D3298: Changepoint analysis of lambda-cyhalothrin efficacy against soybean aphid (Aphis glycines Matsumura): Identifying practical resistance from field efficacy trials

Soybean aphid (Aphis glycines Matsumura) remains the most economically important arthropod pest of soybean in the Upper Midwestern Region of the US. Soybean aphid resistance to the pyrethroid insecticides emerged in 2015; however, the reduction in the efficacy of field applications of pyrethroid insecticides has not been quantified. Based on time series of data from insecticide efficacy trials from two locations, a novel approach of continuous two-phase changepoint-regression models was used to indicate if a change in percent control had occurred, and to provide an indication of when and to what degree the percent control has changed. At both sites examined in this study, a significant change in percent control of λ-cyhalothrin was detected in 2014; thus, signifying the last year prior to the onset of decreasing control. Percent control decreased at a rate of 4.30 and 19.90 percent per year at these locations. In contrast, percent control for chlorpyrifos remained high over time with no significant change point. This research demonstrates that retrospective time series analysis of insecticide efficacy data can identify the onset and magnitude of practical resistance in the field. This further validates and compliments the other lines of evidence related to pyrethroid resistance in soybean aphid.