A comparison of two methods to determine the susceptibility of sawtoothed grain beetle (Oryzaephilus surinamensis) populations to pirimiphos-methyl from Canterbury, New Zealand

Resistance of sawtoothed grain beetle (Oryzaephilus surinamensis) to organophosphate insecticides is documented internationally. There are anecdotal reports of reduced efficacy in New Zealand but to date no empirical assessments have been made. Two-laboratory-based test methods using either a dust (Actellic® Dust) or liquid (Actellic® 50EC) formulations of pirimiphos-methyl, were compared to determine the response of five Canterbury sawtoothed grain beetle populations. A mini-silo method employed grain treated with the recommended application rate (200 g a.i./tonne seed) of pirimiphos-methyl dust. A Petri-dish method treated internal surfaces of 50-mm diameter Petri dishes with liquid pirimiphos-methyl at concentrations from 0–0.1 g a.i./L to determine an estimated LC50 (lethal concentration for 50% mortality) for each population. The rank order of mortality (highest to lowest) in the mini-silo test was similar to the Petri-dish LC50 rankings for the five populations tested. The results illustrate variation in responses to pirimiphosmethyl concentrations by sawtoothed grain beetle populations, indicating both methods are potential options for future resistance testing and will aid the development of management strategies for control of stored insect pests.