Differences in biological characteristics in organophosphorus-resistant strains of the phytoseiid mite Typhlodromus pyri

Strains of Typhlodromus pyri were collected from orchards in SW England, wh ere populations differed in their response to organophosphorus insecticides compared with strains collected in the SE. Biological characteristics of t hese strains were compared with those in OP-resistant strains collected fro m the SE and OP- susceptible strains. There were significant differences in size of eggs and adult mites from the different strains, but these differe nces did not appear to be related to resistance status of the mites. Female OP-susceptible mites had a longer development time than other strains. Ove r all strains total development time for female mites was 0.8 day less than for males. There were significant differences between strains for pre-ovip osition period, but not oviposition or post-oviposition periods. There were significant differences in total numbers of eggs produced between strains, with a standard OP-resistant strain producing the most and an OP-susceptib le strain the fewest eggs. There were significant differences in male longe vity between strains, but this did not appear to be related to resistance s tatus. This study highlights the fact that strains may differ in biological characteristics that contribute to fitness regardless of their pesticide r esistance status.