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

The Colorado potato beetle (CPB) is the most economically important pest of Canadian potato, and if left uncontrolled, it can completely consume the crop. In the past decade, the control of CPB has relied heavily on systemic insecticides, principally the neonicotinoids thiamethoxam and clothianidin. Resistance to neonicotinoids in CPB has been well documented in the past 2 decades and mechanisms underlying the resistance better understood. In contrast, resistance to other insecticide classes, including spinosyns (spinosad and spinetoram) and anthranillic diamides (chlorantraniliprole and cyantraniliprole), have not been studied to the same degree in CPB. Spinosyns are the only insecticide certified for organic potato growers in Canada and are frequently applied as a mid-season foliar spray by conventional growers when seed treatments with neoniconitoid or diamide experience control breaks. Improved knowledge on resistance to spinosyns in CPB would allow for the development of regional management strategies. A survey of insecticide susceptibility in CPB populations from 6 potato growing regions between 2018 and 2022 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: Québec (16%) > Ontario (14%) > Manitoba (13%) > New Brunswick (9%) > Prince Edward Island (2%) > Alberta (0%). There was a significant change in CPB mortality at the diagnostic concentration (DC = LC90) for spinosad and spinetoram in the 6 provinces but only for year 5 relative to the previous 4 years. Moderate cross-resistance was determined between spinosad and spinetoram with the DC mortality for all populations based on a positive and significant correlation (adjusted R2 = 0.3758; P = 1.263e-13). There was also a positive relationship observed between the number of spinosyn applications (years applied at the sampling location) and declining susceptibility to spinosad (R2 = 0.0927; P < 0.002). Cross-resistance was observed between spinosyns and insecticides in the other two classes, the more significant correlation was between spinosad and tetraniliprole (R2 = 0.3025; P < 0.0002). In Québec, 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 and diamides. Spinosyns remain relatively effective, nevertheless growers should be concerned over the increasing cases of reduced susceptibility in conventional potato farms and resistance where organic production occurs. Resistance management should continue to encourage rotation with products from the other classes in season and between years in order to extend spinosyn use for CPB control.

Proteomic Analyses Detect Higher Expression of C-Type Lectins in Imidacloprid-Resistant Colorado Potato Beetle Leptinotarsa decemlineata Say.

The Colorado potato beetle (CPB) is one of the most adaptable insect pests to both plant toxins and synthetic insecticides. Resistance in CPB is reported for over 50 classes of insecticides, and mechanisms of insecticide-resistance include enhanced detoxification enzymes, ABC transporters and target site mutations. Adaptation to insecticides is also associated with changes in behaviour, energy metabolism and other physiological processes seemingly unrelated to resistance but partially explained through genomic analyses. In the present study, in place of genomics, we applied 2-dimensional (2-D) gel and mass spectrometry to investigate protein differences in abdominal and midgut tissue of insecticide-susceptible (S) and -resistant (R) CPB. The proteomic analyses measured constitutive differences in several proteins, but the highest match was identified as a C-type lectin (CTL), a component of innate immunity in insects. The constitutive expression of the CTL was greater in the multi-resistant (LI) strain, and the same spot was measured in both midgut and abdominal tissue. Exposure to the neonicotinoid insecticide, imidacloprid, increased the CTL spot found in the midgut but not in the abdominal tissue of the laboratory (Lab) strain. No increase in protein levels in the midgut tissue was observed in the LI or a field strain (NB) tolerant to neonicotinoids. With the exception of biopesticides, such as Bacillus thuringiensis (Bt), no previous studies have documented differences in the immune response by CTLs in insects exposed to synthetic insecticides or the fitness costs associated with expression levels of immune-related genes in insecticide-resistant strains. This study demonstrates again how CPB has been successful at adapting to insecticides, plant defenses as well as pathogens.