Regional differences in susceptibility to spinosyn insecticides registered for Colorado potato beetle management

Colorado potato beetle (CPB) Leptinotarsa decemlineata Say is the most economically important pest of Canadian potato. In the past decade, the control of CPB has relied heavily on systemic insecticides, principally the neonicotinoids. Resistance to neonicotinoids in CPB populations has been well documented, but this is not the case for spinosyn insecticides. A survey of insecticide susceptibility in CPB populations from 6 provinces between 2018 and 2021 observed: 1) spatial and temporal resistance trends; 2) cross-resistance; and 3) evidence of regional differences in susceptibility to spinosyns. The proportion of populations within each province considered resistant to spinosyns was, in descending order: QC (20.5%) > MB (15.4) > ON (11.4%) > NB (10.7%) > PEI (2.2%) > AB (0%). Overall, there was no significant change in CPB susceptibility at the LC90 or diagnostic concentration (DC) for spinosyns in each province, but cross-resistance was observed between spinosad and spinetoram based on a positive and significant correlation (R2 = 0.364) of DC mortality. There was also a positive relationship observed between the number of spinosyn applications (years applied at the sampling location) and whether a population remained susceptible, less susceptible or resistant to spinosyns. In QC, the greater spinosad use in organic potato farms led to resistance in those CPB populations, but spinosyn resistance at conventional farms was not related to greater application of neonicotinoids. Spinosyns remain relatively effective, nevertheless growers should be concerned over the increasing cases of reduced susceptibility. Resistance management must encourage rotation with products from the other classes in-season and between years in order to extend spinosyn use for CPB control.