Prospects for biological control of the western flower thrips, Frankliniella occidentalis, with the entomopathogenic fungus, Metarhizium anisopliae, on chrysanthemum

Efficacy of entomopathogenic fungi, nematodes and spinetoram combinations for integrated management of Thrips tabaci

BACKGROUND

Two consecutive field trials using a blend of entomopathogens in combination with a new chemistry insecticide were conducted to determine treatment effects on onion thrips (Thrips tabaci Lindeman) populations, crop damage, plant development, crop yield and impact on natural enemies. Products were tested in an onion cropping system and included the insect pathogenic fungus Beauveria bassiana (isolate WG-11), an entomopathogenic nematode Heterorhabditis bacteriophora (strain VS) and the new-chemistry chemical insecticide spinetoram.

RESULTS

In all treatments, a significant decrease in thrips per plant population was detected in both trials. Overall, dual application of entomopathogens and insecticide was more effective than singly applied treatments. The lowest number of thrips larvae (1.96 and 3.85) and adults (0.00 and 0.00) were recorded when treated with dual application of B. bassiana and spinetoram at 7 days post application (DPA) after the second spray application in 2017and 2018, respectively. Damage on onion plants was considerably decreased in all treatments relative to the control. The lowest damage was observed on onion plants treated with B. bassiana + spinetoram at 7 DPA after the second spray application during both years. A significant decrease in the number of natural enemies (beetles, spiders, mites, lacewings, ants and bugs) on onion plants was recorded during both years. Insect pathogens when applied alone and in combination with each other considerably protected arthropod natural enemies compared to insecticide application applied alone. Significant increase in plant agronomic traits was observed compared to the control. Among all the treatments, B. bassiana + spinetoram produced maximum leaf length, leaf weight, total leaves, neck diameter, bulb diameter, number of rings per bulb, bulb weight, dry matter and plant yield following the 2017and 2018 applications, respectively.

CONCLUSION

The findings of the study reveal the potential of using insect pathogens and insecticide for control of T. tabaci. However, combinations containing spinetoram are harmful to nontarget organisms, whereas biological control agents help in protecting biodiversity in onion agroecosystems. © 2023 Society of Chemical Industry.

Biological control of Western flower thrips, Frankliniella occidentalis using a self-sustaining granular fungal treatment

Western flower thrips (WFT), Frankliniella occidentalis, is one of the most destructive pests of vegetables, fruits and ornamental crops worldwide, causing extensive damage by direct feeding of the crop and transmitting economically important viruses. Despite the successes of biocontrol agents to control WFT, more efficient and cost-effective ways must be found to encourage grower adoption of integrated pest management. A sustainable fungal treatment was developed to preserve fungal inoculum in potting soil and reduce thrips populations. Combining cooked, oven-dried millet with BotaniGard® (a commercial form of Beauveria bassiana strain GHA) to potting soil increased spore production and persistence of the fungus in the soil. In treated pots with millet, spore concentrations were 3-4 times greater after 30 days compared with spore yields at 10 days. The number of WFT adults was significantly lower in the marigold pots treated with GHA mix + millet than untreated controls, 12% and 10% in treated pots and 70% and 68% in untreated pots in sterile and non-sterile soil, respectively. Incorporation of millet in the potting mix enhanced the effect of the fungal treatments by providing a nutritive substrate on which the fungus could become established. This method is relatively inexpensive and easy for growers to use in greenhouses because granular formulations of B. bassiana are not commercially available.

A global invasion by the thrip, Frankliniella occidentalis: Current virus vector status and its management

Western flower thrip, Frankliniella occidentalis (Pergande), is among the most economically important agricultural pests globally, attacking a wide range of vegetable and horticultural crops. In addition to causing extensive crop damage, the species is notorious for vectoring destructive plant viruses, mainly belonging to the genera Orthotospovirus, Ilarvirus, Alphacarmovirus and Machlomovirus. Once infected by orthotospoviruses, thrips can remain virulent throughout their lifespan and continue transmitting viruses to host plants when and wherever they feed. These irruptive viral outbreaks in crops will permanently disrupt functional integrated pest management systems, and typically require a remedial treatment involving insecticides, contributing to further development of insecticide resistance. To mitigate against this continuing cycle, the most effective management is early and comprehensive surveillance of the pest species and recognition of plant viruses in the field. This review provides information on the pest status of F. occidentalis, discusses the current global status of the viruses vectored by this thrip species, examines the mechanisms involved in transmitting virus-induced diseases by thrips, and reviews different management strategies, highlighting the potential management tactics developed for various cropping systems. The early surveillance and the utilization of potential methods for control of both F. occidentalis and viruses are proposed.

Imidacloprid Pesticide Regulates Gynaikothrips uzeli (Thysanoptera: Phlaeothripidae) Host Choice Behavior and Immunity Against Lecanicillium lecanii (Hypocreales: Clavicipitaceae)

We attempted to develop an efficient management strategy against gall thrips (Gynaikothrips uzeli Zimmermann (Thysanoptera: Phlaeothripidae)) via the combined application of a systemic insecticide (imidacloprid) and an entomopathogenic fungus (Lecanicillium lecanii Zimmerman (Hypocreales: Clavicipitaceae)). The attraction of G. uzeli to Ficus microcarpa volatiles after imidacloprid treatment was weaker than for untreated plants, which could be due to modulation of volatile metabolite profiles by imidacloprid. The toxicity of L. lecanii against nymph and adult thrips was much higher for those that fed on plants treated with a 50% lethal concentration (LC50) of imidacloprid than for the controls. Phenoloxidase (PO) activity was significantly inhibited in treated G. uzeli, while hemocyte abundances were not different in treated and healthy individuals. Thus, imidacloprid impacted the PO-related humoral immunity of G. uzeli, but not their cellular immunity. Overall, F. microcarpa treated with imidacloprid at LC50 concentrations exhibited volatile profiles that decreased the attraction of G. uzeli and also indirectly increased the pathogenicity of L. lecanni by inhibiting the humoral immunity of gall thrips. The results reported here suggest that combined application of imidacloprid and L. lecanii could be used as a new integrated control strategy against gall thrips.

Sub-lethal concentrations of the entomopathogenic fungus, Beauveria bassiana increase fitness costs of Helicoverpa armigera (Lepidoptera: Noctuidae) offspring

We assessed the effects of exposure of the cotton bollworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) larvae to the entomopathogenic fungus Beauveria bassiana (isolate DC2) on life history parameters of the subsequent generation. Mortality assays against second instar larvae showed B. bassiana isolates to be significantly virulent, causing high mortality. Life history parameters such as developmental time, fecundity and survivorship were affected in the offspring of survivors of exposure to LC₂₅ and LC₅₀ concentrations of B. bassiana DC2. Pre-adult duration of H. armigera was significantly extended and total longevity and female progeny fecundity were decreased. Oviposition duration was shortened compared to offspring of untreated controls. All population parameters including intrinsic (r) and finite (λ) rates of increase and net (R₀) and gross (GRR) reproductive rates were significantly decreased in offspring derived from H. armigera larvae treated with B. bassiana DC2. The intrinsic rate of increase (r) was 0.198 d^-1, 0.120 d^-1 and 0.111 d^-1, respectively, for the F1 generation of controls (untreated), LC₂₅- and LC₅₀-treated cotton bollworm. The morphogenetic effects of B. bassiana observed due to the indirect effect of sub-lethal concentrations included abnormal pupae and adults and mortality resulted from the malformations. Pupal weight was reduced in offspring of treated individuals but sex ratios did not differ. Our results revealed that B. bassiana increased fitness costs of H. armigera in both direct (mortality) and indirect (disruption of normal development) ways. Adverse effects of sub-lethal fungal treatments on the parent generation carried over to the next generation.

Feeding on Beauveria bassiana-treated Frankliniella occidentalis causes negative effects on the predatory mite Neoseiulus barkeri

The entomopathogenic fungus Beauveria bassiana and the predatory mite Neoseiulus barkeri are both potential biocontrol agents for their shared host/prey Frankliniella occidentalis. The combination of the two agents may enhance biological control of F. occidentalis if the fungus does not negatively affect N. barkeri. This study evaluated the indirect effects of B. bassiana strain SZ-26 on N. barkeri mediated by F. occidentalis using the age-stage, two-sex life table. When fed on the first instar larvae of F. occidentalis that had been exposed for 12 h to the SZ-26 suspension, the developmental time of preadult N. barkeri was significantly longer, and the longevity and fecundity were significantly lower than that of N. barkeri fed on untreated F. occidentalis. The mean generation time (T), net reproductive rate (R0), finite rate of increase (λ), intrinsic rate of natural increase (rm) and predation rates were correspondingly affected. The data showed that B. bassiana has indirect negative effects on N. barkeri population dynamics via influencing their prey F. occidentalis larvae, which indicates that there is a risk in combining B. bassiana with N. barkeri simultaneously for the biocontrol of F. occidentalis. The probable mechanism for the negative effects is discussed.

Optimizing Western Flower Thrips Management on French Beans by Combined Use of Beneficials and Imidacloprid

Western flower thrips (WFT), Frankliniella occidentalis (Pergande), is an important pest of vegetable crops worldwide and has developed resistance to many insecticides. The predatory mites Neoseiulus (=Amblyseius) cucumeris (Oudemans), the entomopathogenic fungus Metarhizium anisopliae (Metsch.), and an insecticide (imidacloprid) were tested for their efficacy to reduce WFT population density and damage to French bean (Phaseolus vulgaris L.) pods under field conditions in two planting periods. Metarhizium anisopliae was applied as a foliar spray weekly at a rate of one litre spray volume per plot while imidacloprid was applied as a soil drench every two weeks at a rate of two litres of a mixture of water and imidacloprid per m². Neoseiulus cucumeris was released every two weeks on plant foliage at a rate of three mites per plant. Single and combined treatment applications reduced WFT population density by at least three times and WFT damage to French bean pods by at least 1.7 times compared with untreated plots. The benefit-cost ratios in management of WFT were profitable with highest returns realized on imidacloprid treated plots. The results indicate that M. anisopliae, N. cucumeris, and imidacloprid have the potential for use in developing an integrated pest management program against WFT on French beans.

Colonization of onions by endophytic fungi and their impacts on the biology of Thrips tabaci

Endophytic fungi, which live within host plant tissues without causing any visible symptom of infection, are important mutualists that mediate plant-herbivore interactions. Thrips tabaci (Lindeman) is one of the key pests of onion, Allium cepa L., an economically important agricultural crop cultivated worldwide. However, information on endophyte colonization of onions, and their impacts on the biology of thrips feeding on them, is lacking. We tested the colonization of onion plants by selected fungal endophyte isolates using two inoculation methods. The effects of inoculated endophytes on T. tabaci infesting onion were also examined. Seven fungal endophytes used in our study were able to colonize onion plants either by the seed or seedling inoculation methods. Seed inoculation resulted in 1.47 times higher mean percentage post-inoculation recovery of all the endophytes tested as compared to seedling inoculation. Fewer thrips were observed on plants inoculated with Clonostachys rosea ICIPE 707, Trichoderma asperellum M2RT4, Trichoderma atroviride ICIPE 710, Trichoderma harzianum 709, Hypocrea lixii F3ST1 and Fusarium sp. ICIPE 712 isolates as compared to those inoculated with Fusarium sp. ICIPE 717 and the control treatments. Onion plants colonized by C. rosea ICIPE 707, T. asperellum M2RT4, T. atroviride ICIPE 710 and H. lixii F3ST1 had significantly lower feeding punctures as compared to the other treatments. Among the isolates tested, the lowest numbers of eggs were laid by T. tabaci on H. lixii F3ST1 and C. rosea ICIPE 707 inoculated plants. These results extend the knowledge on colonization of onions by fungal endophytes and their effects on Thrips tabaci.

An entomopathogenic strain of Beauveria bassiana against Frankliniella occidentalis with no detrimental effect on the predatory mite Neoseiulus barkeri: evidence from laboratory bioassay and scanning electron microscopic observation

Among 28 isolates of Beauveria bassiana tested for virulence against F. occidentalis in laboratory bioassays, we found strain SZ-26 as the most potent, causing 96% mortality in adults at 1×10(7) mL(-1)conidia after 4 days. The effect of the strain SZ-26 on survival, longevity and fecundity of the predatory mite Neoseiulus (Amblyseius) barkeri Hughes were studied under laboratory conditions. The bioassay results showed that the corrected mortalities were less than 4 and 8% at 10 days following inoculation of the adult and the larvae of the predator, respectively, with 1×10(7) conidia mL(-1) of SZ-26. Furthermore, no fungal hyphae were found in dead predators. The oviposition and postoviposition durations, longevity, and fecundity displayed no significant differences after inoculation with SZ-26 using first-instar larvae of F. occidentalis as prey in comparison with untreated predator. In contrast, the preoviposition durations were significantly longer. Observations with a scanning electron microscope, revealed that many conidia were attached to the cuticles of F. occidentalis at 2 h after treatment with germ tubes oriented toward cuticle at 24 h, penetration of the insect cuticle at 36 h, and finally, fungal colonization of the whole insect body at 60 h. In contrast, we never observed penetration of the predator's cuticle and conidia were shed gradually from the body, further demonstrating that B. bassiana strain SZ-26 show high toxicity against F. occidentalis but no pathogenicity to predatory mite.

Use of Metarhizium anisopliae chitinase genes for genotyping and virulence characterization

Virulence is the primary factor used for selection of entomopathogenic fungi (EPF) for development as biopesticides. To understand the genetic mechanisms underlying differences in virulence of fungal isolates on various arthropod pests, we compared the chitinase genes, chi2 and chi4, of 8 isolates of Metarhizium anisopliae. The clustering of the isolates showed various groups depending on their virulence. However, the analysis of their chitinase DNA sequences chi2 and chi4 did not reveal major divergences. Although their protein translates have been implicated in fungal virulence, the predicted protein structure of chi2 was identical for all isolates. Despite the critical role of chitin digestion in fungal infection, we conclude that chi2 and chi4 genes cannot serve as molecular markers to characterize observed variations in virulence among M. anisopliae isolates as previously suggested. Nevertheless, processes controlling the efficient upregulation of chitinase expression might be responsible for different virulence characteristics. Further studies using comparative “in vitro” chitin digestion techniques would be more appropriate to compare the quality and the quantity of chitinase production between fungal isolates.

Selection of promising fungal biological control agent of the western flower thrips Frankliniella occidentalis (Pergande)

AIMS

Larval stages of Frankliniella occidentalis are known to be refractory to fungal infection compared with the adult stage. The objective of this study was to identify promising fungal isolate(s) for the control of larval stages of F. occidentalis.

METHODS AND RESULTS

Ten isolates of Metarhizium anisopliae and eight of Beauveria bassiana were screened for virulence against second-instar larvae of F. occidentalis. Conidial production and genetic polymorphism were also investigated. Metarhizium anisopliae isolates ICIPE 7, ICIPE 20, ICIPE 69 and ICIPE 665 had the shortest LT(50) values of 8.0-8.9 days. ICIPE 69, ICIPE 7 and ICIPE 20 had the lowest LC(50) values of 1.1 × 10(7), 2.0 × 10(7) and 3.0 × 10(7) conidia ml(-1), respectively. Metarhizium anisopliae isolate ICIPE 69 produced significantly more conidia than M. anisopliae isolates ICIPE 7 and ICIPE 20. Internally transcribed spacers sequences alignment showed differences in nucleotides composition, which can partly explain differences in virulence.

CONCLUSION

These results coupled with the previous ones on virulence and field efficacy against other species of thrips make M. anisopliae isolate ICIPE 69 a good candidate.

SIGNIFICANCE AND IMPACT OF THE STUDY

Metarhizium anisopliae isolate ICIPE 69 can be suggested for development as fungus-based biopesticide for thrips management.

Development and applications of destruxins: a review

The insecticidal and phytotoxic activities of destruxins (dtxs) have been well studied. The cyclodepsipeptides, which are dtxs mainly isolated from the fungus Metarhizium anisopliae and other fungi, have been well characterized in vitro and in vivo. A succession of important function, such as antitumoral, antiviral, insecticidal, cytotoxic, immunosuppressant, phytotoxic, and antiproliferative effects have been observed. To date, 39 dtxs derivatives have been identified. Dtxs possess a variety of biological activities, including acting as virulence factors for specific insects, a V-ATPase inhibitor that provides a basis for the development of new drug to against osteoporosis, cancer, or biological control agents, etc. Here, we focus on some of the research progress made on understanding dtxs during the last decade, introduce some of the newly identified dtx members, especially from M. anisopliae, and give an overview of the applications of dtxs. Using the dtxs to learn about and moderate biological events has advanced significantly during the past year. We believe that several ongoing dtx application fields may benefit from the reviewed information herein.

Fungal Peptide Destruxin A Plays a Specific Role in Suppressing the Innate Immune Response in Drosophila melanogaster

Destruxins are a class of insecticidal, anti-viral, and phytotoxic cyclic depsipeptides that are also studied for their toxicity to cancer cells. They are produced by various fungi, and a direct relationship has been established between Destruxin production and the virulence of the entomopathogen Metarhizium anisopliae. Aside from opening calcium channels, their in vivo mode of action during pathogenesis remains largely uncharacterized. To better understand the effects of a Destruxin, we looked at changes in gene expression following injection of Destruxin A into the fruit fly Drosophila melanogaster. Microarray results revealed reduced expression of various antimicrobial peptides that play a major role in the humoral immune response of the fly. Flies co-injected with a non-lethal dose of Destruxin A and the normally innocuous Gram-negative bacteria Escherichia coli, showed increased mortality and an accompanying increase in bacterial titers. Mortality due to sepsis was rescued through ectopic activation of components in the IMD pathway, one of two signal transduction pathways that are responsible for antimicrobial peptide induction. These results demonstrate a novel role for Destruxin A in specific suppression of the humoral immune response in insects.

The susceptibility of immature stages of Bemisia tabaci to the entomopathogenic fungus Lecanicillium muscarium on tomato and verbena foliage

Lecanicillium muscarium is a widely occurring entomopathogenic fungus. Laboratory studies were conducted to determine the efficacy of L. muscarium against different instars of Bemisia tabaci on tomato and verbena foliage after two incubation times (3 and 7 days). Significant reduction in B. tabaci numbers were recorded on fungus treated plants (p < 0.001). Second instar B. tabaci proved most susceptible to L. muscarium infection. There was no significant difference in mortality of B. tabaci second instars after either 3 or 7 days exposure to L. muscarium on either host plant. The importance of the speed of pest mortality following treatment and the potential of L. muscarium to be incorporated into an integrated pest management strategy for the biocontrol of B. tabaci on tomato and verbena plants are discussed.