Synergized deltamethrin as a protectant against Sitophilus zeamais Motsch. and S. oryzae (L.) (Coleoptera: Curculionidae) on stored maize

Abstract

The efficacy of deltamethrin applied to maize in combination with different rates of the synergist piperonyl butoxide (PB), was evaluated in the laboratory against Sitophilus zeamais. The test strain had been collected from maize treated with unsynergized deltamethrin in an earlier field trial and a degree of resistance was indicated in impregnated paper assays. Freshly applied deltamethrin was synergized by concentrations of PB above 0.001 mg kg −1 to a maximum of about 15 times at 2 mg kg −1, with no further improvement thereafter. However, when treated grain was stored at 30°C for up to 48 weeks the efficacy of aged treatments improved with increasing application rates of PB up to at least 16 mg kg −1. The results can be explained by breakdown of deltamethrin with concurrent breakdown of PB and consequent reduction in synergism. Application of deltamethrin at 1 mg kg −1 with PB at 8 mg kg −1 prevented F 2 progeny for 24 weeks, whereas PB at 16 mg kg −1 gave 48 weeks protection. However, impregnated paper assays showed that synergized deltamethrin was less effective against a deltamethrin-selected strain of S. zeamais and an organophosphorus-resistant strain of S. oryzae than against the test strain, and so all populations of Sitophilus spp may not be controlled by these treatments.