Resistance to chlorpyrifos in the mealybugs Pseudococcus affinis and P. longispinus in Hawkes Bay and Waikato pipfruit orchards

Abstract

Between 2006 and 2008, 20 populations of Planococcus ficus (Signoret), from Coachella and San Joaquin Valleys of California were measured in the laboratory for susceptibility to buprofezin, chlorpyrifos, dimethoate, methomyl, and imidacloprid. Toxicity was assessed using a petri dish bioassay technique for contact insecticides and by a systemic uptake technique for imidacloprid. Mixed life stages were tested for susceptibility to all insecticides except for buprofezin, which was measured against early and late instars (Þrst, second, and third). DoseÐresponse regression lines from the mortality data established LC 50 and LC 99 values by both techniques. Responses of populations from the two geographical locations to all Þve insecticides varied, in some cases signiÞcantly. Variations in susceptibility to each insecticide among sample sites showed a sevenfold difference for buprofezin, 11-fold to chlorpyrifos, ninefold to dimethoate, 24-fold to methomyl, and 8.5-fold to imidacloprid. In spite of susceptibility differences between populations, baseline toxicity data revealed that all Þve insecticides were quite effective based on low LC 50 s. Chlorpyrifos was the most toxic compound to Planococcus ficus populations as shown by lowest LC 50 s. Buprofezin was toxic to all immature stages but was more potent to Þrst instars. The highest LC 99 estimated by probit analysis of the bioassay data of all 20 populations for each compound was selected as a candidate discriminating dose for use in future resistance monitoring efforts. Establishment of baseline data and development of resistance monitoring tools such as bioassay methods and discriminating doses are essential elements of a sustainable management program for Planococcus ficus.