Effect of Reduced Strength of Pirimiphos-methyl on the Efficacy of Permethrin in Actellic Super Dust against Storage Pests, Larger Grain Borer (Prostephanus truncatus and Maize Weevil (Sitophilus zeamais)

Abstract

Maize (Zea mays L.) is a key staple crop in Kenya used for food, industry, and income generation. However, postharvest losses caused by storage pests such as the larger grain borer (Prostephanus truncatus) and maize weevil (Sitophilus zeamais) pose a major threat to household food security. Control of these pests largely depends on Actellic-based insecticides, but continuous use of the same active ingredients has led to reduced efficacy and resistance. This study evaluated the synergistic effect of pirimiphos-methyl on the efficacy of permethrin in Actellic Super Dust against resistant field strains of P. truncatus and S. zeamais from different regions in eastern Kenya. Bioassays were conducted by exposing insects to treated maize grains containing varying concentrations (1.1–1.6%) of Pirimiphos-methyl combined with 0.3% permethrin for 24 and 72 hours. Mortality and knockdown effects were recorded and analysed using ANOVA. Results showed that S. zeamais remained susceptible to Actellic Super Dust, achieving ≥95% mortality even at 1.1% Pirimiphos-methyl. In contrast, P. truncatus field strains exhibited reduced susceptibility compared to the laboratory strain, requiring ≥1.5% Pirimiphos-methyl and a 72-hour exposure period for satisfactory control. The findings indicate that Pirimiphos-methyl enhances the efficacy of permethrin in Actellic Super Dust, but maintaining a concentration of at least 1.5% is critical for effective management of resistant P. truncatus populations. Regular quality checks and bioassays are recommended to ensure consistent product performance and early detection of resistance