Comparative virulence of Beauveria bassiana isolates against lepidopteran pests of vegetable crops.

Forty-three isolates of the entomopathogenic fungus Beauveria bassiana were screened for virulence 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); 30 of these isolates were tested against beet armyworm (Spodoptera exigua) (BAW). Highly virulent isolates were also tested against black cutworm (Agrotis ipsilon) (BCW), and the most virulent isolate was also assayed against imported cabbage worm (Pieris rapae) (ICW) and cabbage looper (Trichoplusia ni) (CL). All lepidopteran species tested were susceptible to B. bassiana. Corn earworm and beet armyworm were most susceptible to fungal infection, and fall armyworm was least susceptible. Limited testing suggested low susceptibility of black cutworm and cabbage looper. B. bassiana isolate 1200 exhibited virulence against all pest species greater than or equal to commercial strain GHA of B. bassiana currently registered in the USA as BotaniGard. In assays in which larvae were topically sprayed and maintained on the treated substrate for 24h at 100% relative humidity, 6-day (25 degrees C) median lethal concentrations (LC(50)s) of this isolate against CEW, BAW, DBM, FAW, ICW, ECB, CL, and BCW were 4, 5, 7, 11, 12, 98, 125, and 273 conidia/mm(2), respectively. The respective LC(50)s of commercial strain GHA against these pest species were 9, 67, 97, 1213, 29, 1668, 541, and 3504 conidia/mm(2). Use of LC(50) versus median lethal concentration ratios (comparing LC(50)s of each isolate to a "standard" strain) generated similar rankings of isolate virulence. Results from parametric ANOVAs of log LC(50) values followed by Tukey HSD multiple comparisons tests and those from Kruskal-Wallis nonparametric analyses followed by sequential Bonferroni tests for means comparisons were nearly identical.