Survey of Pesticide Resistance in Spider Mite Populations


Grbic, V., I. Scott, T. Baute, D. Lizotte. 2022. Survey of pesticide resistance in spider mite populations. Online platform presentation for the Ontario Soybean and Canola Committee Meeting, February 25, 2022.

Plain language summary

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.


Spider mites are considered a secondary pest in soybean as they are not an annual problem. Often outbreaks are due to hot dry summer weather, and because insecticide sprays have eliminated predators that keep populations under control. The number of eggs laid by TSSM females increases under drought conditions by 3 times. Due to predicted increase in the environmental conditions that favour spider mite growth under climate change, spider mites are considered an emerging pest in Ontario soybean. Currently, dimethoate (Lagon® or Cygon®) is the only active ingredient registered for spider mite control on soybeans in Canada. The incidents of mite resistance to dimethoate have been reported, but the extent of its occurrence in Ontario is not known. The objective of successful pesticide resistance management is to prevent the evolution of resistance by alternating the use of pesticides that act on different targets, but with only one product available this is difficult to achieve.

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