ORCHARD SELECTION FOR RESISTANCE TO A SYNTHETIC PYRETHROID IN ORGANOPHOSPHATE-RESISTANT TYPHLODROMUS-PYRI IN THE UK

Organophosphate resistant Typhlodromus pyri were exposed to deltamethr in in replicated orchard plots from 1986-90. Treatments of 6.3, 3.1, 1 .7 and 0. 8 g active ingredient (ai) ha-1 deltamethrin were applied 14 times over the 5-year period, a 12.5 g ai ha-1 treatment was disconti nued in 1989 after eight applications. Few phytoseiids were found in t he plots receiving the two highest rates of deltamethrin. Bioassays at the end of the five-year period showed a progressive increase in LC50 from the untreated strain (0.40 ppm ai) to the strain that had been e xposed to 3.1 g ai ha-1 (1.50 ppm ai) indicating a four-fold increase in resistance in these selected mites.