Orchard selection for resistance to a synthetic pyrethroid in organophosphate—resistantTyphlodromus pyriin the UK

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Abstract
Organophosphate resistant Typhlodromus pyri were exposed to deltamethrin 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 discontinued in 1989 after eight applications. Few phytoseiids were found in the 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 exposed to 3.1 g ai ha‐1 1 (1.50 ppm ai) indicating a four‐fold increase in resistance in these selected mites.

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