Use of residue profile analysis to identify modes of insecticide activity contributing to control of plum curculio in apples

Residue profile analysis techniques were developed, along with laboratory and field-based bioassays to describe the modes of insecticidal activity responsible for the control of the plum curculio, Conotrachelus nenuphar (Herbst), in apples (Malus spp.). Adult plum curculios were treated in laboratory topical bioassays to determine acute contact activity and lethal time for five insecticides. Azinphosmethyl had the highest levels of toxicity and shortest lethal time values, followed by the neonicotinoids thiacloprid, thiamethoxam, and imidacloprid, whereas indoxacarb had the highest LD50 and LT50 values for topical exposure. Field-based residual activity bioassays assessed adult mortality, and fruit and leaf injury from plum curculio exposed to 4 h, 7 d, and 14 d field-aged residues. All compounds caused significant levels of mortality to plum curculio when adults were exposed to fruit clusters 4 h post-application. Thiacloprid, thiamethoxam, and imidacloprid showed oviposition deterrence, antifeedant, and repellency effects in the 7- and/or 14 d residual bioassays and protected fruit in the absence of significant lethal activity. Indoxacarb maintained lethal activity throughout the study intervals, with the incidence of plum curculio feeding, suggesting that ingestion is an important mode of entry. For the neonicotinoids thiacloprid, thiamethoxam and imidacloprid plum curculio mortality was highly correlated with fruit and leaf surface residues. As surface residues declined, sublethal effects such as oviposition deterrence and antifeedant effect remained. The value of the plant-insect-chemistry triad model for describing the temporal dimensions of insecticidal modes of activity and understanding a compound's critical performance characteristics is discussed.