Supercooling capacity and survival of low temperatures by a pyrethroid-resistant strain of Typhlodromus pyri (Acari : Phytoseiidae)

An organophosphate/pyrethroid resistant strain of the phytoseiid mite Typhl odromus pyri Scheuten was imported from New Zealand in 1988 for use in biol ogical control of European red mite, Panonychus ulmi (Koch), and apple rust mite, Aculus schlechtendali (Nalepa), in Nova Scotia. To better understand the overwintering strategy of T. pyri and the likelihood the New Zealand s train would withstand winter conditions in Canada, we measured supercooling points and mortality of diapausing females held at subzero temperatures. M ites in quartz crucibles were placed in the liquid nitrogen-cooled stage of a cryostage microscope, and temperature was lowered 1 degrees C/min until the mites froze, as indicated by an instantaneous darkening of their body c ontents. Supercooling points of the New Zealand strain averaged -18.2 degre es C for July to September, were -23.2 degrees C in October, and averaged - 28.2 degrees C in the colder months from November to March. The mean superc ooling points for December 1994 did not differ from the mean for the native Nova Scotian strain. However, the mean supercooling point of the New Zeala nd strain for March 1995 was higher than the means for the native strain an d for a T, pyri strain that was imported from Geneva, NY. New Zealand strai n T. pyri taken from cloth bands affixed to orchard trees were placed on ap ple leaves in plastic vials and exposed to low temperatures for various per iods of exposure. At -5 degrees C, mortality at 24 h was less for mites col lected in February than for those collected in November (6% versus 44%) but at -10 degrees C trends were similar, reaching 100% by 24 h in both trials . For the February trial, a logistic function with a coefficient for the pr oduct of time and temperature explained 61% of the variation in mortality. If these results are applied to populations in Nova Scotian orchards, where winter temperatures of -10 degrees C are common, one would predict complet e annihilation of the New Zealand strain. However, populations of the New Z ealand strain, first released in orchards in 1988, have survived every wint er since that date and have proven effective in biological control of Europ ean red mite and apple rust mite. Possible reasons for survival and increas e of populations of this exotic strain of T. pyri, despite apparent suscept ibility to cold-induced mortality, are discussed.