Susceptibility of canegrubs to the insecticide chlorpyrifos

The susceptibility of 10 species of sugarcane white grubs to, chlorpyrifos was measured and effects of grub body weight and soil type on susceptibility assessed. Laboratory data were related to the relative success of field control, and the likelihood of canegrubs being or becoming chlorpyrifos-resistant was assessed. Lethal dosages for the most susceptible species were ... 15 times greater ,than for the least susceptible. The three least susceptible species are also the most difficult to control in the field with controlled-release (CR) chlorpyrifos, and the four most susceptible species are readily controlled with CR chlorpyrifos, suggesting that the test method gives a realistic assessment of field effectiveness. Relatively low slopes of population response lines, and the large doses necessary to kill 75-95% of individuals, suggest at least four species have the potential to be effectively resistant to CR chlorpyrifos. Resistance could be developing in Lepidiota crinita, as previously exposed populations are far less susceptible to chlorpyrifos than a population with no previous exposure. CR granules subject a high proportion of Dermolepida albohirtum and Antitrogus parvulus canegrubs to sub-lethal doses of chlorpyrifos, which may accelerate development of resistance in those species.