The Effect of Mode of Exposure to Beauveria bassiana on Conidia Acquisition and Host Mortality of Colorado Potato Beetle, Leptinotarsa decemlineata

Bioefficacy of engineered Beauveria bassiana with scorpion neurotoxin, LqqIT1 against cotton mealybug, Phenacoccus solenopsis and cowpea aphid, Aphis craccivora

Cotton mealybug, Phenacoccus solenopsis (Tinsley) and cowpea aphid Aphis craccivora (Koch) are notorious polyphagous, hemipteran sap sucking insect pests. A recombinant toxin gene 'LqqIT1' from the scorpion Leiurus quinquestriatus quinquestriatus (Ehrenberg) was cloned in the pAL1 fungal expression vector and then expressed in the entomopathogenic fungus Beauveria bassiana (Balasmo) using genetic modification techniques. The genetically transformed B. bassiana strain (BbLqqIT1-3) and its un-transformed parent strain (Bb-C) were screened to infect the third instar nymphs of P. solenopsis and first instar nymph of A. craccivora through leaf treatment and topical application (spray) method at 1 * 107 spores per ml concentration. The recombinant strain BbLqqIT1-3 was highly pathogenic against A. craccivora but non pathogenic to P. solenopsis. BbLqqIT1-3 induced 72 and 43.33% mortality in A. craccivora nymphs 96 h after leaf treatment and topical application, respectively. The nymphs of A. craccivora infected with BbLqqIT1-3 displayed classical neurotoxic symptoms such as sluggishness, solublize and liquification of the body. Crude soluble toxin protein, BbLqqIT1a-CSE and Bb-WT-CSE was extracted from the BbLqqIT1-3 and Bb-C, respectively using ammonium sulphate precipitation method, and their oral toxicity was analyzed at 5 µg/ml concentration. The survival of the studied insects was negatively affected by the crude soluble toxin extracts. The LT50 values of BbLqqIT1a-CSE against P. solenopsis and A. craccivora were 22.18 and 17.69 h, respectively. Exposure to crude soluble toxin extracts also accounted for the imbalance of ionic concentrations in the hemolymph of treated insects such as hyperpotassemia (3.53-8.18 meq/ml) in the P. solenopsis and hypopotassemia (7.52-0.47 meq/ml) in A. craccivora. The transformed fungus BbLqqIT1-3 strain exhibited promising results in invitro study.

Infectivity of Entomopathogenic Fungal Isolates Against Tarnished Plant Bug Lygus lineolaris (Hemiptera: Miridae)

Twelve isolates of entomopathogenic fungi belonging to Metarhizium robertsii, M. pinghaense, M. brunneum, Beauveria bassiana, and Isaria fumosorosea were screened against tarnished plant bug. All isolates were pathogenic, causing mortality from 28.8 ± 3.4 to 96.3 ± 2.7%. The LT50 values ranged from 2.7 to 6.0 d while the LT90 values varied between 6.6 and 15.0 d. Metarhizium robertsii isolate CPD6 (will be under the trade name NoVil) was among the isolates that caused high mortality within shorter times and was selected for study on developmental stages and greenhouse trial. The third-, fourth-, and fifth-instar nymphs, and adults were inoculated with 106, 107, and 108 conidia per ml of NoVil. All the stages were susceptible to fungal infection. However, third and fourth instars were the most susceptible with no significant differences in mortality across the three concentrations. On the other hand, mortality was dose-dependent with fifth-instar nymph and adult stages. The LT50 and LT90 values were also dose-dependent, with higher concentrations having shorter lethal-time values as compared to the lower concentrations. In the greenhouse, pepper plants were sprayed with NoVil and chemical insecticide Flonicamid (as industrial standard), before releasing adult tarnished plant bug. Mortality of 37.3, 75.5, and 76.3% was recorded in the control, NoVil, and Flonicamid, respectively. This study has identified NoVil as a potential mycoinsecticide candidate for the control of tarnished plant bug under greenhouse conditions. Further field testing on juvenile and adults is needed to evaluate the potential for in-field control.

Efficiency of natural substances to protect Beauveria bassiana conidia from UV radiation

BACKGROUND

Solar radiation is assumed to be a major factor limiting the efficacy of entomopathogenic fungi used as biocontrol agents in open field applications. We evaluated 12 natural UV-protective co-formulants for their effect on the survival of UV-exposed Beauveria bassiana spores on agar plates, colza leaf discs and in the field.

RESULTS

Colony-forming unit (CFU) counts of unformulated conidia on agar plates and leaf discs dropped to ≤ 50% after exposure to UV radiation. The highest UV protection was achieved with humic acid, which provided > 90% protection of UV-B-exposed conidia in laboratory experiments. In the field, 10% humic acid increased spore persistence up to 87% at 7 days after application. Sesame and colza oil also provided high UV protection in both assays (> 73% and > 70%, respectively).

CONCLUSIONS

This study shows that it is possible to increase the persistence of B. bassiana spores under exposure to UV radiation by formulation with natural UV-protective additives. UV protectants might, therefore, increase the efficacy of entomopathogenic fungi as biocontrol agents in open field applications. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Spore Acquisition and Survival of Ambrosia Beetles Associated with the Laurel Wilt Pathogen in Avocados after Exposure to Entomopathogenic Fungi

Laurel wilt is a disease threatening the avocado industry in Florida. The causative agent of the disease is a fungus vectored by ambrosia beetles that bore into the trees. Until recently, management strategies for the vectors of the laurel wilt fungus relied solely on chemical control and sanitation practices. Beneficial entomopathogenic fungi (EPF) are the most common and prevalent natural enemies of pathogen vectors. Laboratory experiments demonstrated that commercial strains of EPF can increase the mortality of the primary vector, Xyleborus glabratus, and potential alternative vectors, Xylosandrus crassiusculus, Xyleborus volvulus and Xyleborus bispinatus (Coleoptera: Curculionidae: Scolytinae). Our study provides baseline data for three formulated commercially-available entomopathogenic fungi used as potential biocontrol agents against X. crassiusculus, X. volvulus and X. bispinatus. The specific objectives were to determine: (1) the mean number of viable spores acquired per beetle species adult after being exposed to formulated fungal products containing different strains of EPF (Isaria fumosorosea, Metarhizium brunneum and Beauveria bassiana); and (2) the median and mean survival times using paper disk bioassays. Prior to being used in experiments, all fungal suspensions were adjusted to 2.4 × 10⁶ viable spores/mL. The number of spores acquired by X. crassiusculus was significantly higher after exposure to B. bassiana, compared to the other fungal treatments. For X. volvulus, the numbers of spores acquired per beetle were significantly different amongst the different fungal treatments, and the sequence of spore acquisition rates on X. volvulus from highest to lowest was I. fumosorosea > M. brunneum > B. bassiana. After X. bispinatus beetles were exposed to the different suspensions, the rates of acquisition of spores per beetle amongst the different fungal treatments were similar. Survival estimates (data pooled across two tests) indicated an impact for each entomopathogenic fungus per beetle species after exposure to a filter paper disk treated at the same fungal suspension concentration. Kaplan⁻Meier analysis (censored at day 7) revealed that each beetle species survived significantly shorter in bioassays containing disks treated with EPF compared to water only. This study demonstrated that ambrosia beetles associated with the laurel wilt pathogen in avocados are susceptible to infection by EPF under laboratory conditions. However, the EPF needs to be tested under field conditions to confirm their efficacy against the beetles.

Optimizing modes of inoculation of Rhipicephalus ticks (Acari: Ixodidae) with a mitosporic entomopathogenic fungus in the laboratory

The process of strain selection is an important step in the development of insect pathogens for biological control. Bioassays were conducted in the laboratory to evaluate the efficacy of different methods of inoculation using Rhipicephalus pulchellus Gerstäcker (Acari: Ixodidae) as a model. Initially, an oil-based formulation of Metarhizium anisopliae (Metsch.) Sorok. (Ascomycota: Hypocreales) titred at 10(9) conidia ml(-1) was applied to R. pulchellus adults using a Burgerjon spray tower or a microapplicator. Inoculation by microapplicator yielded poor results (25.0% tick mortality) compared to Burgerjon's spray tower (52.3% tick mortality), although the mean number of fungal conidia on R. pulchellus adults was lower (1.5 x 10(4) +/- 1.1 x 10(3) conidia ml(-1)) after spraying by Burgerjon's spray tower compared to 1 x 10(6) conidia ml(-1) obtained with the microapplicator. Thus, inoculation by Burgerjon's spray tower was selected for further investigations. Different modes of inoculation were tested and included direct spray of inoculum on the tick and substrate (SS), direct spray on the substrate and tick followed by transfer of the tick to clean uncontaminated Petri dish (SP) or indirect inoculation of ticks through substrate (SW). The LC(50) values following contamination of nymphs (LC(50) = 1.4 x 10(7) conidia ml(-1)) and adults (LC(50) = 6.7 x 10(7) conidia ml(-1)) in SS were significantly lower compared to SP; nymphs (LC(50) = 5.7 x 10(8) conidia ml(-1)) and adults (LC(50) = 5.3 x 10(9) conidia ml(-1)) and SW; nymphs (LC(50) = 5 x 10(8) conidia ml(-1)). Although the LC(50) value in SS was the lowest, it recorded the highest tick mortality among control ticks (24.2% at 2 weeks post-treatment) and (23.3% at 3 weeks post-treatment) in nymphs and adults respectively compared to SP (2.5 and 5.8%, respectively) and SW (0.0 and 0.0). Results show that among the modes of inoculation tested, SP was the most appropriate for inoculating R. pulchellus adults. SW and SP were identified as appropriate techniques for infecting the R. pulchellus nymphs with conidia formulated in oil.

Up-regulation of lysozyme gene expression during metamorphosis and immune challenge of the cotton bollworm, Helicoverpa armigera

Lysozymes act as crucial bacteriolytic enzymes in insect immune system by hydrolyzing the beta (1-->4) bonds between N-acetylglucosamine and N-acetylmuramic acid in the peptidoglycan of prokaryotic cell walls. We have isolated and characterized a Helicoverpa armigera cDNA encoding an insect lysozyme named HaLyz. We amplified a fragment by PCR, using degenerate primers derived from the conservative amino acid sequences for performing 5' and 3' RACE. The full-length cDNA was 661 base pairs. The theoretical pI and molecular weight of the protein were computed to be 9.08 and 15.6 kDa, respectively. Prokaryotic expression of the HaLyz ORF by Escherichia coli confirmed the calculated molecular weight of the protein. The deduced 135 amino acids showed high homology with known lysozymes from other insects, ranging from 47% to 89% by BLASTp search in NCBI. Analyses revealed that this protein has a typical lysozyme C signature among amino acids 93-111, CNVTCAEMLLDDITKASTC. An interesting relation between immunity and larva to pupa metamorphosis in insects was discovered. Real time-PCR showed that HaLyz gene expression was transiently enhanced at the onset of metamorphosis of the cotton bollworm, Helicoverpa armigera. The gene expression was up-regulated after the injection of E. coli or entomopathogenic fungi, Beauveria bassiana, but showed different expression patterns.

COMPORTAMENTO E ASPECTOS CITOLÓGICOS DE BEAUVERIA BASSSIANA APÓS PASSAGEM EM OVO, LARVA E ADULTO DE CHRYSOMYA ALBICEPS

RESUMO O objetivo desta pesquisa foi avaliar o comportamento do fungo Beauveria bassiana Balsamo Vuillemin reisolado de diferentes fases de desenvolvimento de Chrysomya albiceps. O fungo B. bassiana foi originalmente isolado de Castnia licus Drury (Lepidoptera: Castniidae), e está depositado na Coleção de Cultura (Micoteca-URM) do Departamento de Micologia da Universidade Federal de Pernambuco, sob o número de acesso 3.447. O experimento foi realizado em condições artificiais sob umidade e temperatura 60 ± 10% e 27 ± 1o C, respectivamente. O percentual de germinação foi elevado, com o maior índice observado no reisolado de larva; o crescimento radial no intervalo de tempo estipulado foi satisfatório, o número de conídios no 12o dia do reisolado de adulto foi maior do que o reisolado de ovo. O número de colônias de todos os reisolados foi superior ao controle. Não foram observadas diferenças nos aspectos citológicos do ciclo de vida de B. bassiana pós-passagem em ovos, larvas e adultos.

Effect of nutrition on growth and virulence of the entomopathogenic fungus Beauveria bassiana

Three isolates of the entomopathogen Beauveria bassiana along with one strain of Metarhizium anisopliae were cultured on seven media with different carbon/nitrogen (C/N) ratios. The effect of nutrition on virulence of the isolates was evaluated via measurement of colony growth, spore yield, germination speed, conidial C/N ratio and Pr1 (a serine protease) activity. 'Osmotic stress' medium produced the lowest colony growth with low numbers of conidia in all isolates. However, these conidia showed a high germination rate and virulence. However, conidial Pr1 activity was low in some isolates. In most but not in all cases conidia from 1% yeast extract, 2% peptone and low (10 : 1) C/N medium had higher Pr1 activity compared with conidia from other media. However, in some instances we could not conclude that there was a relationship among germination rate, conidial Pr1 activity and virulence. C/N ratio of conidia was statistically different among various media and fungal isolates. Conidia with lower C/N ratio generally produced lower LT(50) (lowest median lethal time) values (more virulent). Insect-passaged conidia from different media had lower C/N ratio compared with similar conidia from artificial cultures. Therefore, they should be more virulent than in vitro produced conidia. As germination rate, conidial Pr1 activity and C/N ratio are independent of host, it seems that host-related determinants such as insect cuticle and physiology and environmental conditions may influence host susceptibility and therefore fungal isolate virulence towards host insects.

Isolation and characterisation of Beauveria bassiana isolates from phylloplanes of hedgerow vegetation

A leaf imprinting technique combined with a selective medium was used to document the natural occurrence of Beauveria bassiana on phylloplanes of typical hedgerow plants (grasses, stinging nettle and hawthorn) in May, July and September in a hedgerow in Denmark. The density of B. bassiana (as measured by numbers of colony forming units) was greatest in September and on lower nettle leaves. B. bassiana was isolated from phylloplanes in a different hedgerow the following year and a similar picture of occurrence was found. Genetic diversity of selected in vitro isolates were characterised by Universally Primed (UP) PCR, and 13 distinguishable banding patterns were found at the two localities. Of these, four were shared between the field sites and all plant species harboured isolates of B. bassiana with at least two different banding patterns. The isolation method described represents a valuable tool for studying naturally occurring B. bassiana and for rapid isolation of indigenous strains of the fungus for future development of biocontrol agents. The significance of the findings for the life-cycle of B. bassiana is discussed.

Acquisition of lethal doses of Beauveria bassiana conidia by western flower thrips, Frankliniella occidentalis, exposed to foliar spray residues of formulated and unformulated conidia

Secondary acquisition of Beauveria bassiana conidia was recorded on the whole bodies and selected body parts of second-instar nymphs and adult female western flower thrips exposed to foliar spray residues of three differently formulated conidial preparations, for 24 h. Conidia were formulated in emulsifiable oil or with clay (wettable powder), or were essentially unformulated conidia (technical grade powder suspended in water with a surfactant). Formulation had no significant effect on dose acquisition and no effect on virulence of acquired conidia. The mean nymphal LC50/LD50 was 116 conidia/mm2 and 52conidia/insect, respectively; the values for adults were 19 conidia/mm2 and 5conidia/insect. Greatest numbers of conidia were recorded on the legs and abdomens of nymphs and on the legs, wings, and thoraces of adults. As would be expected, numbers of conidia acquired increased with residue concentration (application rate). However, an inverse relationship was noted between acquisition rate (conidia acquired/total conidia applied) and residue concentration. The mechanism underlying this response was not determined. However, there was no indication that any body parts (e.g., tarsi) became saturated with spores, which suggests that either the thrips were repelled by the conidial residues or that as the concentrations of conidia on the substrate increased, conidia somehow became more difficult to acquire. Slopes of the LC probit regressions were lower than those of the LD regressions (mean 1.14 vs 1.78), suggesting that the low slopes often obtained in fungal pathogen assays could be partly an artifact of unequal rates of dose acquisition at low vs high application rates.

Conidia production by Beauveria bassiana (for the biocontrol of a diamondback moth) during solid-state fermentation in a packed-bed bioreactor

Conidia of Beauveria bassiana CS-1, which have the potential for the control of the diamondback moth (Plutella xylostella), were produced by solid-state fermentation (SSF) using a packed-bed bioreactor with rice straw and wheat bran. As the packing density and the bed height were increased, the production of conidia decreased. In a packed-bed bioreactor under no aeration and no addition of polypropylene (PP) foam (control), the total average of conidia was 4.9 x 10(8) g-1. The production of conidia was affected more by the addition of PP foam as an inert support than forced aeration and was approx. 23 times higher than that of the control. The total average of conidia produced by B. bassiana was 1.1-1.2 x 10(10) g-1 .

Diatomaceous earth increases the efficacy of Beauveria bassiana against Tribolium castaneum larvae and increases conidia attachment

This research tested the suppressive ability of Beauveria bassiana (Balsamo) Vuillemin alone and in combination with diatomaceous earth against the red flour beetle, Tribolium castaneum (Herbst). Adults did not show a dose response to B. bassiana, and the addition of diatomaceous earth (DE) did not result in a significant increase in mortality. Against larvae, however, DE at 190 mg/kg grain enhanced the efficacy of B. bassiana at all concentrations ranging from 33 to 2,700 mg of conidia per kilogram of grain. The presence of DE resulted in 17- and 16-fold decreases in the median lethal concentration of B. bassiana at 56 and 75% RH, respectively. No significant differences in larval mortality in response to B. bassiana and diatomaceous earth alone or in combination were found between 56 and 75% RH. Conidial attachment to larvae was significantly greater with 190 mg/kg DE than without it. The partial analysis of lipids taken up by DE from the larvae revealed the removal of phospholipids and long-chain fatty acids. These results support the hypothesis that diatomaceous earth enhances the efficacy of B. bassiana against larval T. castaneum, at least in part by damaging the insect cuticle, thus increasing conidial attachment and making nutrients more available to conidia for their germination.

Effect of exposure method to Beauveria bassiana and conidia concentration on mortality, mycosis, and sporulation in cadavers of Chilo partellus (Lepidoptera: Pyralidae)

The effects of exposure methods, conidial concentrations, and temperature on mortality, mycosis and sporulation in second instar Chilo partellus cadavers infected by Beauveria bassiana was investigated in laboratory studies. Larvae directly sprayed with conidia, exposed to conidia-treated leaves, and dipped into conidial suspension resulted in high mortality (98-100%). The longest LT50 (3.5 days) and days to mortality (2.6 days) were observed in the treated-leaves exposure method. The shortest LT50 (1 day) and days to mortality (1 day) were recorded for the dipping method. With increasing conidial concentrations, there were decreasing LT50 and days to mortality. Larvae exposed to treated leaves and larvae directly sprayed with conidial suspensions produced high mycoses in cadavers. Exposure of larvae to treated-leaves resulted in high sporulation. At lower concentrations of conidia, both mycosis and sporulation in cadavers were high. The optimum temperature for mycosis was 20 and 15 degrees C for sporulation.

Characterization of Beauveria bassiana and Metarhizium anisopliae isolates for management of tarnished plant bug, Lygus lineolaris (Hemiptera: Miridae)

Selected morphological and physiological characteristics of four Beauveria bassiana (Balsamo) Vuillemin isolates and one Metarhizium anisopliae (Metschnikoff) Sorokin isolate, which are highly pathogenic to Lygus lineolaris (Palisot de Beauvois) (Hemiptera: Miridae), were determined. There were significant differences in conidial size, viability, spore production, speed of germination, relative hyphal growth, and temperature sensitivity. Spore viability after incubation for 24h at 20 degrees C ranged from 91.4 to 98.6% for the five isolates tested. Spore production on quarter-strength Sabouraud dextrose agar plus 0.25% (w/v) yeast extract after 10 days incubation at 20 degrees C ranged from 1.6x10(6) to 15.5x10(6)conidia/cm(2). One B. bassiana isolate (ARSEF 1394) produced significantly more conidia than the others. Spore germination was temperature-dependant for both B. bassiana and M. anisopliae. The time required for 50% germination (TG(50)) ranged from 25.0 to 30.9, 14.0 to 16.6, and 14.8 to 18.0h at 15, 22, and 28 degrees C, respectively. Only the M. anisopliae isolate (ARSEF 3540) had significant spore germination at 35 degrees C with a TG(50) of 11.8h. A destructive sampling method was used to measure the relative hyphal growth rate among isolates. Exposure to high temperature (40-50 degrees C) for 10min had a negative effect on conidial viability. The importance of these characteristics in selecting fungal isolates for management of L. lineolaris is discussed.

Attachment and germination of Entomophaga maimaiga conidia on host and non-host larval cuticle

The lepidopteran-specific fungal pathogen Entomophaga maimaiga is highly virulent against Lymantria dispar (gypsy moth) larvae, and other members of the family Lymantriidae. Numerous species in the subfamily Cuculliinae (Family Noctuidae) are not susceptible to E. maimaiga due to the inability of this fungus to penetrate the larval cuticle. Conidial attachment and germination were compared among five cuculliine species and L. dispar using bioassays and scanning electron microscopy. Although conidia were showered evenly across larvae during bioassays, on L. dispar conidia were most abundant on segments, where they adhered well to the cuticle and germinated at high percentages. Conidia on cuculliine cuticles were predominantly found in large, loose aggregations in intersegmental areas. Few conidia on cuculliine cuticle germinated and scanning electron microscopy revealed a thick film of mucous enveloping conidia. We hypothesize that the conidia on cuculliines become coated by this film and were only loosely attached to the larval cuticle. No such film was seen on L. dispar larvae where individual conidia appeared well attached. On L. dispar larvae many conidia also adhered to setae. To determine if hydrophobicity affected the ability of E. maimaiga conidia to attach and germinate on a substrate, a goniometer was used to determine relative hydrophobicity of larval cuticles. L. dispar cuticle was more hydrophobic than cuculliine cuticle, suggesting that a high level of hydrophobicity could be a required characteristic for hosts. Cuticles from four cuculliine species and L. dispar were sequentially extracted using hexane, chloroform, and methanol. Conidia were showered onto glass slides coated with the different extracts and germination was quantified. Methanol extracts of cuculliine cuticle consistently decreased germination, compared to all extracts of L. dispar cuticle. For all L. dispar extracts, the majority of conidia produced germ tubes, which is a normal prerequisite for cuticular penetration. For the cuculliines, conidia exposed to hexane and chloroform extracts produced secondary conidia as did all controls, but the conidia exposed to cuculliine methanol extracts that germinated produced germ tubes. These studies demonstrated that a range of factors act in concert to prevent E. maimaiga infection of the cuculliine species investigated.

Factors influencing the infectivity of isolates of Paecilomyces fumosoroseus against diamondback moth, Plutella xylostella

Paecilomyces fumosoroseus isolate 1576 was isolated from an insect, but is avirulent against larvae of diamondback moth, Plutella xylostella, and several other species. Isolate 1576 grew faster and produced more conidia than isolate 4461 on potato dextrose agar. Pregermination of conidia failed to increase the infectivity of isolate 1576, but the procedure did increase the infectivity of isolates 3682, 4461, and 4482. Isolates 1576 and 4461 were both more infective when moisture was high during incubation of inoculated larvae. Starved Pl. xylostella larvae were more susceptible than fed larvae to isolate 1576 (40 and 10% mortality, respectively), but starved and fed larvae were similar in susceptibility to isolate 4461. These results show that isolate 1576 grows vigorously in aerial culture and is capable of infecting stressed Pl. xylostella larvae. Further tests are needed to characterize its pathogenicity toward its original host or closely related species.