Acaricide resistance levels in field and greenhouse populations of Tetranychus urticae in Southwestern Ontario

Abstract: In Canada, the province of Ontario is responsible for the country’s largest soybean
production. Its greenhouse sector is also an essential part of Canadian agriculture. Tetranychus urticae (Koch), the two-spotted spider mite (TSSM), is a chelicerate herbivore with an extensive host range and an ability to rapidly develop resistance to different acaricides. Here we present an assessment of acaricide resistance of 46 TSSM populations from Southwestern Ontario collected in 2021 and 2022. Our study covered greenhouse and field production systems. The assessed active compounds were abamectin, bifenazate, cyflumetofen, dimethoate, etoxazole, pyridaben, and spiromesifen. We used two types of bioassays that were performed using LC90 concentrations determined for the susceptible reference population (LND). We have considered the mortality rate at or below 40 % as a resistance threshold. Our results indicate that only 14/46 populations were fully susceptible to all tested compounds. Resistance to multiple acaricides was present in a significant proportion (21/46) of the sampled populations, with the greenhouse-collected populations being the most resistant. Soybean-collected populations demonstrated frequent resistance to dimethoate (18/32), Ontario’s only registered acaricide for spider mite control on soybeans. These results may be used to prioritize and plan pesticide usage on crops where multiple products are registered.