Research Highlights

Defining Best Practices to Manage Co-Occurring Insects in Minnesota Soybeans

By Barb Baylor Anderson

Soybean aphids have earned the reputation of being the most damaging insect in soybeans. But as Japanese beetles spread across southern Minnesota, they are giving soybean aphids a run for their money when it comes to crop damage. Plus, the two pests co-occur in soybean fields.

“It remains unknown how the combined injury of aphid and Japanese beetle affects soybeans. Farmers need information about how the co-occurring insect pests influence yield to make better informed management decisions,” says Robert Koch, University of Minnesota Extension entomologist and associate professor. “Farmers may be unnecessarily increasing insecticide inputs or incurring yield losses due to damaging infestations not being treated appropriately.”

Koch is principal investigator for a multi-year research project funded by the Minnesota Soybean Promotion & Research Council to first quantify the effects of combined infestation of soybean aphid and Japanese beetle on soybean yield and quality. Secondly, the goal is to characterize the effects of aphid and beetle feeding individually and combined on plant spectral reflectance to increase the accuracy of drone-based remote sensing for soybean insect pests.

For the work, experimental plots of soybean were established in two locations in southern Minnesota where soybean aphid and Japanese beetle are likely to occur. Per plant aphid counts, visual defoliation, computer calculated defoliation and hyperspectral measurements were taken of all plots over the course of several weeks. Plots were harvested for yield comparisons as well.

Preliminary data analyses suggest low levels of soybean aphid quantified as about 100 aphids per plant and Japanese beetle defoliation of less than five percent did not reduce soybean yield or seed quality. High levels of soybean aphid – upwards of 1,000 per plant – reduced seed weight. Intense defoliation of 33 percent decreased total yield and the weight and number of seeds.

Going into the research project, Koch says it was unclear how Japanese beetle feeding might also affect efforts to use drone-based scouting for aphids. A second year of evaluation on the impact of combined infestation on soybean yield, quality and spectral reflectance of the canopy helped address how co-infestation by insects like Japanese beetle affect spectral reflectance.

Specifically, Japanese beetle defoliation of less than five percent was found to increase red-edge, but not near infrared reflectance of soybean plants, says Koch. Near infrared reflectance was previously identified as promising for detecting soybean aphids.

“We found typical levels of Japanese beetle defoliation in the field are unlikely to affect detection of soybean aphids with remote sensing,” he says. “However, intense defoliation of 33 percent decreased reflectance at near infrared and therefore could affect remote sensing for aphids. Scouting for pests is a costly aspect of crop management, so drone-based remote sensing could greatly improve the efficiency of these scouting efforts.”

Koch says while some Minnesota soybean farmers may think low levels of soybean aphid might need to be treated if another pest is present, he recommends continuing to rely on the threshold of 250 aphids per plant to decide when to apply insecticides. Additional research will allow Koch to offer even more robust management recommendations in the future.

“We need to examine this relationship between soybean aphid and Japanese beetle under more environments, locations and years,” he says. “However, it can be challenging to get both pests to cooperate with us in our research. Ultimately, we hope the results of this project will contribute to greater economic and environmental sustainability of soybean production.”

Published: Oct 25, 2021