DETECTING INSECTICIDE-RESISTANT APHIDS BEFORE FIELD FAILURES COST FARMERS

The goal of this project is to help farmers grow soybeans more profitably while dealing with insecticide-resistant populations of the soybean aphid. Insecticide resistance is a challenge for all agriculture, especially for commodities with small profit margins and afflicted by multiple insect pests, like soybean. Combatting the impacts of insecticide resistance is a task that requires data on several fronts. Those who support farmers need to know the frequency and level of resistance among members of a pest population and the impact resistance will have on crop production to develop recommendations for mitigating this impact. Sustainable soybean production is at risk when these challenges are unaddressed. We will provide soybean farmers with information regarding local prevalence of resistance needed to make informed decisions on effective insecticide use, thereby maximizing farmer profit.Soybean is an example of a commodity challenged by an invasive insect pest that recently developed resistance to insecticides. Historically, soybean were grown in the Midwest with limited insecticide use. Soybean production changed with the arrival of soybean aphid in the US. During 2001-2010, insecticide use on soybean quadrupled, while during this same period insecticide use decreased for other US commodities. Although recommendations for using insecticides to manage soybean aphid include scouting and using an economic threshold (i.e. Integrated Pest Management [IPM] principles), farmers do not routinely use IPM for this pest. Surveys of farmers in the Midwest during the initial soybean aphid invasion suggest a reliance on regional summaries of aphid outbreaks, often applying insecticides on a calendar basis. Given the low cost of insecticides and the relatively high value of soybean during this period, farmers produced soybeans profitably. Regardless of whether a soybean aphid outbreak occurs and if it is insecticide-resistant, farm profitability in Iowa is challenging. A majority of Iowa farmers experience an economic loss when growing crops based on a recent summary of USDA crop production data and soil-quality in Iowa.The widespread, continual use of inexpensive, pyrethroid insecticides resulted in the development of resistance. The occurrence and frequency of pyrethroid resistance is relatively low, given the multi-state range of soybean aphid in the US and Canada. However, in the four years in which resistance has been tracked, resistance has spread from isolated fields in Minnesota, to fields in North Dakota, South Dakota, and Iowa. Given the capacity of soybean aphid to move across states to colonize soybeans and continued use of insecticides to treat infestations, the economic impacts will expand into the future.Resistance makes it hard for farmers to select an insecticides to manage a pest. The economic impact of resistance from re-application costs and yield loss are only realized after a failure. Management of soybean aphids would benefit from genetic markers that detect resistance so farmers can tailor pest management to avoid these economic impacts. The frequency and range of resistant aphids is highly variable, and scouting tools do not yet exist for predicting if a population is resistant to pyrethroids. Our first objective is to develop genetic markers for detecting resistance within individual aphids.We have identified a mutation (knockdown resistance, kdr) and developed markers that explain variability in susceptibility to pyrethroids. However, the magnitude of resistance varies among different populations identified with this marker, suggesting other mechanisms of resistance are present. Recent studies revealed resistance involves increased rates of insecticide breakdown by multiple detoxification enzymes. Therefore, we will develop genetic markers for the kdr mutation and detoxification enzymes to predict pyrethroid resistance among field-collected soybean aphids. Markers will be used to reveal changes in the frequency of resistant genotypes before and after an insecticide application.Our second objective is to use these markers to describe the frequency of insecticide resistance among field collected soybean aphids. Entomologists at Iowa State University have conducted a insecticide efficacy trial for over 15 years. These markers will be used on aphids collected before and after insecticides are sprayed. We will measure the change in the frequency of resistance, as well as the effect on the insecticides on yield. These data will be included in economic models to determine when the frequency of resistance results in sufficient yield loss to justify switching to insecticides that are more expensive.Our final objective is to share these data in real time to inform farmers and agribusiness about the insecticide-resistant soybean aphids. An extension program will use economic models to help farmers see the value in monitoring for resistance and to avoid insecticides that do not provide optimal protection. We will share this approach with farmers, Certified Crop Advisors, and agribusiness to improve the management of resistance for this pest and other field crop pests.Develop genetic markers for detecting resistance within individual aphidsUse molecular marker(s) to describe changes in the frequency of pyrethroid resistance among field collected soybean aphids. Share these data in real time to inform farmers and agribusiness about the insecticide-resistant soybean aphids