Entomologists provide update on Iowa soybean pests

December 3, 2020 | Bethany Baratta

Investment in soybean checkoff funding and collaboration through Iowa Soybean Association (ISA) and the North Central Soybean Research Program (NCSRP) are advancing understanding of two soybean pests in the state: soybean gall midge and soybean aphids.

Iowa State University (ISU) Extension entomologists Erin Hodgson and Ashley Dean discussed ongoing research and management recommendations for soybean aphids and soybean gall midge during the first episode of CropsTV. This online program replaces the popular Integrated Crop Management Conference and Crop Advantage Series this winter. It’s sponsored in-part by the Iowa Soybean Association.

$1.8 billion impact

Soybean gall midge was first found in northeast Nebraska in 2011 and was first documented in Iowa in 2016. Since then, soybean gall midge injury has been reported in 5 states and 114 counties. At the end of the 2020 growing season, there were 31 counties in Iowa with gall midge infestations. Hodgson expects that gall midge is affecting 3.7 million acres of soybeans with a $1.8 billion impact on soybean value.

Through a collaborative project in Iowa, Minnesota, and Nebraska, researchers were able to collect over 4,500 adults throughout 900 locations in the 2020 growing season. This helped collaborators understand the emergence and life-cycle patterns of these pests.

The first emergence was reported on June 12 in Cass County, Iowa, 2 days sooner than last year. The duration of gall midge emergence was 25.6 days in the 2020 growing season, 9 days longer than in the 2019 growing season.

“We were hoping to have a really narrow window of emergence of 3 to 5 days,” Hodgson said.

That wasn’t the case.

The longer emergence window creates a significant challenge for protecting early-vegetative soybean varieties, she notes.

Because of the longer emergence window, finding a chemical solution to beating the pest becomes much more challenging.

Hodgson and her team evaluated 767 soybean lines through a soybean germplasm screening project in Iowa and Nebraska.

Of the 767 lines, 1% to 2% of the lines were not infested by soybean gall midge.

The genetic screening research was looking for ‘clean’ resistance genes that could later be used in modern soybean lines.

With the backing of soybean checkoff and other funding sources, Hodgson and her team are also working to develop an injury-yield model in relation to soybean gall midge infestation.

“We’re hoping to be able to provide some predictive values to farmers to help them decide whether a treatment would be worth it,” Hodgson said.

This tool is a prime example of how checkoff-funded projects return value back to farmers, said Ed Anderson, senior director of research at ISA.

“Like so many checkoff investments, farmers directly benefit because checkoff funding enables researchers and extension folks to do research, trials and evaluations that generate knowledge and data; results that they can put right back into the hands of farmers,” Anderson said.

An injury-yield loss model, another component of the project, further helps identify how many acres are affected and the loss associated with the affected acres.

One of the most common questions Hodgson hears from agronomists and farmers: “We see soybean gall midge, what can we do about it?”

Hodgson and the project’s collaborators are evaluating the efficacy of various treatments of gall midge.

Highly variable injury scores were observed among seed treatment, in-furrow and foliar treatments in 2019 and 2020. Iowa faced more severe gall midge infestations in 2019, but saw less severe infestation rates than Nebraska in 2020. The team is still investigating what causes the severity of the pest from year to year.

Research showed that early-vegetative applications did not prevent egg laying or improve the efficacy score of treatment options.

Application sites did not improve the efficacy score in the evaluation. Seed treatment showed a slight improvement in efficacy in studies in Nebraska, Hodgson noted.

Iowa Soybean Association research showed that Thimet had suppressive effects on soybean gall midge.

To watch for and assist with management strategies of soybean gall midge, the Soybean Gall Midge (SGM) Alert Network has been established. It’s a network of 36 sites across four states. Go to soybeangallmidge.org to learn more.

Aphid management modeling

ISU entomologist Ashley Dean’s work has been focused on developing a model to predict yield and revenue based on management decisions for soybean aphids.

Her model considers the probability of an outbreak and the probability that aphids are resistant.

Take the 2018 growing season for example:

As of 2018, the outbreak frequency was 43.48% across high risk areas in Iowa. In those high-risk areas in 2018, the average yield loss was 12.7% during outbreak years.

Knowing this information helps farmers explore the management scenarios to decide whether treating a field within a high- or low-risk area makes economic sense for them.

Findings related to Dean’s initial aphid modeling research relating to the economic decisions regarding treatment, funded by NCSRP, can be found here. 

There are multiple management tactics for soybean aphid, including insecticide application and host-plant resistance. These tactics have shown to be equal in terms of yield protection unless aphids are resistant to an insecticide (e.g., pyrethroids).

Host plant resistance

Dean’s modeling shows the benefit of host plant resistance is even greater with pyrethroid-resistant aphids. Host plant resistance in soybean, specifically aphid-resistant soybean, works by suppressing aphid growth and reproduction on the plant. This reduces the number of insecticide applications needed to manage soybean aphids, an economic benefit to farmers.

The first gene that showed resistance to soybean aphid and was commercialized, named Rag1, was first identified by scientists at the U.S. Department of Agriculture. Soybean varieties expressing the Rag1 gene have been evaluated throughout the North Central region. In Iowa, there was a significant difference in soybean aphid population growth when comparing resistant and susceptible plots.

When not treated with an insecticide, aphid-resistant soybean had higher yields than aphid-susceptible soybeans, according to research by Hodgson and ISU entomologist Matt O’Neal.

Soybean aphid will be more challenging and expensive to manage in the future, Dean said. She recommends that farmers consider host-plant resistant varieties of soybean and the economics of any management decisions made for soybean aphid.

Bringing value back to the farmers

Dean’s work is another example of how checkoff funding and collaboration brings value back to the farmer, Anderson said.

“This excellent work demonstrates how checkoff funding has helped bring significant benefit to farmers in managing soybean aphid and now insecticide-resistant aphids, especially through checkoff-funded identification and characterization of these host plant resistant genes,” Anderson said.

For more information on CropsTV, go to https://www.aep.iastate.edu/cropstv/.

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