Effect of deltamethrin and spinosad on phosphine resistant strains in comparison with laboratory strains of four stored product pest species

Abstract: Scope of the study was information on alternative treatment against the most reported phosphine resistant species, C. ferrugineus, R. dominica, S. oryzae and L. serricorne. In Germany deltamethrin (DM) containing pesticides are registered for commodity and empty room treatment since 2011. Spinosad (SP) has been reported as promising active substance for grain protection. Tests were performed with commercially available products K-Obiol® EC 25 and Spintor®. Wheat was impregnated with solutions containing insecticide at a dose of 0.25 mg/kg DM and 1 mg/kg SP, respectively. Adults of laboratory and phosphine resistant strains from the insect stock of Julius Kühn-Institut were used for bioassays. Adults were placed on insecticide treated wheat at 25 °C and 62-65% r.h. Mortality was checked after 24 h, 48 h, 1 and 2 weeks. Results after short exposure (24 h and 48 h, respectively) indicate mortalities below 35% in all cases. R. dominica generally showed highest susceptibility to the insecticides: After 2 weeks 100% mortality was achieved on DM treated substrate and about 95-98% mortality on SP treated substrate for both strains. L. serricorne was not notably affected by exposure to SP within its comparatively short lifetime. Differences between laboratory and resistant strains were a slightly higher mortality for resistant strains of C. ferrugineus, S. oryzae and R. dominica due to SP exposure and after 2 weeks C. ferrugineus phosphine resistant strain showed a higher mortality (36%) compared to the laboratory strain (8%). Exposure to DM had nearly no effect to L. serricorne phosphine resistant strain. Results indicate that deltamethrin and spinosad principally have potential in control of phosphine resistant strains. However, a successful treatment depends on the individual species. As found by chance the coexistent resistance to phosphine and high tolerance to deltamethrin in a strain of L. serricorne needs further consideration.