Integrating Host-Plant Resistance and Insecticides for Sustainable Soybean Aphid Management

Integrating Host-Plant Resistance and Insecticides for Sustainable Soybean Aphid Management

Summary

The goal of this project is to determine if there are combined effects of insecticide application and using 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. Ongoing experiments are examining this interaction 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.

Objectives/Performance Targets

Three primary objectives were planned for this experiment in 2014:

1. 1. Measure an interaction between resistant plants and insecticides in field conditions.

1. 1. Also detect an interaction in lab assays under controlled greenhouse conditions where potential confounding factors such as weather and beneficial insects can be ruled out.

1. 1. 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 useage 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 plots were a 2×4 factorial design consisting of susceptible or resistant (Rag1) plots where each variety received one of four insecticide treatments with active ingredients: lambda-cyhalothrin (trade name: Warrior II), chlorpyrifos (trade name: Lorsban), pyrethrum+azadirachtin (trade name: Azera), or no insecticide treatment. Plots were sampled for the number of aphids and aphid predators or parasitoids per plant after plant emergence and after insecticide treatments. Plots were treated August 8 at labeled field rates after aphids in susceptible plots had surpassed an average of 250 aphids per plant and sampled for an additional 21 days after treatment.

Another variety was previously planned for inclusion in this study that had two resistance traits (Rag1+Rag2), but was excluded because aphids did not typically reach high enough population sizes to detect an effect of insecticide application in a previous field experiment in 2013. Aphid predator exclusion cages were also originally planned in each plot for this experiment, but were not included because data from the previous year didn’t indicate differences in effects on aphids between caged or uncaged plants. 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 the time of insecticide application.

Lab assays also began in spring 2014. Concentrations that caused approximately 30% aphid mortality on susceptible plants after dipping leaves into lambda-cyhalothrin, chlorpyrifos, and Azera were determined. 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) susceptible and resistant plant leaves by confining them in a clip cage attached to the leaf. Mortality and number of nymphs produced per aphid were recorded 48 hours after. Assays for lambda-cyhalothrin, and pyrethrum + azadirachtin are completed and currently being analyzed, while assays with chlorpyrifos are ongoing.

Growers and crop consultants were surveyed at Minnesota extension meetings during winter in 2013-14 and 2014-15 on their knowledge and use of soybean aphid resistant plants and insecticides.2013-14 meeting surveys are complete (see Table 1), while 2014-15 surveys are currently being summarized.

Accomplishments/Milestones

In the 2014 field plots, aphid populations (measured in cumulative aphid days at the end of the reason) 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 resistance and lamda-cyhalothrin on soybean aphids are independent of one another. However, aphid populations were lower than expected on resistant plants treated with chlorpyrifos (p = 0.03). This indicates chlorpyrifos and the resistant plants may be improving each other’s efficacy (Fig. 1).

Lab assays are ongoing, but are expected to conclude in spring 2015.

• Figure 1. 2014 field data

Impacts and Contributions/Outcomes

Information on soybean aphid host-plant resistance and insecticide use was presented at extension meetings in the winter of 2013-14 . After the presentation, 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). Surveys for 2014-2015 are still being processed where attendees were additionally presented with information found in this study on interactions between host-plant resistance and insecticides.

A field day was held on August 12, 2014 for farmers near Rosemount, MN where the atendees were given presentations and plot tours of projects at the University of Minnesota’s Rosemount Experiment Station. During one of the tours, farmers were able to walk through our plots to see high aphid populations on susceptible plants, and lower aphid populations on resistant untreated and resistant insecticide treated plots.

The findings from this study were also presented at the 2015 Farmers Forum at the Northern Plains Sustainable Agriculture Society in Aberdeen, SD.

• Table 1

Collaborators: