Effects of inoculation method on efficacy of wettable powder and oil dispersion formulations of Beauveria bassiana against Colorado potato beetle larvae under low-humidity conditions

The effects of inoculation method on efficacy of two formulations of Beauveria bassiana strain GHA against Colorado potato beetle larvae were investigated. Under dry greenhouse conditions, ca. 58% mortality was observed among second-instar larvae exposed directly to sprays of B. bassiana conidia, whereas mortality among larvae exposed to similarly treated foliage (either leaf dorsal or ventral surfaces) was <10%. Mortality was ca. 64% among larvae exposed to both direct sprays and foliar spray deposits. Equivalent rates of mortality were observed among larvae treated with a clay-based wettable powder versus an emulsifiable oil-based formulation of B. bassiana conidia; however, this was observed despite application of an approximately 40% greater dose of WP-formulated conidia, indicating greater efficacy of the emulsifiable oil formulation. These results suggest that, under dry conditions, potato beetle larvae do not readily acquire an effective dose of conidia from treated foliage and that development of improved application technologies to more effectively target the larvae may ultimately prove more beneficial than development of formulations with greater foliar persistence.