Abstract
Five populations of mealybug, Planococcus kraunhiae (Kuwana), were collected from commercial persimmon orchards in Wakayama Prefecture: four populations including the Katsuragi population from four orchards with conventional insecticide spraying, and the Hashimoto population from an insecticide-free orchard. First instar nymphs were transferred onto a kidney bean leaf on 0.5% agar gel in a plastic Petri dish. A Petri dish which contained 20–40 nymphs was sprayed once with 6 ml of insecticide through a spraying tower on day 2 (1st instar), day 9 (2nd instar), day 16 (3rd instar) or day 25 (adult), and examined for susceptibility to insecticide. LC50 of the 1st instar Hashimoto population and the resistance ratio at LC50 of the other populations collected from conventional spraying orchards to that of the Hashimoto population were, respectively, 0.637 ppm and 8.0–12.2 for cypermethrin, 1.15 ppm and 6.0–7.8 for methidathion, and 0.029 and 15.4–20.2 for acetamiprid. Neonicotinoids were the most toxic to the Katsuragi population, followed by organophosphates, while the synthetic pyrethroids were less effective. The susceptibility to prothiofos and methidathion decreased as the growth stage advanced whereas susceptibility to acetamiprid remained high.
