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

Authors: Wraight, Stephen; RAMOS, MARK - Former ARS Employee

Submitted to: Biocontrol Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2017
Publication Date: 2/28/2017
Citation: Wraight, S.P., Ramos, M.E. 2017. Effects of mode of inoculation on efficacy of wettable powder and oil dispersion formulations of Beauveria bassiana applied against Colorado potato beetle larvae under low-humidity greenhouse conditions. Biocontrol Science and Technology. 27:348-363.

Interpretive Summary: Development and marketing of biopesticides based on insect-pathogenic fungi has long been constrained by inconsistent and/or lower, slower activity compared to the chemical insecticides commonly relied upon for pest control. Much previous research has focused on development of UV-protectant formulations and formulations with superior spreading/sticking properties to enhance survival and persistence of the active ingredient (fungal spores) in the field environment. One premise upon which these efforts are based is that insects feeding on or otherwise in contact with a plant surface contaminated with a fungal pathogen will acquire a lethal dose of the pathogen. In this study, Colorado potato beetle (CPB) larvae were exposed to either a wettable powder or emulsifiable oil formulation of the fungal pathogen Beauveria bassiana in three different ways: 1) larvae on potted potato plants were exposed to direct spray applications and left to feed on the treated plants, 2) larvae were treated as above, but then transferred immediately to untreated plants, and 3) larvae were exposed only to treated plants. Tests were conducted under low-humidity conditions. Mortality was 57% among sprayed larvae, but less than 10% among those exposed only to treated plants. Mortality was 62% among larvae that were exposed to both the spray and the spray residue. Results were the same for both formulations. These findings indicate that, under dry conditions, CPB larvae do not readily acquire a lethal dose of Beauveria from treated foliage and that development of improved application technologies to more effectively target CPB larvae and other insect pests could ultimately prove more beneficial than development of formulations with greater foliar persistence.

Technical Abstract: The effects of inoculation method on efficacy of two formulations of Beauveria bassiana strain GHA against Colorado potato beetle larvae were investigated. Under low-humidity greenhouse conditions, 57% mortality was observed among groups of second-instar larvae exposed directly to sprays of B. bassiana conidia (mean application rate of 332 conidia/mm2), whereas mortality among larvae exposed to similarly treated foliage (either leaf dorsal or ventral surfaces) was < 10%. Mortality was 62% among larvae exposed to both direct sprays and foliar spray deposits. No significant differences in mortality were observed among larvae treated with a clay-based wettable powder versus an emulsifiable oil-based formulation of B. bassiana conidia. 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 beetle larvae may ultimately prove more beneficial than development of formulations with greater foliar persistence.