Impact of short-term heat pre-treatment at 50 °C on the toxicity of contact insecticides to adults of three Sitophilus granarius (L.) populations

Abstract The effect of short-term heat pre-treatment on the toxicity of contact insecticides was tested on Sitophilus granarius adults from three populations in order to examine possible interactions. In the laboratory: (a) weevil paralysis time (PT) after start of direct exposure to 50 °C; (b) the impact of three short-term heat pre-treatments at 50 °C (PT 20 , PT 50 and PT 80 ) on the 24 and 72 h recovery rates of laboratory (Lab) strain adults exposed to one of five insecticides on filter papers; and (c) deltamethrin and dichlorvos toxicity to weevils of two populations with altered insecticide susceptibility after exposure to 50 °C over the PT 80 interval, were determined. The PT 99 values for weevils after exposure to 50 °C showed that the population selected with pirimiphos-methyl had the longest paralysis time. Previous heat treatments of the Lab strain for the PT 20 and PT 50 intervals had little impact on the toxicity of the insecticides, chlorpyrifos-methyl being the most toxic with the first interval, deltamethrin with the second, while malathion was the least toxic. However, after heat treatment of the Lab strain over the PT 80 and after a 72-h recovery period, deltamethrin and dichlorvos showed reduced toxicity, while chlorpyrifos-methyl was the most toxic insecticide and malathion again the least toxic. For field and selected weevils, heat pre-treatment at 50 °C over the PT 80 increased deltamethrin and dichlorvos toxicity to selected populations, but not to field populations when compared to their toxicity without pre-heating. Deltamethrin and dichlorvos toxicity after a 72-h recovery period from heat treatment over the PT 80 was higher to the selected population than to the resistant field population. Compared to the Lab strain, deltamethrin demonstrated lower toxicity to selected weevils and especially to a field population, while dichlorvos was considerably more toxic to both populations.