Efficacy of Insecticides and Biopesticides against Rhopalosiphum maidis (Fitch) and their Toxicity to the Predatory Coccinellid, Coccinella septempunctata L.

Abstract

The wheat aphid, Rhopalosiphum maidis, is an important sap-feeding insect-pest that adversely affects wheat growth and productivity. Although synthetic insecticides provide rapid aphid suppression, their adverse effects on predators necessitate the evaluation of comparatively safer pest-management options. The present study evaluated the efficacy of selected insecticides and biopesticides against R. maidis using a leaf-dip bioassay at recommended doses. Subsequently, their toxicity to the predatory coccinellid, Coccinella septempunctata (adults and fourth-instar grubs), was assessed using an atomisation method under laboratory conditions at the recommended and twice the recommended field doses. Thiamethoxam was the fastest-acting treatment against R. maidis, resulting in 66.00, 80.00 and 91.00% population reduction at 12, 18 and 24 hours after treatment (HAT), respectively, and complete reduction after 48 HAT. Quinalphos resulted in 48.75, 61.25 and 93.75% reduction at the corresponding intervals and achieved complete reduction after 72 HAT. Among the biopesticides, B. bassiana was the most effective, resulting in 21% reduction at 72 HAT. In contrast, quinalphos and thiamethoxam caused substantial mortality of C. septempunctata adults and fourth-instar grubs. However, biopesticides caused considerably lower mortality, while Neemastra caused no mortality of predators. The findings indicate that B. bassiana offers potential for inclusion in integrated management of R. maidis, particularly where conservation of coccinellid predators is a priority. Further field studies are required to validate its efficacy and compatibility with other integrated pest management (IPM) components.