Characterization of the synergistic interaction between Beauveria bassiana strain GHA and Bacillus thuringiensis morrisoni strain tenebrionis applied against Colorado potato beetle

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

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2017
Publication Date: 1/17/2017
Citation: Wraight, S.P., Ramos, M.E. 2017. Characterization of the synergistic interaction between Beauveria bassiana strain GHA and Bacillus thuringiensis morrisoni strain tenebrionis applied against Colorado potato beetle. Journal of Invertebrate Pathology. 144:47-57.

Interpretive Summary: In a previous study, we reported discovery of synergism between two biopesticides applied against Colorado potato beetle (CPB). The scope of the study was limited, however, and much of the fundamental nature of the interaction could not be determined. Investigations were thus undertaken aimed at further elucidation of underlying causes. A flowable concentrate of Bacillus thuringiensis (Bt) (Novodor® FC) and a wettable powder of Beauveria bassiana (Bb) (Mycotrol® 22WP) were applied against CPB larvae infesting potato in field plots. Novodor FC and an emulsifiable oil formulation of Bb (Mycotrol ES) were applied against larvae on potted potato plants in greenhouse tests under low humidity, variable-temperature conditions. The biopesticides were applied separately and in combination (tank mixed). In the field tests, each biopesticide was also combined with the spray-carrier of the other formulation (formulation without active ingredient). Greenhouse tests showed that under warm, dry conditions, low activity of Mycotrol was counterbalanced by high activity of Novodor, and under cool, somewhat more humid conditions, low Novodor activity was balanced by high activity of Mycotrol, the result being a constant level of synergism (CPB mortality ca. 20 percentage points higher than predicted by independent action). Similar levels of synergism were observed under the markedly different conditions of the field environment, and the synergism was confirmed as arising from interaction of the two micobes, as the Bt spray carrier had no significant effect on efficacy of the Mycotrol product and the Bb spray carrier had no effect on the efficacy of Novodor. These findings further underscore the strong complementary action of these control agents when applied jointly against CPB.

Technical Abstract: Studies were undertaken to further characterize the previously identified synergistic activity of Bacillus thuringiensis- and Beauveria bassiana-based biopesticides against Colorado potato beetle (CPB). A flowable concentrate of B. thuringiensis morrisoni strain tenebrionis (Bt) (Novodor® FC) and a wettable powder of B. bassiana strain GHA (Bb) (Mycotrol® 22WP) were applied against CPB larvae infesting potato in field plots. Novodor FC and an oil-dispersion formulation of Bb (Mycotrol ES) were applied against second-instar CPB larvae on potted potato plants in greenhouse tests under low relative humidity, variable-temperature conditions. Each pathogen was applied alone and in combination (tank-mixed) with the other pathogen. In the field tests, each biopesticide was also combined with the spray-carrier of the other pathogen (formulation without active ingredient). The greenhouse tests showed that under warm, dry conditions, low activity of Mycotrol was counterbalanced by high activity of the Novodor, and under cool, somewhat more humid conditions, low Novodor activity was balanced by high activity of Mycotrol, the result being a constant level of synergism (CPB mortality ca. 20 percentage points higher than predicted by independent action). Similar levels of synergism were observed under the markedly different conditions of the field environment, and the synergism was confirmed as arising from interaction of the two micobes, as the Bt spray carrier had no significant effect on efficacy of the Mycotrol product and the Bb spray carrier had no effect on the efficacy of Novodor. These findings further underscore the strong complementary action of these agents applied jointly against CPB.