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.

Investigation of Metabolic Resistance to Soybean Aphid (Aphis glycines Matsumura) Feeding in Soybean Cultivars.

Soybean aphid (Aphis glycines) is a major soybean (Glycine max) herbivore pest in many soybean growing regions. High numbers of aphids on soybean can cause severe reductions in yield. The management of soybean aphids includes monitoring, insecticide applications when required, and the use of resistant cultivars. Soybean aphid-resistant soybean varieties are associated with genes that confer one or more categories of resistance to soybean aphids, including antibiosis (affects survival, growth, and fecundity), antixenosis (affects behaviour such as feeding), and tolerance (plant can withstand greater damage without economic loss). The genetic resistance of soybean to several herbivores has been associated with isoflavonoid phytoalexins; however, this correlation has not been observed in soybean varieties commonly grown in southern Ontario, Canada. Isoflavonoids in the leaves of 18 cultivars in the early growth stage were analyzed by HPLC and the concentration by fresh weight was used to rate the potential resistance to aphids. Greenhouse and growth cabinet trials determined that the cultivars with greater resistance to aphids were Harosoy 63 and OAC Avatar. The most susceptible cultivar was Maple Arrow, whereas Pagoda and Conrad were more tolerant to aphid feeding damage. Overall, there was a low correlation between the number of aphids per leaf, feeding damage, and leaf isoflavonoid levels. Metabolite profiling by high-resolution LC-MS determined that the most resistant cultivar had on average lower levels of certain free amino acids (Met, Tyr, and His) relative to the most susceptible cultivar. This suggests that within the tested cultivars, nutritional quality stimulates aphid feeding more than isoflavonoids negatively affect aphid feeding or growth. These findings provide a better understanding of soybean host plant resistance and suggest ways to improve soybean resistance to aphid feeding through the breeding or metabolic engineering of leaf metabolites.

Soybean (Glycine max L Merr) host-plant defenses and resistance to the two-spotted spider mite (Tetranychus urticae Koch).

In southern Ontario, Canada, the two-spotted spider mite (Tetranychus urticae) is an emerging pest of soybean (Glycine max) due to the increasing incidence of warmer, drier weather conditions. One key strategy to manage soybean pests is breeding resistant cultivars. Resistance to pathogens and herbivores in soybean has been associated with isoflavonoid phytoalexins, a group of specialized metabolites commonly associated with root, leaf and seed tissues. A survey of 18 Ontario soybean cultivars for spider mite resistance included evaluations of antibiosis and tolerance in relation to isoflavonoid and other metabolites detected in the leaves. Ten-day and 4-week trials beginning with early growth stage plants were used to compare survival, growth, fecundity as well as damage to leaves. Two-spotted spider mite (TSSM) counts were correlated with HPLC measurements of isoflavonoid concentration in the leaves and global metabolite profiling by high resolution LC-MS to identify other metabolites unique to the most resistant (R) and susceptible (S) cultivars. Within 10 days, no significant difference (P>0.05) in resistance to TSSM was determined between cultivars, but after 4 weeks, one cultivar, OAC Avatar, was revealed to have the lowest number of adult TSSMs and their eggs. Other cultivars showing partial resistance included OAC Wallace and OAC Lakeview, while Pagoda was the most tolerant to TSSM feeding. A low, positive correlation between isoflavonoid concentrations and TSSM counts and feeding damage indicated these compounds alone do not explain the range of resistance or tolerance observed. In contrast, other metabolite features were significantly different (P<0.05) in R versus S cultivars. In the presence of TSSM, the R cultivars had significantly greater (P<0.05) concentrations of the free amino acids Trp, Val, Thr, Glu, Asp and His relative to S cultivars. Furthermore, the R cultivar metabolites detected are viable targets for more in-depth analysis of their potential roles in TSSM defense.