Influence of temperature on the insecticidal effect of Beauveria bassiana in combination with diatomaceous earth against Rhyzopertha dominica and Sitophilus oryzae on stored wheat

Laboratory experiments were carried out in order to evaluate the insecticidal effect of Beauveria bassiana (Balsamo) Vuillemin formulations against adults of Rhyzopertha dominica (F.) and Sitophilus oryzae (L.) on stored wheat. To assess B. bassiana effect, the formulated product Naturalis® was applied at three dose rates, 2500, 5000, and 10,000 ppm. Also, wheat was treated with the diatomaceous earth (DE) formulation SilicoSec® (Biofa, Germany), at two dose rates, 0.2 and 0.5 g/kg of wheat either alone or in combination with each fungal rate. Dead adults were counted 24 h, 48 h, 7 days, and 14 days later. The bioassays were conducted at three temperatures, 22, 26, and 30 °C and one RH level, 55%. Generally, the increase of temperature increased the insecticidal effect of DE against R. dominica and S. oryzae, but B. bassiana was more effective at 26 °C than at 30 °C, for both species examined. The simultaneous use of DE in the fungal formulation moderated this negative effect. Both species were almost equally susceptible to SilicoSec®. In contrast, R. dominica was more susceptible than S. oryzae to B. bassiana. Some combination treatment had an additive effect on the fungal efficacy by DE while in others a negative effect was observed.