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.
Field plots were planted in June 2014 near Rosemount, MN in a randomized complete block design with eight treatments. Treatments structure comprised a 2×4 factorial design consisting of aphid-susceptible or aphid-resistant (Rag1) soybean and each soybean line received one of four insecticide treatments [lambda-cyhalothrin (trade name: Warrior II), chlorpyrifos (trade name: Lorsban), pyrethrum+azadirachtin (trade name: Azera), or untreated]. Soybean plants in plots were sampled approximately weekly and the number of aphids, aphid predators and parasitoids were recorded per plant from emergence to 21 days after treatment. Plots were treated with insecticides on August 8 at labeled field rates after aphids in susceptible plots had surpassed an average of 250 aphids per plant. Cumulative insect days were calculated to summarize the abundance of aphids and ladybeetles over time in plots.
Lab assays were performed by determining concentrations of lambda-cyhalothrin, chlorpyrifos, and Azera that caused approximately 35% aphid mortality on susceptible plants after dipping leaves into each insecticide. These concentrations were used in lab assays where aphids were first reared on susceptible or resistant (Rag1) plants. Aphids were then transferred to treated and untreated (i.e., water only) leaves of susceptible and resistant plants and aphids were confined in a clip cage attached to the leaves. Mortality and number of nymphs produced per aphid were recorded 48 hours after.
In the 2014 field plots, aphid populations (measured in cumulative aphid days at the end of the season) were lower on resistant plants than susceptible plants (p < 0.01), and lamda-cyhalothrin and chlorpyrifos treatments significantly decreased aphid population levels on both susceptible and resistant varieties (p < 0.01 in both cases). However, Azera did not reduce aphid populations on susceptible or resistant varieties (p = 0.45). Aphid populations were not lower than expected on resistant plants treated with lamda-cyhalothrin (p = 0.23), which indicates the effects of aphid-resistance and lamda-cyhalothrin on soybean aphids were independent of one another (i.e., additive effect). However, aphid populations were lower than expected on aphid-resistant plants treated with chlorpyrifos (p = 0.03). This indicates chlorpyrifos and the aphid-resistant plants may synergistically improve efficacy (Fig. 2). Cumulative lady beetle days were not significantly different between insecticide or no-insecticide plots, but fewer lady beetles were found in resistant plots compared to susceptible plots. The field experiment will be repeated again in 2015 to supplement the data gathered within the scope of this grant for 2014 and a preliminary experiment conducted in 2013 to account for variability in conditions across years such as weather during and after insecticide application.
Lab assays also began in spring 2014 and concluded in the spring of 2015. Assays for lambda-cyhalothrin, and Azera are completed. Mortality was higher on both aphid-resistant untreated and aphid-susceptible lamda-cyhalothrin treated plants than aphid-susceptible untreated plants, while mortality was not significantly different between the individual treatments alone. Mortality was also significantly higher on aphid-resistant lamda-cyhalothrin treated plants than both individual treatments. However, aphids were less susceptible to lambda-cyhalothrin on aphid-resistant plants than they were on aphid-susceptible plants, as even higher mortality was expected from the combined effects of each treatment (Fig. 3). Mortality from the Azera experiment was very similar to the lamda-cyhalothrin experiment, but there was no change in aphid susceptibility to Azera between susceptible and resistant plants (Fig. 4).
We could not determine a concentration of chlorpyrifos that consistently caused around 35% mortality on susceptible plants for use in the assay. There was high variation in the amount of mortality a given concentration of chlorpyrifos caused as we recorded mortality ranging from 0 to 100% mortality in different experiments over time with the same dose. Consultation with insect toxicologists from the north central region confirmed the difficulties in working with chlorpyrifos and other organophosphate insecticides due to very steep dose-response curves.
Educational & Outreach Activities
A field day was held on August 12, 2014 for farmers near Rosemount, MN where the attendees were given presentations and plot tours of projects at the University of Minnesota’s Rosemount Experiment Station. During one of the tours, farmers and agricultural professionals were encouraged to walk through our plots to see witness the efficacy of aphid-resistant plants in suppressing aphid population growth and to see how the different insecticides performed on these soybean lines. Attendees of this event were very interested in the research and asked many questions.
The findings from this study were also presented at the 2015 Farmers Forum at the Northern Plains Sustainable Agriculture Society in Aberdeen, SD in addition to multiple extension meetings for growers and crop consultants in Minnesota.
We are planning to submit our findings to a peer-reviewed journal in fall 2015.
Results of this research indicate that insecticides and aphid-resistant plants can be used in tandem without strong antagonistic interactions between these tactics. It appears that synergistic interactions between these tactics can occur. However, further follow-up research is being performed in our laboratory to better determine when such synergistic interactions might be expected to occur. If the use of resistant plants can lead to fewer insecticide applications, or use of less toxic insecticides, there are several potential long-term outcomes for farmers. Operating costs may decrease due to less pesticide use. There may also be fewer non-target effects on beneficial insect populations, which reduce soybean aphid population densities. Environmental and human-health risks may decrease with decreased insecticide exposure. Finally, organic farmers may gain an effective combination of tactics for managing soybean aphid with organic-approved insecticides used on aphid-resistant soybean. The organic insecticide had low efficacy in the field experiment and required recommended field rates to obtain about 10% mortality in the lab experiment, so other organic insecticides could be examined for synergistic effects.
Information on soybean aphid host-plant resistance and insecticide use was presented at extension meetings in the winter of 2013-14 and 2014-15. After the presentations, attendees completed a written survey asking if they have used resistant soybeans, would expect fewer insecticide applications on resistant varieties, and are aware beneficial insects may be affected by insecticide applications (Table 1). Results indicating low adoption of aphid-resistant soybean for soybean aphid management are disheartening. However, this trend is likely due to the limited and apparently decreasing availability of aphid-resistant soybean varieties. Additional work in our laboratory, in collaboration with the soybean breeder at the U of MN, is focused on searching for new sources of aphid resistance to increase the availability of aphid resistant soybean for Minnesota and other northern states. We were pleased to see that an increasing percentage of growers expect to use less insecticide if they utilize aphid-resistant soybean. Most importantly, we documented that nearly all the growers surveyed recognize the importance of natural enemies (e.g., predators) for soybean aphid suppression. This suggests that we could potentially leverage this knowledge to increase adoption of management tactics that are more compatible with aphid natural enemies and more sustainable overall.
Areas needing additional study
We found fewer lady beetles in resistant plots than susceptible plots in our field experiment. This could be due to direct effects of the resistant plant on lady beetles biology. However, fewer lady beetles might be found on resistant plants because there are also fewer soybean aphids. Further studies could examine potential direct of Rag genes on natural enemies such as lady beetles and population dynamics of natural enemies associated with soybean aphid.
Overall, we did not find evidence that combined use of resistant plants and insecticide use for soybean aphid control was incompatible (i.e. no strong antagonistic interactions were observed). We did find one instance of a synergistic interaction with chlorpyrifos under field conditions. Chlorpyrifos is an organophosphate, so we hope that other insecticide classes are found that are synergistic with resistant soybean in the future.