Performance of a pyrethroid-resistant strain of the predator mite Typhlodromus pyri (Acari : Phytoseiidae) under different insecticide regimes

An organophosphate pyrethroid-resistant strain of Typhlodromus pyri Scheute n imported from New Zealand was reared on potted apple trees in an outdoor insectary. From 1988 to 1995, the population was selected one to three time s per year with a dilute solution (1.7 ppm) of the pyrethroid cypermethrin. Petri dish bioassays with cypermethrin in 1995 indicated that the insectar y-reared T. pyri had an LC50 of 81 ppm versus 0.006 ppm for native T. pyri taken from a research orchard. The bioassays suggested that recommended orc hard rates of cypermethrin would cause heavy mortality in native population s of T. pyri hut only moderate losses in the imported New Zealand strain. B ioassays in 1996 with the organophosphate insecticide dimethoate indicated both New Zealand and native T. pyri were susceptible and that recommended o rchard rates of dimethoate likely would cause high mortality of T. pyri in apple orchards. These findings from bioassays were supported by data from o rchard trials. In June and July 1993, insectary-reared New Zealand T. pyri were placed on ave apple trees in each of eight 38-tree plots in the resear ch orchard. In late August 1994, New Zealand T. pyri from orchard trees tha t had been sprayed twice by airblast sprayer with the full recommended rate of 50 g (AI)/ha (83 ppm) cypermethrin were placed on the other 33 trees in each of six plots. In the summers of 1994-1996, plots were treated with on e of the following insecticide regimes: (1) conventional integrated pest ma nagement (IPM) (registered neurotoxic insecticides considered harmless or s lightly toxic to T. pyri); (2) aclvanced IPM (use of newer, more selective insecticides); (3) pyrethroid (at least one full-rate application of cyperm ethrin); (4) dimethoate; and (5) dimethoate plus pyrethroid. Densities of E uropean red mite, Panonychus ulmi (Koch),were highest in all plots treated with dimethoate and in pyrethroid plots not yet inoculated with New Zealand T. pyri. Densities of apple rust mite, Aculus schlechtendali (Nalepa), and of the stigmaeid predator Zetzellia mali (Ewing) were highest in plots tre ated with dimethoate and were nearly absent in the IPM plots. Densities of T. pyri were high enough for effective biocontrol in the IPM plots and in t he pyrethroid plots 1-2 qr after release of the New Zealand strain. provide d pyrethroid was applied just before the resistant strain was released in t he orchard. A recurring theme of this study was the generally negative asso ciation between densities of phytophagous mites and those of T. pyri. sugge sting the ability of this predator to suppress their prey. In contrast, the positive association between phytophagous mites and Z. mali suggests the i nability of this predator to regulate their prey at least under the conditi ons of this study.