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

Physiological and Molecular Response Modifications by Ultraviolet-C Radiation in Plutella xylostella and Its Compatibility with Cordyceps fumosorosea

Ultraviolet-C (UV-C) radiation significantly impacts living organisms. UV-C radiation can also be used as a pest management tool. Therefore, this study was designed to investigate the effect of UV-C radiation on the physiology and gene expression level of Plutella xylostella, a destructive vegetable pest. Results showed that, after exposure to UV-C radiation for 3, 6, 12, and 24 h, the activity of SOD (superoxide dismutase) and CAT (catalase) of P. xylostella increased, while the activity of PPO (polyphenol oxidase), POD (peroxidase), AChE (acetylcholinesterase), CarE (carboxylesterase), and ACP (acid phosphatase) decreased with increased exposure time. Correlation coefficient analyses indicated that the activity of CAT correlated positively, while PPO and CarE correlated negatively, with exposure time. Gene regulation analysis via qRT-PCR confirmed a significant increase in regulation in CAT, CarE, and PPO-related genes. We also investigated the effect of UV-C exposure on the virulence of Cordyceps fumosorosea against P. xylostella. Here, results indicated that when the fungal treatment was applied to larvae before UV-C radiation, the virulence of C. fumosorosea was significantly reduced. However, this decline in virulence of C. fumosorosea due to UV-C exposure remained only for one generation, and no effect was observed on secondary infection. On the other hand, when larvae were exposed to UV-C radiation before fungal application, the mortality rate significantly increased as the exposure time to UV-C radiation increased. From the current study, it could be concluded that UV-C exposure suppressed the immunity to P. xylostella, which later enhanced the virulence of entomopathogenic fungi. Moreover, the study also suggested that UV irradiation is an effective pest management tool that could be incorporated into pest management strategies, which could help reduce pesticide application, be economically beneficial for the farmer, and be environmentally safe.

Lack of resistance development in Bemisia tabaci to Isaria fumosorosea after multiple generations of selection

The emergence of insecticide resistant insect pests is of significant concern worldwide. The whitefly, Bemisia tabaci, is an important agricultural pest and has shown incredible resilience developing resistance to a number of chemical pesticides. Entomopathogenic fungi such as Isaria fumosorosea offer an attractive alternative to chemical pesticides for insect control, and this fungus has been shown to be an effective pathogen of B. tabaci. Little is known concerning the potential for the development of resistance to I. fumosorosea by B. tabaci. Five generations of successive survivors of B. tabaci infected by I. fumosorosea were assayed with I. fumosorosea. No significant differences in susceptibility to I. fumosorosea, number of ovarioles, or ovipostioning were seen between any of the generations tested. Effects of I. fumosorosea and cell-free ethyl acetate fractions derived from the fungus on the B. tabaci fat body, ovary, and vitellogenin were also investigated. These data revealed significant deformation and degradation of ovary tissues and associated vitellogenin by the fungal mycelium as well as by cell-free ethyl acetate fungal extracts. These data indicate the lack of the emergence of resistance to I. fumosorosea under the conditions tested and demonstrate invasion of the insect reproductive tissues during fungal infection.

Evaluation of alternative Plutella xylostella control by two Isaria fumosorosea conidial formulations - oil-based formulation and wettable powder - combined with Bacillus thuringiensis

BACKGROUND

Entomopathogenic fungi are potential candidates for controlling the diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae). The control efficacy of two Isaria fumosorosea conidial formulations - wettable powder and oil-based formulation - combined with Bacillus thuringiensis against P. xylostella was tested.

RESULTS

In the laboratory, the combined application of two pathogens increased larval mortality either in an additive or a synergistic way. P. xylostella larvae treated with oil-based formulation died sooner than larvae infected with wettable powder. For pot and field experiments, each formulation was applied alone or combined with B. thuringiensis 668 µg mL(-1) , and then larval mortality, pupation rate, adult emergence rate, female longevity and fecundity were recorded. In pot experiments there was no evidence of any antagonistic effects between the two pathogens. Combined application of B. thuringiensis and a high concentration of the two I. fumosorosea formulations resulted in higher mortality (84.4 and 86.2%) with minimum pupation (15.6 and 11.9%) and adult emergence rates (8.7 and 7.0%). Female longevity and fecundity were significantly reduced by the two formulations at high concentration compared with the control. Similar results were also observed in field experiments.

CONCLUSION

The combined application of I. fumosorosea and B. thuringiensis is a promising alternative strategy for P. xylostella control. © 2015 Society of Chemical Industry.

Evaluation of the time-concentration-mortality responses of Plutella xylostella larvae to the interaction of Isaria fumosorosea with the insecticides beta-cypermethrin and Bacillus thuringiensis

BACKGROUND

Entomopathogenic fungi are potential candidates for controlling Plutella xylostella, a cosmopolitan pest of crucifers. In this study, bioassays were conducted to evaluate the interaction between Isaria fumosorosea and sublethal doses of two insecticides, beta-cypermethrin and Bacillus thuringiensis, against P. xylostella.

RESULTS

Data of each assay were in good agreement with the time-concentration-mortality model, indicating a strong dependence of the fungus and insecticide interaction on both concentration and post-exposure time. Using beta-cypermethrin 58-116 µg mL(-1) or B. thuringiensis 222.5-890 µg mL(-1) with the fungus significantly enhanced fungal efficacy. The LC50 values of the fungus declined over a 1-7 day period after exposure, and the LT50 values decreased with increasing concentration. Based on LC50 or LC90 estimates, synergism between the fungus and beta-cypermethrin resulted in a 2.7-28.3-fold reduction in LC50 values and a 12.1-19.6-fold reduction in LC90 values, while synergism of the fungus with B. thuringiensis led to a 2.4-385.0-fold reduction in LC50 values and a 4.4-151.7-fold reduction in LC90 values.

CONCLUSION

Results show that sublethal doses of B. thuringiensis and beta-cypermethrin can synergise I. fumosorosea activity on P. xylostella, suggesting that combination of I. fumosorosea with the two insecticides might offer an integrated approach to controlling P. xylostella in practice. © 2014 Society of Chemical Industry.

Dietary stress increases the susceptibility of Tribolium castaneum to Beauveria bassiana

Sanitation being an important component of chemical-free management of stored-products pests, the nutritional stress on insects that results from a clean environment may prove advantageous to the use of microbial controls. Dietary stress by food deprivation or suboptimal diet increased susceptibility of the red flour beetle, Tribolium castaneum (Herbst), larvae to Beauveria bassiana (Balsamo) Vuillemin. Fungus-related mortality increased linearly with the number of days of food deprivation. Rearing of larvae on a rice meal diet resulted in slower development and greater susceptibility to B. bassiana than rearing on whole wheat flour with brewer's yeast. Larvae that were fed for 24 h on flour with B. bassiana conidia consumed significantly less and weighed significantly less than those that were fed fungus-free flour. Thus, the presence of B. bassiana conidia is itself a cause of dietary stress.

Evaluation of the time-concentration-mortality responses of Plutella xylostella larvae to the interaction of Beauveria bassiana with a nereistoxin analogue insecticide

Six bioassays were conducted to evaluate the interaction between Beauveria bassiana SG8702 and a nereistoxin analogue insecticide, diammonium S,S'-(2-dimethylaminotrimethylene)di(thiosulfate), which is highly compatible with the fungal biocontrol agent against diamondback moth Plutella xylostella (L.). Second-instar larvae were exposed to sprays of B. bassiana alone (assay 1) at concentrations of 21-38, 157-232 and 822-1133 conidia mm(-2) or together with the insecticide at the low application rates of 5, 10, 25, 50 and 100 microg AI ml(-1) (assays 2-6), and then maintained at 25 degrees C and 12:12 h light:dark photoperiod for daily monitoring of mortality for 8 days. Based on the modelling of the resultant time-concentration-mortality data sets, the fungal agent was highly virulent to P. xylostella with an LC50 decreasing from 269 conidia mm(-2) on day 4 to 107 on day 8. Lower lethal concentrations or shorter median lethal times resulted from fungal sprays including the tested chemical rates; the latter never caused higher mortalities than the fungal treatments alone. The fungal action over 3-7 days after spray was significantly enhanced by including in the fungal sprays the chemical at rates of > or =25 microg ml(-1) for 2.6- to 1756-fold reduction of LC50 values, > or =50 microg ml(-1) for 4- to 274-fold reduction of LC70 values and 100 microg ml(-1) for 9- to 33-fold reduction of LC90 values respectively. These rates were equivalent to 5-20% of the chemical rate labelled for field application. The fungal and chemical interaction outlined above highlights the feasibility of combined formulation or application of B. bassiana and the chemical insecticide for P. xylostella control.

Hydrophobic and electrostatic cell surface properties of blastospores of the entomopathogenic fungus Paecilomyces fumosoroseus

The physicochemical surface properties of blastospores of the entomopathogenic fungus Paecilomyces fumosoroseus were examined. Contact angle measurements were performed on microbial lawns composed of blastospores of P. fumosoroseus to quantify their cell surface energy components. In addition, suspensions of the blastospores were characterized with the microbial adhesion to solvents assay. Zeta potential measurements were used to quantify the surface charge and determine the zero potential of the blastospores. The results show blastospores of P. fumosoroseus are best described as having a basic monopolar surface and classified as hydrophilic. Blastospores are also negatively charged under neutral conditions with an isoelectric point of 3.4.

Comparison of blastospores of two Paecilomyces fumosoroseus isolates: in vitro traits and virulence when injected into fall armyworm, Spodoptera frugiperda

Blastospores of two isolates of Paecilomyces fumosoroseus were compared to determine their productivity in vitro and their relative pathogenicity after injection of fall armyworm, Spodoptera frugiperda. Blastospores of less virulent P. fumosoroseus isolate 1576 are smaller than those of isolate 4461, and they germinate and proliferate more slowly in vitro. The pathogenicity of injected blastospores of isolate 1576 against S. frugiperda varies with larval size. In small larvae, percentage mortality was lower among those injected with isolate 1576 than among those injected with isolate 4461. Large larvae were equally susceptible to both isolates. Survival times were higher for isolate 1576 and were higher at lower doses for both isolates. Those larvae injected late in development that ultimately died, regardless of treatment, did not lose weight typical of developing pupae. Injection of large larvae with isolate 4461 resulted in a significantly longer time to pupation among apparently uninfected larvae.