Antagonism between Beauveria bassiana and imidacloprid when combined for Bemisia argentifolii (Homoptera : Aleyrodidae) control

Imidacloprid and the entomopathogenic fungus Beauveria bassiana (Balsamo) V uillemin are both used to control the whitefly Bemisia argentifolii Bellows & Perring. We tested whether the two control strategies acted additively, synergistically, or antagonistically when combined for whitefly control. We found antagonism in that B. bassiana inhibited the effectiveness of imidac loprid. When B. bassiana was combined with imidacloprid, insect response wa s either less than or similar to (depending on B. bassiana rates) that when imidacloprid was used alone. Adding imidacloprid to B. bassiana treatments always increased mortality, but the increase was less than additive. Beauv erin bassiana spore germination and colony formation were not inhibited by imidacloprid in vitro, and B. bassiana did not adsorb or degrade imidaclopr id in a tank mix. We hypothesize that B. bassiana caused a behavioral respo nse that reduced insect feeding and uptake of imidacloprid.