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

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.

A tritrophic effect of host plant on susceptibility of western flower thrips to the entomopathogenic fungus Beauveria bassiana

Adult female western flower thrips (Frankliniella occidentalis) were exposed 12-24h to bean (Phaseolus vulgaris) and impatiens (Impatiens wallerana) leaf disks treated with Beauveria bassiana conidia and then transferred to clean bean or impatiens at various times post-treatment. Significantly greater levels of fungal infection were observed when thrips were treated on bean versus impatiens, but exposure to impatiens following treatment had no effect on fungal infection (percent mortality). This result, combined with observations of no inhibition of germination of conidia exposed to intact or macerated impatiens foliage, indicated that the negative effect of the impatiens host plant was not due to plant chemical compounds (antibiosis). Further observations revealed that insects acquired (picked-up) 75% more conidia from treated bean disks than from treated impatiens disks. This difference in dose acquisition was determined to account for the observed difference in percent mortality (15%) following treatment on the two host plants. Median lethal doses (LD(50)) of B. bassiana were not significantly different on the two host plants, but median lethal concentrations were nearly 7-fold greater on impatiens. This difference was explained by disproportionate rates of conidial acquisition at measured rates of conidial deposition (an inverse relationship was observed between application rate expressed as conidia/mm(2) and the number of conidia acquired). The mechanism underlying the differential rates of conidial acquisition from bean versus impatiens was not determined.