A comparison of the toxicity of pirimiphos-methyl and malathion to Typhaea stercorea (L.) when applied to stored maize

The toxicity of freshly applied and aged residues of pirimiphos-methyl and malathion were assessed against adult Typhaea stercorea (L.). Maize was treated with each pesticide at doses of 2, 4, 6, and 8 mg kg^?1 and stored at a constant 25 °C and 70% r.h. for 12 weeks. All fresh deposits of pirimiphos-methyl produced 100% mortality as did malathion at 4, 6 and 8 mg kg^?1. After four weeks storage pirimiphos-methyl still gave 100% mortality at 6 and 8 mg kg^?1 but this dropped to 96% at 4 mg kg^?1 and 60% at 2 mg kg^?1, while mortality for malathion was less than 31% even at 8 mg kg^?1. After 12 weeks storage only pirimiphos-methyl gave effective control with 78% mortality at the highest dose of 8 mg kg^?1, while control by malathion had completely broken down. The effect of exposure time on T. stercorea for both pesticides at a single dose of 4 mg kg^?1 was also assessed. Beetles were left in contact with treated maize for 2, 4, 6, 8 and 10 d. Fresh and four-week-old pirimiphos-methyl residues produced over 98% mortality at all exposure periods but on 12-week-old residues mortality had dropped and was only 61% following 10 d exposure. Only freshly applied malathion gave 100% mortality and even the maximum exposure of 10 d only produced 51% mortality of T. stercorea at four weeks and 39% at 12 weeks.