Effect of combining imidacloprid and diatomaceous earth with Beauveria bassiana on mole cricket (Orthoptera: Gryllotalpidae) mortality.

Sublethal doses of three orthopteran-derived strains of Beauveria bassiana (Balsamo) Vuillemin were topically applied to adult southern mole crickets, Scapteriscus borellii Giglio-Tos (Orthoptera: Gryllotalpidae), and tested in combination with substrate treatments of diatomaceous earth (DE) and imidacloprid. Crickets treated only with the high doses (10(8) conidia per cricket) of each of the three B. bassiana strains exhibited the shortest survival times as well as the highest percentage mortality at 28 d after treatment. However, these treatments did.not differ significantly from any of the diatomaceous earth combination treatments. Two of the strains tested, 5977 and 3622, exhibited synergistic interactions with DE, whereas the third strain, GHA, was not significant for synergy. Mortality caused by the combination treatment was still greater than the expected additive effect. DE abrades the insect cuticle and absorbs cuticular lipids, aiding the entry of germinating conidia into the mole cricket hemocoel. None of the three strains exhibited synergy when combined with imidacloprid, and mortality of all combination treatments was less than additive. For strain 5977, there was an antagonistic interaction with imidacloprid. It was difficult to obtain <30% mortality for imidacloprid only treatments, which was considered the upper limit for sublethal doses. The mean percentage mortality caused by imidacloprid was 37.5%, and this high percentage made it difficult for any combination treatment to cause significantly more mortality than the expected additive effect. These results clarify the interactions of other control products with B. bassiana and provide a basis for a reduced pesticide approach to mole cricket control.