By Angie Peltier, UMN Extension crops educator & Robert Koch, UMN Extension soybean entomology specialist
On March 22, 2023, Robert Koch joined UMN Extension crops educator Anthony Hanson for a conversation about new and not-so-new soybean pests in Minnesota. This was an episode in the 2023 Strategic Farming: Let’s talk crops! series of webinars.
To watch this episode: https://www.youtube.com/watch?v=srH9XWjpy9Q
Soybean aphids (Figure 1) are by no means a new pest of Minnesota soybean; it has been more than 20 years since its arrival in the state. What is new about this pest is the shrinking list of insecticides that remain effective and labeled for use.
Soybean gall midge (SGM; Figure 2) is a newer pest of Minnesota soybean, found in the US and Minnesota for the first time in 2018. The adult SGM is a very small fly. Adult flies mate and lay their eggs in the expansion cracks that tend to form on stems towards the base of the plant early in the growing season. SGM eggs hatch into initially clear to white-colored larvae that turn an orange color as they grow and develop. SGM feed on tissue inside the outer layers of stem and feeding injury can result in dark stem lesions toward the base of the plant, brittle stems susceptible to lodging, plant wilt and death. Infestations tend to be most severe on plants growing near the edge of fields closest to the previous year’s soybean crop.
There is another small, orange larva called the white-mold gall midge (WMGM) that could be easily confused with the SGM. The WMGM feeds not on plants, but rather on the fungus that causes white mold. A couple of characteristics that can help distinguish between these two types of gall midges include that while the SGM larvae can be observed beneath the outer stem tissue near the base of the plant as early as the two or three leaf growth stage and is most prevalent on field edges, WMGM larvae tend to be observed on or in plants (stems and pods) infected with the white mold fungus, after flowering.
The SGM host range is still being investigated and is now known to include dry bean and lima bean, in addition to soybean, sweet clover and alfalfa. It is not known whether the host range also includes legume species found in natural or prairie areas of the state, but other researchers are examining this.
While many management tactics have been or are currently being investigated, effective and practical means to manage SGM have not yet been identified. There are many insecticides labeled for use on soybean in Minnesota; however, the protection afforded by SGM larvae occurring within stem tissue and a long period of adult emergence has resulted in inconsistent and generally low levels of control with foliar insecticides. While researchers at the University of Nebraska in Lincoln have identified several soybean lines that appear resistant to SGM, the basis of this resistance is unknown and to transfer the genes responsible for this resistance into elite soybean varieties adapted to Minnesota is going to take many years.
Additional research efforts on biological control agents have involved determining whether ground-dwelling predatory insects eat SGM. In survey efforts using traps made small plastic containers buried in the soil and filled with antifreeze (called pitfall traps), the team determined that one species of predatory beetle was very abundant in the SGM-infested fields. In laboratory experiments, the team showed that this species of beetle was capable of feeding on SGM larvae. However, to determine if the predator is actually feeding on SGM in natural conditions in the field, more advanced experimental techniques were needed. A subset of the beetles collected in pitfall traps were dissected and their gut contents were analyzed to determine if they contained DNA of the SGM, which would indicate that the predators had eaten SGM. Preliminary results from just one sample date indicated that at least 7% of the beetles tested had consumed SGM.
Another new pest of soybean found in Minnesota in 2021 is called the soybean tentiform leafminer (STL; Figure 3). While this pest had been known to infest the native legumes, American hog peanut and slickseed fuzzy bean, across eastern North America, it had previously not been known to be a pest of soybean. In 2021, STL was found widespread in soybean across southern Minnesota and into eastern South Dakota.
STL adults are small moths (about the size of George Washington’s nose on a quarter) that lay their eggs on the underside of leaves. Upon hatching, STL larvae chew their way into the leaf tissue to form a linear “mine” as they tunnel within the leaf. As larvae molt and grow over time, they remain inside the leaves create larger mines that are constrained by leaflets veins. As the larvae near maturity, the upper surface of the leaf where they have been mining becomes raised or “tented” (hence the “tentiform” part of their name) and light colored spots can be seen on the upper surface of the leaf where light shines through the small holes they make within the leaves. The leaf area that is fed upon by STL larvae tends to die, likely resulting in an effect on plants similar to that caused by defoliating insects.
In 2022, Dr. Koch’s team visited a STL-infested field looking for additional information about this new pest. The team found that STL injury tended to be greater toward the bottom of soybean plants and on field edges near wooded areas, although additional work in 2023 is needed to confirm this finding.
While more research on STL is necessary, preliminary assays with potted soybean plants has shown that some insecticides that can move through the leaf (translaminar activity) and kill the young STL larvae inside the leaves. Additional research will be needed in 2023 to confirm these results with actual field infestations of this STL. Other promising work has found that several species of parasitic wasps attack STL. Lastly, a survey effort during the 2023 growing season will help to reveal how large of a footprint STL currently has on the Minnesota landscape.
Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
On March 22, 2023, Robert Koch joined UMN Extension crops educator Anthony Hanson for a conversation about new and not-so-new soybean pests in Minnesota. This was an episode in the 2023 Strategic Farming: Let’s talk crops! series of webinars.
To watch this episode: https://www.youtube.com/watch?v=srH9XWjpy9Q
Insecticides labeled for managing soybean aphid
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| Figure 1. Soybean aphid adult giving live birth to nymph. Photo: James Menger |
The US Environmental Protection Agency revoked the tolerances for the organophosphate insecticide chlorpyrifos (e.x. Lorsban, etc.). In 2022, it became no longer legal to use this active ingredient on crops. While there are still a handful of other Group 1 organophosphate insecticides on the market, none were as widely used as chlorpyrifos.
For several years now, the Minnesota Department of Agriculture (MDA) has been monitoring surface waters throughout the state for pesticides used in crop production. Two active ingredients in the neonicotinoid class of insecticides (Group 4; imidacloprid and clothianidin) used as seed treatments and/or foliar-applied insecticides have been detected in surface waters at high enough concentrations to trigger the MDA to designate these insecticides as “surface water pesticides of concern.” This designation triggered the development of a series of best management practices (BPMs) that include the use of integrated pest management principles and setbacks near surface waters. The goal of these BPMs is to reduce the frequency and magnitude of these surface water detections and forestall additional regulatory actions or restrictions of these active ingredients.
For many years the pyrethroid class (Group 3) of insecticides were very effective. They were widely used whether or not soybean aphid population densities reached treatment thresholds, with many considering them a “cheap” insurance. The widespread and often unnecessary use of this class of insecticides in soybean and other crops led to the detection in 2015 of pyrethroid-resistant soybean aphids in Minnesota. Soybean aphid populations resistant to pyrethroid insecticides have been detected in the state ever since, rendering some of the most common products in this class of insecticides ineffective.
In recent years, newer insecticide active ingredients have come on the market including afidopyropen, a Group 9 insecticide (ex. trade name: Sefina) and sulfoxaflor (ex. trade name: Transform) and flupyradifurone (ex. trade name: Sivanto), which are separate subgroups of insecticides related to the neonicotinoids (all in group 4). While these active ingredients are new enough that many growers don’t yet have much experience with them, research shows that they are generally just as effective against soybean aphid as chlorpyrifos had been. These three new active ingredients also tend to be more selective in targeting insects with piercing-sucking mouthparts like aphids and are much less toxic to predatory insects like lady beetles and lacewings, and parasitic wasps that act as naturally occurring biocontrol agents of aphids.
For several years now, the Minnesota Department of Agriculture (MDA) has been monitoring surface waters throughout the state for pesticides used in crop production. Two active ingredients in the neonicotinoid class of insecticides (Group 4; imidacloprid and clothianidin) used as seed treatments and/or foliar-applied insecticides have been detected in surface waters at high enough concentrations to trigger the MDA to designate these insecticides as “surface water pesticides of concern.” This designation triggered the development of a series of best management practices (BPMs) that include the use of integrated pest management principles and setbacks near surface waters. The goal of these BPMs is to reduce the frequency and magnitude of these surface water detections and forestall additional regulatory actions or restrictions of these active ingredients.
For many years the pyrethroid class (Group 3) of insecticides were very effective. They were widely used whether or not soybean aphid population densities reached treatment thresholds, with many considering them a “cheap” insurance. The widespread and often unnecessary use of this class of insecticides in soybean and other crops led to the detection in 2015 of pyrethroid-resistant soybean aphids in Minnesota. Soybean aphid populations resistant to pyrethroid insecticides have been detected in the state ever since, rendering some of the most common products in this class of insecticides ineffective.
In recent years, newer insecticide active ingredients have come on the market including afidopyropen, a Group 9 insecticide (ex. trade name: Sefina) and sulfoxaflor (ex. trade name: Transform) and flupyradifurone (ex. trade name: Sivanto), which are separate subgroups of insecticides related to the neonicotinoids (all in group 4). While these active ingredients are new enough that many growers don’t yet have much experience with them, research shows that they are generally just as effective against soybean aphid as chlorpyrifos had been. These three new active ingredients also tend to be more selective in targeting insects with piercing-sucking mouthparts like aphids and are much less toxic to predatory insects like lady beetles and lacewings, and parasitic wasps that act as naturally occurring biocontrol agents of aphids.
Soybean gall midge
 |
| Figure 2. Soybean gall midge larvae. Photo: Sarah Lisak |
There is another small, orange larva called the white-mold gall midge (WMGM) that could be easily confused with the SGM. The WMGM feeds not on plants, but rather on the fungus that causes white mold. A couple of characteristics that can help distinguish between these two types of gall midges include that while the SGM larvae can be observed beneath the outer stem tissue near the base of the plant as early as the two or three leaf growth stage and is most prevalent on field edges, WMGM larvae tend to be observed on or in plants (stems and pods) infected with the white mold fungus, after flowering.
The SGM host range is still being investigated and is now known to include dry bean and lima bean, in addition to soybean, sweet clover and alfalfa. It is not known whether the host range also includes legume species found in natural or prairie areas of the state, but other researchers are examining this.
While many management tactics have been or are currently being investigated, effective and practical means to manage SGM have not yet been identified. There are many insecticides labeled for use on soybean in Minnesota; however, the protection afforded by SGM larvae occurring within stem tissue and a long period of adult emergence has resulted in inconsistent and generally low levels of control with foliar insecticides. While researchers at the University of Nebraska in Lincoln have identified several soybean lines that appear resistant to SGM, the basis of this resistance is unknown and to transfer the genes responsible for this resistance into elite soybean varieties adapted to Minnesota is going to take many years.
Biological control research
Dr. Koch’s lab is focusing their work on investigating biological control of the SGM. This work resulted in identifying a new species of parasitic wasp (parasitoid) that had never been observed before. Initial work on identifying SGM parasitoids relied on collecting infested plants in the field and bringing them back to the laboratory to see whether anything other than SGM emerged. Once the wasp was found, the team used molecular techniques to determine whether there was wasp DNA inside any SGM larvae collected from the field. These DNA-based techniques appear more effective for assessing levels of parasitism of SGM larvae in Minnesota fields.Additional research efforts on biological control agents have involved determining whether ground-dwelling predatory insects eat SGM. In survey efforts using traps made small plastic containers buried in the soil and filled with antifreeze (called pitfall traps), the team determined that one species of predatory beetle was very abundant in the SGM-infested fields. In laboratory experiments, the team showed that this species of beetle was capable of feeding on SGM larvae. However, to determine if the predator is actually feeding on SGM in natural conditions in the field, more advanced experimental techniques were needed. A subset of the beetles collected in pitfall traps were dissected and their gut contents were analyzed to determine if they contained DNA of the SGM, which would indicate that the predators had eaten SGM. Preliminary results from just one sample date indicated that at least 7% of the beetles tested had consumed SGM.
Soybean tentiform leafminer
 |
| Figure 3. Soybean tentiform leafminer larva. Photo: R. Koch |
STL adults are small moths (about the size of George Washington’s nose on a quarter) that lay their eggs on the underside of leaves. Upon hatching, STL larvae chew their way into the leaf tissue to form a linear “mine” as they tunnel within the leaf. As larvae molt and grow over time, they remain inside the leaves create larger mines that are constrained by leaflets veins. As the larvae near maturity, the upper surface of the leaf where they have been mining becomes raised or “tented” (hence the “tentiform” part of their name) and light colored spots can be seen on the upper surface of the leaf where light shines through the small holes they make within the leaves. The leaf area that is fed upon by STL larvae tends to die, likely resulting in an effect on plants similar to that caused by defoliating insects.
In 2022, Dr. Koch’s team visited a STL-infested field looking for additional information about this new pest. The team found that STL injury tended to be greater toward the bottom of soybean plants and on field edges near wooded areas, although additional work in 2023 is needed to confirm this finding.
While more research on STL is necessary, preliminary assays with potted soybean plants has shown that some insecticides that can move through the leaf (translaminar activity) and kill the young STL larvae inside the leaves. Additional research will be needed in 2023 to confirm these results with actual field infestations of this STL. Other promising work has found that several species of parasitic wasps attack STL. Lastly, a survey effort during the 2023 growing season will help to reveal how large of a footprint STL currently has on the Minnesota landscape.
Fielding audience questions
Koch answered numerous audience questions on soybean aphid, soybean gall midge, parasitic wasps, grasshoppers and soybean tentiform leafminer including: Do parasitic wasps cause damage to other crops or humans? How would buckthorn removal impact soybean aphid pressure? Does the same species of parasitic wasp attack soybean aphid and soybean gall midge? What about other organophosphate insecticides for soybean aphid management? Why is there resistance to pyrethroids in so many insect pests? What about the effectiveness of organic insecticides? Where did the SGM come from?Join us for the final session
Join us for the final session of the University of Minnesota’s Strategic Farming: Let’s talk crops! webinar series this Wednesday, March 29. We will welcome special guest Dr. Brian Luck, Extension specialist, Machinery systems and precision agriculture from UW-Madison, and Dr. Seth Naeve , UMN Extension soybean agronomist, to discuss getting your planter ready for spring. For more information and to register, visit z.umn.edu/strategic-farming.Thanks to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!
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