- Agronomic: soybeans
- Education and Training: extension
- Natural Resources/Environment: biodiversity
- Pest Management: biological control, botanical pesticides, chemical control, economic threshold, field monitoring/scouting, genetic resistance, integrated pest management
- Sustainable Communities: sustainability measures
The goal of this project was to determine if there are combined effects of insecticide application and resistant plants for soybean aphid control, with the hope that the efficacy of insecticides is improved on resistant plants. We tested three different insecticides. Two were conventional insecticides with active ingredients lambda-cyhalothrin and chlorpyrifos, which are a pyrethroid and an organophosphate, respectively. We also tested an insecticide available for organic growers containing pyrethrum and azadirachtin, which is referred to by its trade name Azera in this summary.
In our 2014 field experiment, we found that the combined use of resistant plants (containing the Rag1 gene) and chlorpyrifos produced a synergistic effect. In those plots, the decrease in aphids compared to susceptible untreated plants was lower than combined effects of plots with resistant untreated plants or susceptible treated plants alone. Other experiments examined interactions in controlled greenhouse settings. This grant funded a portion of a larger project covering three years of field experiments and lab assays. The 2014 field experiment and lab assays discussed below were funded by SARE.
Soybean aphid (Aphis glycines Matsumura) is a major pest of soybean in the Midwest. Insecticides, such as pyrethroids and organophosphates, are used to suppress soybean aphid outbreaks to prevent yield loss. Another management tactic is host-plant resistance. Genes have been found that confer resistance to soybean aphid (e.g., Rag1 and Rag2), and are available individually or combined in commercial varieties of soybean. However, aphid populations on aphid-resistant soybean plants can still build to economically damaging levels, which require treatment with insecticides to protect yields. In some situations, resistant soybean may need to be treated with insecticides. Pest-resistant plants can affect pest susceptibility to insecticides in three ways: synergistic effects, antagonistic effects or no effects. Synergistic effects increase the susceptibility of insect pests to insecticides when the pests feed on those plants (Fig. 1). Such an effect on soybean aphid could lead to fewer insecticide applications during the year. The increased aphid susceptibility may allow for the use of less toxic insecticides that would otherwise be less effective against soybean aphid on susceptible soybean, but would be more compatible with natural enemies of the soybean aphid. An opposite effect can also occur called antagonistic effects where insecticide susceptibility decreases on resistant plants (Fig. 1). A weak antagonistic effect can still be compatible with an integrated pest management program as it still offers increased control over a single management tactic, but a strong antagonistic effect results in less control than if only one control method is used. The resistant plant also might not affect insecticide susceptibility, and an additive effect would be observed instead (Fig. 1).
Three primary objectives were planned for this experiment:
- Measure an interaction between resistant plants and insecticides in field conditions.
- Examine the interaction between resistant plants and insecticides in laboratory assays under controlled greenhouse conditions where potential confounding factors such as weather and beneficial insects can be ruled out.
- Survey growers at extension meetings before the completion of this study and after presenting the findings of this study to assess current trends in resistant plant and insecticide usage and any potential changes over time.