Assessment of Different Bioassay Techniques for Resistance Monitoring in the Diamondback Moth (Lepidoptera: Plutellidae)

Abstract

Topical application, leaf residue, and filter paper residue bioassay methods were used to assess the toxicity of permethrin and methomyl to larvae of the diamondback moth, Plutella xylostella (L.). The objective was to develop an on-farm insecticide resistance monitoring technique for diamondback moth that required only simple, inexpensive equipment and was easy to use. The results of tests using the three bioassay methods led to similar resistance ratios for three diamondback moth strains. Leaf residue and filter paper residue tests using discriminating concentrations could detect the relative susceptibility to permethrin and methomyl in three strains, indicating that both techniques were practical for on-farm resistance monitoring. Third instars were more suitable for the detection of resistance than fourth instars. A monitoring system incorporating concentrations f both the LC90 for susceptible diamondback moth and the recommended field concentration of insecticide is suggested to detect resistance at low levels and to help choose insecticides for the control of diamondback moth strains with higher resistance levels.