusive to subdiffusive walking pattern. Furthermore, these diseased ants did not actively head towards directions that were opposite to the nest entrance. This study found no evidence for early spatial distancing by contaminated M.rubra workers that would fit to the actual risk of colony-wide contagion. Coupled to a lower mobility and area-reduced walking patterns, the late distancing of moribund individuals appears to be a symptom of sickness resulting from fungus-induced physical and physiological dysfunctions. Besides questioning the truly altruistic nature of death in isolation in this system (and potentially others), we discuss about the ecological and physiological constraints that explain the absence of early distancing when some ant species are exposed to pathogens.

Side effects of a fungus-based biopesticide on stingless bee guarding behaviour

Pathogenic fungi have been used worldwide to control crop pests and are assumed to pose negligible threats to the survival of pollinators. Although eusocial stingless bees provide essential pollination services and might be exposed to these biopesticides in tropical agroecosystems, there is a substantial knowledge gap regarding the side effects of fungal pathogens on behavioural traits that are crucial for colony functioning, such as guarding behaviour. Here, we evaluated the effect of Beauveria bassiana on the sophisticated kin recognition system of Tetragonisca angustula, a bee with morphologically specialized entrance guards. By combining behavioural assays and chemical analyses, we show that guards detect pathogen-exposed nestmates, preventing them from accessing nests. Furthermore, cuticular profiles of pathogen-exposed foragers contained significantly lower amounts of linear alkanes than the unexposed ones. Such chemical cues associated with fungal conidia may potentially trigger aggression towards pathogen-exposed bees, preventing pathogen spread into and among colonies. This is the first demonstration that this highly abundant native bee seems to respond in a much more adaptive way to a potentially infectious threat, outweighing the costs of losing foraging workforce when reducing the chances of fungal pathogen outbreaks within their colonies, than honeybees do.

Low- or high-white light irradiance induces similar conidial stress tolerance in Metarhizium robertsii

White light during mycelial growth influences high conidial stress tolerance of the insect-pathogenic fungus Metarhizium robertsii, but little is known if low- or high-white light irradiances induce different stress tolerances. The fungus was grown either in the dark using two culture media: on minimal medium (Czapek medium without sucrose = MM) or on potato dextrose agar (PDA) or PDA medium under five different continuous white light irradiances. The stress tolerances of conidia produced on all treatments were evaluated by conidial germination on PDA supplemented with KCl for osmotic stress or on PDA supplemented with menadione for oxidative stress. Conidia produced on MM in the dark were more tolerant to osmotic and oxidative stress than conidia produced on PDA in the dark or under the light. For osmotic stress, growth under the lower to higher irradiances produced conidia with similar tolerances but more tolerant than conidia produced in the dark. For oxidative stress, conidia produced under the white light irradiances were generally more tolerant to menadione than conidia produced in the dark. Moreover, conidia produced in the dark germinated at the same speed when incubated in the dark or under lower irradiance treatment. However, at higher irradiance, conidial germination was delayed compared to germination in the dark, which germinated faster. Therefore, growth under light from low to high irradiances induces similar conidial higher stress tolerances; however, higher white light irradiances cause a delay in germination speed.

Virulence and horizontal transmission of Metarhizium anisopliae by the adults of the greenhouse whitefly Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) and the efficacy of oil formulations against its nymphs

The pathogenicity of dry conidia and fungal suspensions of 16 entomopathogenic fungal isolates (10 Metarhizium anisopliae and six Beauveria bassiana) was evaluated against adults and second instar nymphs of the greenhouse whitefly, Trialeurodes vaporariorum respectively. All the tested isolates were pathogenic to T. vaporariorum and caused mortality of 45-93% against the adults and 24-89% against the nymphs. However, M. anisopliae strains showed higher virulence to both developmental stages as compared to B. bassiana strains. The three most virulent isolates that caused high mortalities in adults were M. anisopliae ICIPE 18, ICIPE 62 and ICIPE 69, with cumulative mortalities of 82, 91 and 93%, and median lethal times (LT50) of 5.20, 5.05 and 4.78 days, respectively. These isolates were further assessed for spore acquisition and retention by the adult insects at 0, 24, 48 and 72 h after exposure to dry conidia spores. There was no significant difference among isolates on their acquisition by the insects, although the effect of time on the number of spores retained by each insect was significant. For M. anisopliae ICIPE 62 and ICIPE 69, spore number was significantly higher immediately after exposure at 0 h than at 24, 48 and 72 h, whereas for M. anisopliae ICIPE 18, the spore number remained constant for all the days. The infected "donor" insects were able to horizontally transmit the acquired spores to uninfected "recipient" insects causing high mortality rates in both donor and recipient groups. Metarhizium anisopliae ICIPE 7, ICIPE 18 and ICIPE 62 were the most virulent isolates against the nymphs in aqueous formulation during the first screening with >80% mortality. However, in 2% (v/v) oil formulations at 1 × 108 conidia/ml, canola formulated ICIPE 62, ICIPE 18 and olive formulated ICIPE 18 were the most effective, resulting in 87.8, 88.1 and 99.4% nymphal mortalities respectively and with lower LT50. Oil formulations significantly enhanced the efficacy and virulence of the isolates against the nymphs compared to aqueous formulations.

Antifungal roles of adult-specific cuticular protein genes of the red flour beetle, Tribolium castaneum

The insect cuticle is a composite structure that can further be divided into a few sub-structural layers. Its large moiety comprises a lattice of chitin fibrils and structural proteins, both of which are stabilized by covalent bonding among them. The cuticle covers the whole surface of insect body, and thus has long been suggested for the involvement in defense against entomopathogens, especially entomopathogenic fungi that infect percutaneously. We have been addressing this issue in the past few years and have so far demonstrated experimentally that chitin synthase 1, laccase2 as well as benzoquinone synthesis-related genes of Tribolium castaneum have indispensable roles in the antifungal host defense. In the present study we focused on another major component of the insect cuticular integument, structural cuticular proteins. We chose three genes coding for adult-specific cuticular proteins, namely CPR4, CPR18 and CPR27, and examined their roles in forming immunologically sound adult cuticular integuments. Analyses of developmental expression revealed that the three genes showed high level expression in the pupal stage. These results are consistent with their proposed roles in constructing cuticle of adult beetles. The RNA interference-mediated gene knockdown was employed to silence these genes, and the administration of double strand RNAs in pupae resulted in the adults with malformed elytra. The single knockdown of the three genes attenuated somewhat the defense of the resulting adult beetles against Beauveria bassiana and Metarhizium anisopliae, but statistical analyses indicated no significant differences from controls. In contrast, the double or triple knockdown mutant beetles displayed a drastic disruption of the host defense against the two entomopathogenic fungal species irrespective of the combination of targeted cuticular protein genes, demonstrating the important roles of the three cuticular protein genes in conferring robust antifungal properties on the adult cuticle. Scanning electron microscopic observation revealed that the germination of conidia attached on the adult body surface was still suppressed after the gene knockdown as in the case of wild-type beetles, suggesting that the weakened antifungal phenotypes resulted from the combined knockdown of the adult-specific cuticular protein genes could not be accounted for by the disfunction of secretion/retention of fungistatic benzoquinone derivatives.

Novel associations in antibiosis stemming from an insect pupal cell

The pupal soil cell of the pecan weevil, Curculio caryae (Coleoptera: Curculionidae), was reported previously to exhibit antibiosis to an entomopathogenic fungus, Beauveria bassiana. The objectives of this study were to examine 1) if the antimicrobial effect occurs in other insects that form pupal cells, 2) whether the effect extends to plant pathogenic fungi, and 3) identify the source of antibiosis in pupal soil cells of C. caryae. Antibiosis of pupal cells against B. bassiana was confirmed in-vitro in three additional curculionids, Diaprepes abbreviatus, Conotrachelus nenuphar, and Pissodes nemorensis, all of which had fewer fungal colonies relative to controls. Pupal soil cells were found to suppress phytopathogenic fungi in-vitro, including suppression of Alternaria solani by D. abbreviatus pupal cell, and that of Monilinia fructicola by C. caryae. The detection of antibiosis of soil cells formed by surface-sterilized insects using sterile soil implies the antimicrobial effect stemmed from inside the insect. Further, a novel biotic mechanism was identified: a bacterium related to Serratia nematodiphila was isolated from C. caryae pupal soil cells and was found to be associated with antibiosis. The bacterial cultures with or without autoclave had similar effects but were not as potent as pupal soil cells for suppressing B. bassiana. Also, autoclaved soil cells and autoclaved bacterial culture suppressed M. fructicola but were not as inhibitory as non-autoclaved soil cells. This indicates that antibiosis may be due to bacterial metabolites, although other factors may also be involved. Our findings suggest potential to develop the antibiotic compounds as novel bio-fungicides to control plant diseases.

Development of Metarhizium humberi in Aedes aegypti eggs

The entomopathogenic fungus Metarhizium humberi affects Aedes aegypti adults, larvae and eggs, but its ovicidal activity is not yet well documented. Conidia of this fungus adhered to the chorion, initiated germination within 12 h, and germinating conidia were detected for up to 10 d after contact with the egg. Germ tubes either penetrated the chorion directly or formed appressoria at the end of a short hypha (<5 μm) or, subsequently, on longer, branched hyphae. Thin layers of what was most probably a fungal mucilaginous excretion were detected on the chorion adjacent to germ tubes, appressoria and hyphae. After 5 d eggs frequently appeared shriveled with ruptures in the chorion, and with the interior filled with hyphae that eventually produced mycelium and new conidia on the egg surfaces. Findings demonstrated that this fungus can infect A. aegypti eggs and subsequently recycle on their surface by producing large numbers of new conidia that should be infective for further generations of eggs, larvae and adults.

Persistence and efficacy of enhanced diatomaceous earth, imidacloprid, and Beauveria bassiana against three coleopteran and one psocid stored-grain insects

The residual efficacy of the enhanced diatomaceous earth (DE) formulation DEBBM alone and in combination with Beauveria bassiana (Hypocreales: Cordycipitaceae) or with the neonicotinoid insecticide imidacloprid against Tribolium castaneum (Coleoptera: Tenebrionidae), Rhyzopertha dominica (Coleoptera: Bostrychidae), Cryptolestes ferrugineus (Coleoptera: Laemophloeidae), and Liposcelis paeta (Psocoptera: Liposcelididae) was investigated in the laboratory. The combination treatments were more effective compared to the single treatments against all examined species. The combinations of DEBBM and imidacloprid and imidacloprid with the highest dose rate of B. bassiana provided the highest mortality values against all tested species for 90 days of storage period. The combination of DEBBM plus B. bassiana resulted to the highest mortalities and to the lowest offspring production of all combinations tested after 180 days of storage. Mortality of adults for each test insect species was decreased over the storage period of 6 months, and the progeny production was increased with the extended storage period. Among the tested insect species, L. paeta was the most susceptible to all three grain protectants followed by C. ferrugineus, R. domina, and T. castaneum. The findings of the current study suggest that the use of DEBBM, imidacloprid, and B. bassiana as grain protectants may provide elevated control of major stored-grain insect species during a prolonged period of storage.

Relative humidity impacts development and activity against Aedes aegypti adults by granular formulations of Metarhizium humberi microsclerotia

The impact of ambient relative humidity (RH) on conidial production of Metarhizium humberi IP 46 microsclerotia (MS) formulated in pellets or granules was investigated, and a promising granular formulation was tested against Aedes aegypti adults to confirm its efficacy. Microcrystalline cellulose (MC) and diatomaceous earth (DE) or a combination of vermiculite (VE), DE and silicon dioxide (SD) were tested as carriers in granular formulations containing MS. A range of 93-96.5% RH was critical for fungal development, and at least 96.5-98.5% RH was required for high conidial production on pellets or granules. Conidial production was clearly higher on pellets and granules prepared with VE than MC as the main carrier. VE granules containing MS were highly active against A. aegypti adults. Most mosquitoes were killed within 6 days after treatment regardless of the exposure time of adults to the formulation (1 min-24 h) or ambient humidity (75 or >98%). Production of conidia on dead adults varied between 7.3 × 10⁶ and 2.2 × 10⁷ conidia/individual, when exposed to MS granules for 12 h and 1 min, respectively. Granular formulations containing VE as the main carrier and MS as the active ingredient of M. humberi have strong potential for use against A. aegypti. KEY POINTS: • High conidial production on granular microsclerotial formulations at >96.5% RH • Vermiculite is more appropriate as a carrier than microcrystalline cellulose • Granules with IP 46 microsclerotia are highly active against Aedes aegypti adults.

Susceptibility of All Nymphal Stages of Bemisia tabaci Biotype B (Hemiptera: Aleyrodidae) to Three Brazilian Isolates of Cordyceps sp. (Hypocreales: Cordycipitaceae) in a Screenhouse Under Variable Temperature and Moisture Conditions

The susceptibility of 1st to 4th instars of Bemisia tabaci (Gennadius, 1989) (Hemiptera: Aleyrodidae) to three isolates of Cordyceps sp. (Hypocreales: Cordycipitaceae) was evaluated in screenhouse experiments under variable temperatures and moisture conditions. No differences in susceptibility to the Cordyceps sp. isolates were observed among 1st, 2nd, and 3rd instar nymphs with respect to median lethal time (LT₅₀) values. Confirmed mortalities ranged from 63.7 to 87.8% when the isolates were tested at 5 × 10⁷ conidia mL^-1. The 4th instar was the least susceptible to the fungal isolates (≤ 36.6% mortality). However, 60.0 to 99.5% of the adults that emerged from 4th instar nymphs previously treated with the fungus succumbed to the infection. Temperature was more detrimental to Cordyceps sp. virulence towards B. tabaci nymphs than relative humidity (RH). At similar RH, median LT₅₀ for 1st instar (9.4 days) was higher than for 3rd instar (5.3 days) when the fungus was tested at 5 × 10⁷ conidia mL^-1; minimal temperatures of ≥ 12.6°C compared to ≥ 17.0°C were registered for experiments with 1st and 3rd instars, respectively. However, temperatures ≥ 35°C for 4 to 6 h daily did not affect the efficacy of the fungus against nymphs. Cordyceps sp. showed high virulence to all life stages of B. tabaci at relatively low RH, and an ability to grow extensively over the leaf surface and to produce high amounts of conidia on infected hosts. These attributes certainly boost its potential as an important pest control component of B. tabaci biotype B, especially for management of populations resistant to synthetic insecticides.

Fumigant activity of essential oils from Cinnamomum and Citrus spp. and pure compounds against Dermanyssus gallinae (De Geer) (Acari: Dermanyssidae) and toxicity toward the nontarget organism Beauveria bassiana (Vuill.)

Dermanyssus gallinae(De Geer) (Acari: Dermanyssidae) is the main ectoparasite associated with laying poultry. This mite is commonly controlled by the application of synthetic chemical insecticides, wich lead to the selection of resistant populations and formation of residues in eggs. Thus, new molecules must be developed to control D. gallinae. This work evaluated the toxicity of essential oils (EOs) from Cinnamomum cassia, Cinnamomum camphora, Cinnamomum camphora var. linalooliferum, Citrus aurantium, Citrus aurantium var. bergamia, Citrus aurantifolia and Citrus reticulata var. tangerine against D. gallinae. Additionally, the chemical profiles of the most bioactive EOs were analyzed by gas chromatography coupled with mass spectrometry (GC-MS) and the major compounds were subjected to new tests using D. gallinae. The most toxic EOs against D. gallinae were evaluated for the nontarget entomopathogenic fungus Beauveria bassiana (Unioeste 88). The EOs from C. cassia (LC₅₀ = 25.43 ± 1.0423 μg/cm³) and C. camphora var. linalooliferum (LC₅₀ = 39.84 ± 1.9635 μg/cm³) were the most active in the fumigant bioassay and caused mortality rates of 96 and 61%, respectively. The GC-MS analysis revealed that the major constituents of EOs from C. cassia and C. camphora var. linalooliferum were trans-cinnamaldehyde and linalool, respectively. The pure compounds, trans-cinnamaldehyde (LC₅₀ = 68.89 ± 3.1391 μg/cm³) and linalool (LC₅₀ = 51.45 ± 1.1967 μg/cm³), were tested on D. gallinae and showed lower toxicity than the EOs. Thus, the compounds were not the only active substances produced by C. cassia and C. camphora var. linalooliferum; moreover synergism may have occurred between the substances. The EOs from C. cassia and C. camphora var. linalooliferum were also toxic to B. bassiana (Unioeste 88). Thus, EOs from C. cassia and C. camphora var. linalooliferum are promising candidates for use in D. gallinae control, but cannot be used in conjunction with the fungus B. bassiana.

Changes in transcriptomic and metabolomic profiles of morphotypes of Ophiocordyceps sinensis within the hemocoel of its host larvae, Thitarodes xiaojinensis

Background

Ophiocordyceps sinensis (Berk.) is a well-known entomopathogenic and medicinal fungus. It parasitizes and mummifies the underground ghost moth larvae to produce a fruiting body named Chinese cordyceps. Specific for the fungus, O. sinensis experiences a biotrophic vegetative growth period spanning over 5 months. During this vegetative growth, it appears successively in the host hemocoel in three/four morphotypes, namely, the yeast-like blastospores (subdivided into proliferative (BP) and stationary phase (BS)), prehyphae (PreHy) and the hyphae (Hy). This peculiar morphogenesis has been elucidated through morphological and ultrastructural observations, but its molecular basis remains cryptic. In this study, transcriptome and metabolome profiling of BP, BS, PreHy and Hy stages were performed to characterize the key genes, metabolites, and signaling pathways that regulated the vegetative development of O. sinensis in Thitarodes xiaojinensis larva.

Results

The molecular events and metabolic pathways that regulated different intracellular processes at various stages were examined. Cluster analyses of differentially expressed genes across the four stages revealed the stage specifically enriched pathways. Analysis of metabolome profiles showed that carbon metabolism and several amino acids biosynthesis were significantly perturbed during the tested development stages of O. sinensis in the host hemocoel. Genes homologous to Saccharomyces cerevisiae MAPK cascade were significantly up-regulated during the transition from blastospore to hypha. The up-regulation of Sho1, a regulator protein, suggested nutrient starvation act a role in activation of MAPK pathway and filamentous growth. In addition, up-regulation of several fatty acid synthesis genes and their corresponding products accumulation in the samples of BS might explain more lipid droplets were observed in BS than in BP. Coupled with the up-regulation of fatty acid degradation during PreHy and Hy stages, it is presumed that lipid accumulation and mobilization play important roles in filamentous development.

Conclusions

This is the first report comprehensively describing developmental transcriptomics and metabolomics of O. sinensis in vivo. Our findings provide new perspectives into the key pathways and hub genes involved in morphological changes of fungus developed in the hemocoel of its host, and are expected to guide future studies on morphogenesis and morphotype changes of entomopathogenic fungi in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07209-2.

Characterization and biocontrol potential of a naturally occurring isolate of Metarhizium pingshaense infecting Conogethes punctiferalis

An entomopathogenic fungus was isolated from an infected larva of Conogethes punctiferalis (Guenée) (Crambidae: Lepidoptera), a highly polyphagous pest recorded from more than 120 plants and widely distributed in Asia and Oceanic countries. The fungus was identified as Metarhizium pingshaense Q.T. Chen & H.L. Guo (Ascomycota: Hypocreales) based on morphological characteristics and molecular studies. Scanning electron microscopic studies were conducted to study the infection of C. punctiferalis by M. pingshaense. Bioassay studies with purified conidial suspension proved that the isolate was highly virulent to C. punctiferalis, causing more than 86 % mortality to fifth instar larvae at 1 × 10⁸ spores/mL, under laboratory conditions. The median lethal concentration (LC₅₀) of the fungus against late instar larvae was 9.1 × 10⁵ conidia/mL and the median survival time (MST) of late instar larvae tested at the doses of 1 × 10⁸ and 1 × 10⁷ conidia/mL were 4.7 and 6.4 days, respectively. The optimal temperature for fungal growth and sporulation was found to be 25 ± 1 °C. This is the first report of M. pingshaense naturally infecting C. punctiferalis. Isolation of a highly virulent strain of this fungus holds promise towards development of a potential mycoinsecticide against this pest.

Roles of autophagy-related genes in conidiogenesis and blastospore formation, virulence, and stress response of Beauveria bassiana

Autophagy is a conserved intracellular recycling mechanism, in which autophagy-related genes 12 and 16 (ATG12 and ATG16) function in a complex controlling the ubiquitin-like conjugation system. In the insect-pathogenic fungus Beauveria bassiana, ATG12 and ATG16 were functionally characterized. Disruption of BbATG12 or BbATG16 resulted in the absence of autophagic bodies under starvation stress. ΔBbATG12 and ΔBbATG16 mutant strains displayed similar defects in asexual development (conidiation and blastospore formation) and tolerance to oxidative stress. ΔBbATG16 strain exhibited the impaired growth on the media with gelatin or chitin as a single nitrogen source, and ΔBbATG12 displayed decreased growth on the media with sucrose, fructose or maltose as a single carbon source. Both BbATG12 and BbATG16 were required for fungal virulence. BbATG16 mutation had more effects on fungal virulence than BbATG12 in topical infection assay, although both genes had similar contributions to fungal virulence in intrahemocoel injection assay. This study indicates that BbATG12 and BbATG16 mediate diverse biological functions in addition to their convergent roles in autophagy.

Genetic diversity among naturally-occurring strains of Beauveria bassiana associated with the introduced coffee berry borer, Hypothenemus hampei, (Coleoptera: Curculionidae) on Hawai'i Island

The coffee berry borer (CBB), Hypothenemus hampei, is considered the most important insect pest of coffee worldwide. CBB was discovered on Hawai'i Island in 2010 and soon thereafter on the islands of O'ahu (2014) and Maui (2016). As part of an areawide effort to manage CBB in Hawai'i, we conducted a survey of naturally-occurring Beauveria associated with the beetle to complement field efficacy studies of the commercial B. bassiana strain GHA. Sampling of CBB from coffee farms or unmanaged sites in various districts on the islands of Hawai'i and O'ahu, and also from Puerto Rico, resulted in >1800 Beauveria isolates. These were initially characterized using colony morphology to differentiate strain GHA, registered for use in Hawai'i, from indigenous congenerics. A total of 114 isolates representative of these indigenous morphotypes were selected for further characterization. Sequencing of the intergenic regions B locus and EFutr identified all as Beauveria bassiana sensu stricto. Sixteen haplotypes were observed, with one more common haplotype present in 12 of 16 sites sampled on Hawai'i Island. This B locus-EFutr haplotype, designated Bb1, was the only haplotype observed in 2016 epizootics on two high-elevation coffee farms on Hawai'i Island with no history of GHA application. Many of the haplotypes showed genetic similarity to those collected from CBB from other countries, including Brazil, Columbia, Nicaragua, and Kenya, but a few were identical to those from other insect species collected in Hawai'i before 2010. This diversity suggests a mixed lineage among B. bassiana strains associated with CBB in the three Hawaiian islands.

Isolation and identification of entomopathogenic fungus from Eastern Ghats of South Indian forest soil and their efficacy as biopesticide for mosquito control

The repeated usage of chemical insecticides, responsible for insecticide resistance in mosquitoes and environmental toxicity. Currently effective and environmental-safe control strategies are needed for the control disease-vector mosquitoes. Entomopathogens can be an effective alternative to chemical insecticide. Herein we isolated and tested 46 soil-borne entomopathogenic fungi belonging to six genera, namely Beauveria sp., Metarhizium sp., Fusarium sp., Aspergillus sp., Trichoderma sp., and Verticillium sp., fungi conidia were tested on Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus larvae. Bioassays results show that M. anisopliae fungal isolate causes a 100%, 98.6% and 92% mortality within six days, on Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus, respectively. M. anisopliae treated three mosquito larvae have lower lifetime with LT₅₀ values in A. stephensi, 2.931 days; A. aegypti, 2.676 days and C. quinquefasciatus, 3.254 days. 18 s rDNA sequence analysis confirmed that the isolated fungus are belonging to the genus of M. anisopliae-VKKH3, B. bassiana-VKBb03, and V. lecanii-VKPH1. Our results clearly show that M. anisopliae has good potential, as a low-cost, environmentally safe tool for the control of A. aegypti, A. stephensi, and C. quinquefasciatus mosquitoes.

Activity of Metarhizium brunneum and Beauveria bassiana against early developmental stages of the false codling moth Thaumatotibia leucotreta

This study evaluated the efficacy of two entomopathogenic Hypocrealean fungi, Metarhizium brunneum (laboratory isolate) and Beauveria bassiana (the commercial product Botanigard), for preventative control of the false codling moth, Thaumatotibia leucotreta. The mortality of eggs and first instar larvae was studied in three different assays. First, fungal virulence was examined under optimal laboratory conditions (25 °C, 85% RH) by placing T. leucotreta eggs on conidia-impregnated filter paper. One-day-old eggs and first instar larvae were susceptible to both fungi. In contrast, 5-day-old eggs (advanced embryo development) were susceptible to M. brunneum, but not to B. bassiana. The activity of both fungi against eggs was assessed under two humidity regimes: 85% RH-optimal for fungal germination, and 60% RH-the average humidity in the laboratory. Pieces of parchment paper serving as oviposition surfaces were treated with each of the fungi and introduced to gravid females at different time points after inoculation (0, 2, 7 and 14 days). Although the tested fungal species differed in their virulence to T. leucotreta eggs, both reduced hatching rate under both humidity regimes to 8.3-58.3%, compared to 71.7-83.3% in the control treatments. To evaluate reduction of T. leucotreta infestation of fruit, 'Ori' citrus fruit (easy peeler Citrus sinensis) were treated with each of the fungi. Eggs were placed on the fruit peels and the fruit were maintained under room conditions (25 °C, 60% RH). Between 41.7% and 54.1% of fruit in control groups were infested by the T. leucotreta larvae. Treatments with either of the fungi resulted in about 16% infestation of the fruit with larvae, a marked (3.3-fold) reduction.

Benzoquinone synthesis-related genes of Tribolium castaneum confer the robust antifungal host defense to the adult beetles through the inhibition of conidial germination on the body surface

Insects fight against invading microbial pathogens through various immune-related measures that comprise 'internal', 'external' as well as 'social' immunities. The defenses by external immunity associated with the cuticular integument are supposed to be of particular importance in repelling entomopathogenic fungi that infect host insects transcutaneously. Among such integument-related defenses, external secretions of benzoquinone derivatives typical of tenebrionid beetles have been suggested to play important roles in the antimicrobial defenses. In the present study, by utilizing the experimental infection system composed of the red flour beetle Tribolium castaneum and generalist ascomycete entomopathogens Beauveria bassiana and Metarhizium anisopliae, we performed the functional assays of the three T. castaneum genes whose involvement in benzoquinone synthesis in the adults has been reported, namely GT39, GT62 and GT63. Observations by scanning electron microcopy (SEM) revealed that the conidia of the two fungal species did not germinate on the wild-type adult body surface but did on the pupae. The expression analyses demonstrated that the levels of GT39 and GT62 mRNA increased from middle pupae and reached high in early adults while GT63 did not show a clear adult-biased expression pattern. The RNA interference-based knockdown of any of the three genes in pupae resulted in the adults compromised to the infection of the both fungal species. SEM observations revealed that the gene silencing allowed the conidial germination on the body surface of the knockdown beetles, thereby impairing the robust antifungal defense of adult beetles. Thus, we have provided direct experimental evidence for the functional importance in vivo of these benzoquinone synthesis-related genes that support the antifungal defense of tenebrionid beetles.

Lack of involvement of chitinase in direct toxicity of Beauveria bassiana cultures to the aphid Myzus persicae

The fungal insect pathogen Beauveria bassiana produces a range of insecticidal metabolites and enzymes, including chitinases and proteases, which may assist the disease progression. The enzymes often play a predominant role in the pathogenicity pathway and both chitinases and proteases have previously been shown to be important in host infection. Spray application of supernatants of B. bassiana broth cultures of an isolate from New Zealand caused significant mortality in the green peach aphid, Myzus persicae, within 24 h, demonstrating an apparent contact toxicity. Three-day-old broth cultures were the most effective, with less insect mortality seen using six-day-old broth. However, aphicidal activity increased again when treating aphids with seven-day-old broth. Cultures grew substantially better and produced more potent aphicidal cultures when cultured in media with an initial pH above 5.5. Chitinase was produced a day earlier than the serine protease Pr1, but the peak production periods of these enzymes did not correlate with the aphicidal activities of three- or six-day-old cultures. Cultures treated with EDTA or heated to inactivate the enzymes still showed strong insecticidal activity. Neither beauvericin nor bassianolide, two known insecticidal metabolites, were detected in the supernatants. Therefore the key aphicidal components of B. bassiana cultures were not associated with chitinase nor Pr1 and are yet to be identified.

Metarhizium humberi sp. nov. (Hypocreales: Clavicipitaceae), a new member of the PARB clade in the Metarhizium anisopliae complex from Latin America

A new species, Metarhizium humberi, from the M. anisopliae complex and sister lineage of the M. anisopliae s.str. in the PARB clade, including M. pingshaense, M. anisopliae, M. robertsii and M. brunneum, is described based on phylogenetic analyses [translation elongation factor 1-alpha (5'TEF and 3'TEF), RNA polymerase II largest subunit (RPB1a), RNA polymerase II second largest subunit (RPB2a) and β-tubulin (BTUB)]. Metarhizium humberi was first collected in 2001 in the Central Brazilian state of Goiás, later found to be a common fungus in soils in Brazil, and since then has also been isolated from coleopteran, hemipteran and lepidopteran insects in Brazil and Mexico. This new species, named in honor of Richard A. Humber, a well-known insect pathologist and taxonomist of entomopathogenic fungi, is characterized by a high insecticidal activity against different developmental stages of arthropod pests with importance in agriculture and vectors of diseases to human and animals.

First record of Aspergillus oryzae as an entomopathogenic fungus against the poultry red mite Dermanyssus gallinae

The poultry red mite, Dermanyssus gallinae, is a blood-feeding ectoparasite that affects egg-laying hens worldwide. Strategies to control this parasite have focused in the use of entomopathogenic fungi, such as Metarhizium anisopliae. However, only a few studies have evaluated the use of Aspergillus oryzae to control D. gallinae and none of them have employed native strains. In the work presented here, a novel entomopathogenic fungus was isolated from a dead D. gallinae. The results of phylogenetic analysis showed 100% similarity between the isolated strain and those of two species, A. oryzae and Aspergillus flavus, and 99.82% similarity with A. parvisclerotigenus, which were in the same branch of the Flavi section of the genus Aspergillus. This entomopathogenic fungus was a non-aflatoxin B1 producer, as shown by the presence of aflatoxin B1 in the conidial infection suspension. Morphological features of fungus in comparison with A. oryzae and A. flavus indicated that the isolated strain belonged to A. oryzae, and was named Aspergillus sp. Dg-1. The pathogenicity of Aspergillus sp. Dg-1 on D. gallinae at different life stages was then assessed under laboratory conditions. The experiments showed that the isolated strain significantly increased the mortality rate in adult mites, up to 24.83 ± 2.25, compared to the mortality rates in the control group, which were 15.17 ± 2.75 (P ＜ 0.05). However, Aspergillus sp. Dg-1 did not have pathogenic effects on the second nymph stage of D. gallinae. Our findings demonstrate that Aspergillus sp. Dg-1 has pathogenic effects on D. gallinae in their adult stage, presenting biocontrol potential against D. gallinae.

Activity of additives and their effect in formulations of Metarhizium anisopliae s.l. IP 46 against Aedes aegypti adults and on post mortem conidiogenesis

BACKGROUND

Oil formulations of entomopathogenic fungi have interest for biological mosquito control.

OBJECTIVES

The activities of M. anisopliae s.l. IP 46 conidia were tested in Aedes aegypti adults either without any formulation or formulated with vegetable or mineral oil and in combination with diatomaceous earth.

FINDINGS

IP 46 was highly active against adults, the vector of important arboviruses in the tropics and subtropics. At an exposure of adults to 3.3 × 10⁷ conidia/cm², values of lethal times TL₅₀ and TL₉₀ reached minimal 3.8 and 4.6 days, respectively, and lethal concentrations LC₅₀ and LC₉₀ were 2.7 × 10⁵ and 2.4 × 10⁶ conidia/cm², respectively, after 10 days of exposure. Activity against adults was improved by diatomaceous earth (KeepDry^® KD) combined with mineral oil (Naturol^® N) or vegetable oil (Graxol^® G). Additives KD or N separately (and G to a lesser extent) or in combination, KD + N and KD + G without conidia had also a clear adulticidal effect. Efficacy of conidia formulated or not with KD + N decreased somewhat at shorter exposure periods. Time of exposure (0.017, 12, 48, 72 or 120 h) of adults to KD and N or IP 46 or conidia and KD and N had no significant effect on mortality. M. anisopliae s.l. recycled on fungus-killed mosquitoes producing high quantities of new conidia regardless of the conidial concentrations or formulations tested. Additives tested had no clear effect on quantitative conidiogenesis on cadavers.

MAIN CONCLUSIONS

Formulations of IP 46 conidia with mineral oil and diatomaceous earth represent a promising tool for the development of potent strategies of focal control of this important vector with entomopathogenic fungi.

Within-host interactions of Metarhizium rileyi strains and nucleopolyhedroviruses in Spodoptera frugiperda and Anticarsia gemmatalis (Lepidoptera: Noctuidae)

Members of the family Baculoviridae have been quite successfully used as biocontrol agents against some lepidopterans. Likewise, a number of fungi are important natural enemies of these pests. An interesting approach to increase control efficacy could be the combination of a given nucleopolyhedrovirus (NPV) and a fungus, since they possess distinct modes of action. As a first step towards this goal, we assessed the interaction between NPV (either AgMNPV-79 or SfMNPV-6nd) and the entomopathogenic fungus Metarhizium rileyi (either CG1153 or CG381), using Anticarsia gemmatalis and Spodoptera frugiperda as hosts. In sequential applications of these pathogens, per os inoculation of an NPV (leaf discs with 2.5 × 10⁴ occlusion bodies) either two days before or two days post-spraying of its counterpart fungal strain (5 × 10³ conidia.cm^-2 sprays) usually resulted in an antagonistic effect. When both pathogens were simultaneously applied at different combined dosages, usually an additive effect was seen. Interestingly, a number of dead larvae showing signs of co-infections (partially with soft integument and partially mummified) were recorded. However, mixes with lower dosages of both pathogens did not cause significantly higher insect mortalities compared to low dosages of the fungus applied alone. The advantages and disadvantages of the simultaneous applications of NPV and M. rileyi aiming at the management of either A. gemmatalis or S. frugiperda were discussed.

Oosporein, an abundant metabolite in Beauveria caledonica, with a feedback induction mechanism and a role in insect virulence

Oosporein was first identified from the insect pathogen Beauveria bassiana >50 y ago. Here, we investigate the insecticidal, anti-feedant and immunomodulation effects of oosporein produced by Beauveria caledonica on the forestry pest Hylobius abietis and model insect Galleria mellonella. We report a novel feedback induction mechanism regulating oosporein production in B. caledonica; exogenous oosporein induces the expression of the oosporein cluster, leading to increased abundance of oosporein biosynthetic enzymes, as shown by label-free quantitative proteomics. Oosporein did not have an anti-feedant effect on H. abietis adults - on the contrary, insects exposed to oosporein-treated food fed more than those exposed to untreated food only. Injected oosporein did not kill insect larvae but increased susceptibility of H. abietis to a subsequent infection. Oosporein did not act as a contact toxin on H. abietis adults and G. mellonella larvae at the concentrations tested. Therefore, it appears that oosporein promotes infection rather than directly killing insects; this could be mediated both by a reduction in haemocyte numbers and by alterations to the humoral immune system. This work makes a case for future research into the potential use of B. caledonica as a biocontrol agent through combinations with oosporein or with enhanced production of oosporein.

In vitro efficacy of Metarhizium anisopliae sensu lato against unfed Amblyomma parvum (Acari: Ixodidae)

Amblyomma parvum Aragão (Acari: Ixodidae) is a tick species found with wide distribution in the Neotropical region. Even though it is a wildlife-related tick, it is also a frequent parasite of domestic animals, is aggressive to human beings and may harbor pathogenic microorganisms. Therefore, it is a target species for control on domestic animals, particularly those at the rural-wildlife interface. Herein, the efficacy of two isolates (E9 and IBCB 425) of an entomopathogenic fungus, Metarhizium anisopliae sensu lato, already evaluated for ticks that parasitize domestic animals, was tested against unfed A. parvum adults. Both isolates displayed high acaricidal efficacy after immersion in fungal conidial suspensions for 5 min. Isolate E9 killed all ticks by the 7th day post-treatment, and isolate IBCB 425 did so by the 11th day. Tick mortality of 80 and 90% was achieved as early as the 3rd and 4th days, respectively, with both treatments. Thus, if a commercial M. anisopliae s.l. acaricide against domestic animal ticks is developed, it would also be effective against A. parvum.

Metarhizium anisopliae infection reduces Trypanosoma congolense reproduction in Glossina fuscipes fuscipes and its ability to acquire or transmit the parasite

Background

Tsetse fly-borne trypanosomiasis remains a significant problem in Africa despite years of interventions and research. The need for new strategies to control and possibly eliminate trypanosomiasis cannot be over-emphasized. Entomopathogenic fungi (EPF) infect their hosts through the cuticle and proliferate within the body of the host causing death in about 3–14 days depending on the concentration. During the infection process, EPF can reduce blood feeding abilities in hematophagous arthropods such as mosquitoes, tsetse flies and ticks, which may subsequently impact the development and transmission of parasites. Here, we report on the effects of infection of tsetse fly (Glossina fuscipes fuscipes) by the EPF, Metarhizium anisopliae ICIPE 30 wild-type strain (WT) and green fluorescent protein-transformed strain (GZP-1) on the ability of the flies to harbor and transmit the parasite, Trypanosoma congolense.

Results

Teneral flies were fed T. congolense-infected blood for 2 h and then infected using velvet carpet fabric impregnated with conidia covered inside a cylindrical plastic tube for 12 h. Control flies were fed with T. congolense-infected blood but not exposed to the fungal treatment via the carpet fabric inside a cylindrical plastic tube. Insects were dissected at 2, 3, 5 and 7 days post-fungal exposure and the density of parasites quantified. Parasite load decreased from 8.7 × 10⁷ at day 2 to between 8.3 × 10⁴ and 1.3 × 10⁵ T. congolense ml^− 1 at day 3 post-fungal exposure in fungus-treated (WT and GZP-1) fly groups. When T. congolense-infected flies were exposed to either fungal strain, they did not transmit the parasite to mice whereas control treatment flies remained capable of parasite transmission. Furthermore, M. anisopliae-inoculated flies which fed on T. congolense-infected mice were not able to acquire the parasites at 4 days post-fungal exposure while parasite acquisition was observed in the control treatment during the same period.

Conclusions

Infection of the vector G. f. fuscipes by the entomopathogenic fungus M. anisopliae negatively affected the multiplication of the parasite T. congolense in the fly and reduced the vectorial capacity to acquire or transmit the parasite.

Activity of native and commercial strains of Metarhizium spp. against the poultry red mite Dermanyssus gallinae under different environmental conditions

The poultry red mite (PRM), Dermanyssus gallinae, is a major pest of laying hens with extremely limited control means. To evaluate the potential of natural and commercial entomopathogenic fungi (EPF) for use against D. gallinae, we tested four wild populations of D. gallinae from Israeli farms. The genus Aspergillus was identified as the most abundant isolates from the mites. Additionally, eight new isolates of Metarhizium belonging to the specie M. brunneum were identified. At all sites from which mites were collected in both seasons, the abundance of fungi on D. gallinae was greater during the winter season than during the summer season. Through indirect inoculations of adult D. gallinae, we examined the virulence of the native Metarhizium species, the commercial strain Ma-43 and a previously described acaropathogenic strain (Ma-7). All of the Metarhizium strains caused 56-95% mortality of adult mites by seven days after inoculation at a concentration of 5 × 10⁵ conidia per cm². The efficacies of Ma-43, Ma-7 and the most promising native strain were tested under optimal abiotic conditions (28°C; 85-100% RH) and abiotic conditions similar to those typically found in a poultry house (30 °C; 60% RH). Under optimal conditions, the efficacy of all three stains ranged between 85 and 92%. In contrast, under poultry-house conditions, the efficacy of control ranged between 30 and 40%. The incidence of mycoses on mite cadavers was significantly decreased under poultry-house conditions. These results demonstrate the potential of native and commercial Metarhizium strains for use as biopesticides. Future research should address suitable delivery methods and formulations for the effective control of D. gallinae under poultry-house conditions.

The interplay between dose and immune system activation determines fungal infection outcome in the African malaria mosquito, Anopheles gambiae

The Toll pathway is a central regulator of antifungal immunity in insects. In mosquitoes, the Toll pathway affects infections with the fungal entomopathogen, Beauveria bassiana, which is considered a potential mosquito biopesticide. We report here the use of B. bassiana strain I93-825 in Anopheles gambiae to analyze the impact of Toll pathway modulation on mosquito survival. Exposure to a narrow dose range of conidia by direct contact decreased mosquito longevity and median survival. In addition, fungal exposure dose correlated positively and linearly with hazard ratio. Increased Toll signaling by knockdown of its inhibitor, cactus, decreased survivorship of uninfected females, increased mosquito survival after low dose B. bassiana exposure, but had little effect following exposure to higher doses. This observed trade-off could have implications for development of B. bassiana as a prospective vector control tool. On the one hand, selection for small increases in mosquito immune signaling across a narrow dose range could impair efficacy of B. bassiana. On the other hand, costs of immunity and the capacity for higher doses of fungus to overwhelm immune responses could limit evolution of resistance.

The immune strategies of mosquito Aedes aegypti against microbial infection

Yellow fever mosquito Aedes aegypti transmits many devastating arthropod-borne viruses (arboviruses), such as dengue virus, yellow fever virus, Chikungunya virus, and Zika virus, which cause great concern to human health. Mosquito control is an effective method to block the spread of infectious diseases. Ae. aegypti uses its innate immune system to fight against arboviruses, parasites, and fungi. In this review, we briefly summarize the recent findings in the immune response of Ae. aegypti against arboviral and entomopathogenic infections. This review enriches our understanding of the mosquito immune system and provides evidence to support the development of novel mosquito control strategies.