Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/4/2010
Publication Date: 1/7/2010
Citation: Wraight, S.P., Ramos, M., Williams, J.E., Avery, P.B., Jaronski, S., Vandenberg, J.D. 2010. COMPARATIVE VIRULENCE OF BEAUVERIA BASSIANA ISOLATES AGAINST LEPIDOPTERAN PESTS OF VEGETABLE CROPS. Journal of Invertebrate Pathology. 103:186-199.
Interpretive Summary: Caterpillar larvae, including diamondback moth, armyworms, cutworms, loopers, and bud, fruit, and stem borers are important pests of vegetable crops. Crops are commonly attacked by multiple species, and these pest complexes pose a difficult control problem. Compounding this problem in recent years has been the loss of many of the broad-spectrum synthetic chemical insecticides long relied upon for control (removed from registrations due to long-term risks posed to consumers). This has stimulated development of biological and other alternatives for pest management. A number of fungus-based biopesticides have been developed in recent years, and some have demonstrated good efficacy against a few key pests of vegetable crops, including diamondback moth. Commercialized pathogens, however, typically have exhibited limited host ranges, being effective against only one or two members of a caterpillar pest complex. It is generally economically unfeasible for growers to target each of several pests with a different control agent (especially with relatively costly microbial control agents). In this study, strains of the common insect pathogenic fungus Beauveria bassiana were screened with the objective of finding new strains with higher virulence against a broader range of lepidopteran pests. Bioassays of 40 strains of this fungus against larvae of eight common caterpillar pests of vegetable crops resulted in identification of a unique strain with virulence against all pest species greater than or equal to that of the most important commercial strain of B. bassiana currently registered in the U.S. In addition to the discovery of a potentially useful biological control agent, this work demonstrated that screening of even modest numbers of fungal strains from the large culture collections of the USDA and other institutions can be highly productive.
Technical Abstract: Approximately 40 isolates of the entomopathogenic fungus Beauveria bassiana were screened against second-instar larvae of diamondback moth (Plutella xylostella) (DBM), European corn borer (Ostrinia nubilalis) (ECB), corn earworm (Helicoverpa zea) (CEW), and fall armyworm (Spodoptera frugiperda) (FAW), and 30 of these isolates were tested against beet armyworm (Spodoptera exigua) (BAW). Highly virulent isolates identified in the screening assays were also tested against black cutworm (Agrotis ipsilon) (BCW), and the top isolate was also assayed against imported cabbage worm (Pieris rapae) (ICW) and cabbage looper (Trichoplusia ni) (CL). B. bassiana was pathogenic against all lepidopteran species tested, and numerous highly virulent isolates were identified. Corn earworm and beet armyworm were most susceptible to fungal infection, and fall armyworm was least susceptible. Limited testing suggested low susceptibility also of black cutworm and cabbage looper. A unique isolate (strain BB1200) exhibited virulence against all pest species greater than or equal to the most important commercial strain of B. bassiana currently registered in the U.S. (strain GHA). In assays in which larvae were topically sprayed and maintained on the treated substrate for 24 h at 100% relative humidity, 6-day (25ºC) median lethal rates (LR50s) of this isolate against CEW, BAW, DBM, FAW, ICW, ECB, CL, and BCW were 4, 5, 7, 11, 12, 98, 125, and 273 conidia/mm2, respectively. The respective LR50s of commercial strain GHA against these pest species were 9, 67, 97, 1,213, 29, 1,668, 541, and 3,504 conidia/mm2. Use of LR50 versus median lethal dose ratios (comparing LR50s of each isolate to a `standard' strain) generated similar rankings of isolate virulence.