Survey of Pesticide Resistance in Spider Mite Populations

The two-spotted spider mite (TSSM) is a global generalist pest that feeds on more than 150 crops. In Canada and Ontario, it is one of the major pests of greenhouse and horticultural crops, and is an established pest on soybean and corn. The objectives of the project were to identify population structure of the TSSM infesting soybean fields in Southern Ontario and to identify the pesticide resistance status. Results from bioassays conducted on TSSM populations collected in soybean fields across Ontario show the prevalence of the insecticide dimethoate (Cygon or Lagon) resistance is high in some areas. Out of 31 fields tested, 18 (58% of sites) were found to have resistant TSSM populations. Fields close to one another tend to have similar levels of resistance. As part of the study, other miticides currently registered on horticultural crops were also tested against these populations as potential candidates for registration on soybeans. In general, populations collected on soybeans were susceptible to these miticides, except for a few localized populations that show tolerance to 2 other classes of acaricides. This isn’t entirely surprising, given TSSM are pests of almost all horticultural crops too and are easily carried by wind from neighbouring fields where these products are already in use. Some active ingredients are showing promise as good candidates for registration.