Eric Yu, UMN Extension Crops Educator, and Fei Yang, UMN Extension Entomologist
On February 26, 2025, Dr. Fei Yang, UMN Corn Extension Entomologist, joined UMN Extension Crops Educator Dr. Anthony Hanson for a discussion on corn insect pests. Dr. Yang provided an update on the current challenges Minnesota faces with these pests and shared insights from his research on corn insect pest management. This was the eighth episode of the 2025 Strategic Farming: Let’s Talk Crops! webinar series, which will continue through March.
View recordings of this and all Strategic Farming sessions at:
https://z.umn.edu/StrategicFarmingRecordings
Despite these resistance concerns, Bt technology remains a vital component of corn rootworm management. This is supported by a 2023 study conducted by Dr. Yang in Lamberton, MN, where he compared pyramided hybrid corn varieties, including SmartStax, SmartStax PRO, and VT4PRO, with non-Bt corn. The study found that pyramided hybrids had significantly less node injury compared to non-Bt corn, consistently staying below the economic threshold for rootworm larval damage (node injury score of 0.25). However, one field with SmartStax showed a node injury score of 0.40, suggesting some resistance to Cry3Bb1 and Cry34/35Ab1 in that corn rootworm population.
A multi-year study conducted in Minnesota from 2021 to 2024 examined the impact of crop rotation on both WCR and NCR populations using yellow sticky traps. The study compared fields with rotated corn to fields with continuous corn, including fields with two consecutive years of corn and fields with three or more consecutive years of corn plantings, and found that crop rotation was still the effective tool for managing corn rootworm populations across the landscape.
Although rotation-resistant WCR variants have not yet been reported in Minnesota prior to 2024, a case in July 2024 has raised concerns. A first-year corn field planted with Bt corn (mCry3A + Cry34/35Ab1), following seven years of alfalfa, experienced WCR root injury. This injury was confirmed in corn plants expressing Bt traits. This suggests the possible presence of rotation-resistant WCR populations in Minnesota. Nearby soybean fields were observed with adult WCR beetles feeding on the soybean plants, suggesting that the rootworms may be laying their eggs in these fields, further indicating rotation resistance in these populations. Toward the end of the 2024 growing season, three additional reports from Minnesota indicated first-year corn fields experiencing damage from WCR, raising concerns about the spread of rotation-resistant WCR populations in the state.
Dr. Yang also conducted a separate study on the efficacy of insecticide seed treatments for managing corn rootworm. This study focused on different neonicotinoids at varying concentrations. For thiamethoxam, in non-Bt corn, increasing the concentration to 1.25 mg per seed significantly reduced root injury. However, Bt corn was effective in controlling corn rootworm regardless of whether a seed treatment was applied, indicating that the Bt trait worked well against the local corn rootworm population. When comparing three different neonicotinoid seed treatments in both non-Bt and Bt corn, Dr. Yang found that thiamethoxam and clothianidin significantly reduced root injury in non-Bt corn compared to untreated seed, while chlorantraniliprole did not reduce root injury. As with the previous findings, Bt corn was effective in controlling corn rootworm, regardless of the seed treatment used. In the presence of Bt resistant populations, seed treatments with higher concentrations of thiamethoxam or clothianidin may offer additional protection.
In response, Dr. Yang collected larvae from the affected field to study whether this population exhibited resistance to any Bt traits targeting ECB. The collected larvae were reared, and the resulting adults were mated. Their offspring were then exposed to each Bt protein. The study found that when larvae were exposed to Cry1 proteins, no survivors emerged, indicating that Cry1 resistance is low in the field. However, the study also revealed high resistance to the Cry2Ab2 protein. This mirrors findings from other populations in Minnesota and Wisconsin, where ECB larvae show low mortality even when exposed to very high concentrations of Cry2Ab2. While the overall proportion of ECB populations resistant to Bt remains low, the risk of resistant populations spreading still exists. Despite this, Cry1 proteins continue to be an effective tool for controlling ECB.
Cornsilk fly, a pest primarily affecting sweet corn rather than field corn, was also first discovered in September 2024 in Rosemount, MN. The larvae were found in a sweet corn plot that was being monitored for corn earworm and ECB resistance to Bt corn. Cornsilk fly can cause significant damage to sweet corn, as evidenced by previous outbreaks in Florida. While both the corn leafhopper and cornsilk fly are new and currently present in low numbers, there is minimal concern at this time. The University of Minnesota Extension will continue to closely monitor these emerging insect pests.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program!
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| Western corn rootworm beetles feeding on corn silks. |
View recordings of this and all Strategic Farming sessions at:
https://z.umn.edu/StrategicFarmingRecordings
Corn rootworm
Background and damage
Two corn rootworm species are problematic in Minnesota: western corn rootworm (WCR) and northern corn rootworm (NCR), both of which have similar life cycles. Adult beetles lay their eggs from late summer to early fall, and the eggs overwinter (undergoing diapause) until warmer temperatures trigger hatching, typically around late May to early June the following year. The larval stage is the most damaging, as it can cause significant root injury. Corn rootworm larvae, particularly in the later instars, can severely prune roots, reducing the plant’s ability to take up water and nutrients, which can lead to corn lodging. Lodging poses several challenges for the corn plant, including hindering photosynthesis, pollination, and complicating harvest. Root injuries caused by corn rootworms can also make the plant more susceptible to secondary infections, such as crown rot. While the larval stage is the most destructive, adult beetles can also cause damage by feeding on tassels and silks, leading to poor pollination and increasing the risk of ear mold diseases, both of which can reduce corn yields.Corn rootworm management options
Several tools are available to manage corn rootworm, including corn with Bt traits, crop rotation, and insecticides. Insecticides can be applied in various ways, such as soil-applied insecticides, seed treatments for larval control, and foliar insecticides for adult control. Currently, there are four Bt proteins targeting corn rootworm, as well as a relatively new RNAi trait. These four Bt proteins and one RNAi work through three different modes of action (MOAs). Many Bt hybrids express two or more MOAs in a single plant, which is particularly effective when the traits come from different MOAs. This approach reduces the likelihood of resistance, as the rootworm would need to develop resistance to several traits simultaneously in order to survive.Corn rootworm resistance challenges
Resistance to Bt corn
Resistance to Bt proteins, particularly Cry3 and Cry34/35, has been widely documented in WCR populations, with the first case discovered in 2009, six years after Bt-RW corn commercialization. In Minnesota, Bt resistance has been reported and confirmed in Yellow Medicine, Brown, Mower, and Redwood Counties, as published in peer-reviewed journals. Dr. Yang also reported several other cases of Bt resistance across the state. However, reports of Bt resistance in NCR are less common, with only two publications documenting resistance in NCR populations. The first, published in 2019, described resistance in a 2016 NCR population from North Dakota, while the second, published in 2023, reported resistance in a 2019 NCR population from Meeker County, Minnesota.Despite these resistance concerns, Bt technology remains a vital component of corn rootworm management. This is supported by a 2023 study conducted by Dr. Yang in Lamberton, MN, where he compared pyramided hybrid corn varieties, including SmartStax, SmartStax PRO, and VT4PRO, with non-Bt corn. The study found that pyramided hybrids had significantly less node injury compared to non-Bt corn, consistently staying below the economic threshold for rootworm larval damage (node injury score of 0.25). However, one field with SmartStax showed a node injury score of 0.40, suggesting some resistance to Cry3Bb1 and Cry34/35Ab1 in that corn rootworm population.
Resistance to crop rotation
The WCR is most abundant in continuous corn fields and is less mobile, with adult beetles typically staying within the same field or moving between neighboring corn fields. However, a variant of WCR has developed rotation resistance by laying eggs in non-corn fields, allowing them to hatch the following spring when corn is planted in those fields. The NCR, in contrast, can be found in both continuous and rotated corn fields and is more mobile, meaning populations can travel some distance to other corn fields. The NCR also exhibits rotation resistance, but through extended diapause, a mechanism that allows eggs to overwinter for additional years, causing them to hatch only when corn is planted again.A multi-year study conducted in Minnesota from 2021 to 2024 examined the impact of crop rotation on both WCR and NCR populations using yellow sticky traps. The study compared fields with rotated corn to fields with continuous corn, including fields with two consecutive years of corn and fields with three or more consecutive years of corn plantings, and found that crop rotation was still the effective tool for managing corn rootworm populations across the landscape.
WCR Resistance to crop rotation in Minnesota
For WCR, the study found that insect pressure was low in rotated corn fields. However, insect pressure increased in fields with two consecutive years of corn, and pressure rose significantly in fields with three or more years of continuous corn, even with Bt corn planting. Similar results were observed in fields with non-Bt corn, with low pressure in rotated corn fields and high pressure in continuous corn fields. These findings underscore the importance of crop rotation, and confirm the Bt resistance in WCR.Although rotation-resistant WCR variants have not yet been reported in Minnesota prior to 2024, a case in July 2024 has raised concerns. A first-year corn field planted with Bt corn (mCry3A + Cry34/35Ab1), following seven years of alfalfa, experienced WCR root injury. This injury was confirmed in corn plants expressing Bt traits. This suggests the possible presence of rotation-resistant WCR populations in Minnesota. Nearby soybean fields were observed with adult WCR beetles feeding on the soybean plants, suggesting that the rootworms may be laying their eggs in these fields, further indicating rotation resistance in these populations. Toward the end of the 2024 growing season, three additional reports from Minnesota indicated first-year corn fields experiencing damage from WCR, raising concerns about the spread of rotation-resistant WCR populations in the state.
NCR Resistance to crop rotation in Minnesota
For NCR, the study showed that in Bt corn fields, insect pressure was very low regardless of whether the field had been rotated or had consecutive years of corn. This suggests that Bt traits are still effective in controlling NCR populations in Minnesota. However, in non-Bt corn fields, insect pressure was significantly higher in first-year corn fields (rotated fields) than in fields with consecutive years of corn planting in 2023 and 2024. This pattern suggests that NCR populations in Minnesota have developed crop rotation resistance through extended diapause. This issue is not new to the state, as extended diapause in NCR populations was first documented in southern Minnesota in the late 1970s. Since then, NCR populations with extended diapause have spread as far north as Grant, Wilkin, and Otter Tail Counties in 2024.Insecticides for rootworm management
Dr. Yang and his colleagues conducted a study from 2020 to 2024 in Minnesota to evaluate various soil insecticides at different rates for controlling corn rootworm. The insecticides tested included products with active ingredients from several groups: pyrethroids, neonicotinoids, diamides, isoxazolines, and combinations of organophosphates and pyrethroids. The study found that increasing the concentration of any product, regardless of the type, did not reduce corn rootworm damage. Additionally, when six different pyrethroid-based products were compared to treatments without insecticides, no significant reduction in corn rootworm damage was observed, indicating that pyrethroids are not effective for managing corn rootworm. However, the study did show that the combination of organophosphates and pyrethroids was the only treatment to significantly reduce corn rootworm damage. It's important to note that environmental factors also influenced corn rootworm populations. In 2024, heavy rainfall and flooding during late May through mid-June coincided with the corn rootworm egg hatch period, resulting in significant rootworm mortality due to the flooded conditions.Dr. Yang also conducted a separate study on the efficacy of insecticide seed treatments for managing corn rootworm. This study focused on different neonicotinoids at varying concentrations. For thiamethoxam, in non-Bt corn, increasing the concentration to 1.25 mg per seed significantly reduced root injury. However, Bt corn was effective in controlling corn rootworm regardless of whether a seed treatment was applied, indicating that the Bt trait worked well against the local corn rootworm population. When comparing three different neonicotinoid seed treatments in both non-Bt and Bt corn, Dr. Yang found that thiamethoxam and clothianidin significantly reduced root injury in non-Bt corn compared to untreated seed, while chlorantraniliprole did not reduce root injury. As with the previous findings, Bt corn was effective in controlling corn rootworm, regardless of the seed treatment used. In the presence of Bt resistant populations, seed treatments with higher concentrations of thiamethoxam or clothianidin may offer additional protection.
European corn borer resistance concerns in the U.S.
The ECB larvae can cause significant crop damage through feeding, which may result in stalk breakage, ear drop, and increased susceptibility to diseases. Transgenic Bt corn, which contains Cry1 and Cry2 proteins, has effectively controlled ECB for over 28 years in the U.S. However, in 2018, resistance to Bt corn was first documented in Nova Scotia, Canada, specifically against the Cry1F protein. Since this discovery, resistance has spread across three Canadian provinces, affecting all available Bt corn traits, including Cry1Ab, Cry1A.105, and Cry2Ab2. In 2023, the first report of ECB resistance to Bt corn was recorded in the U.S., in Connecticut. The ECB can be highly mobile which poses a significant threat to the efficacy of Bt technology, as resistance can spread over long distances. This is especially concerning for Minnesota, where resistant populations have already been confirmed in nearby Manitoba, Canada. Additionally, hot spots of ECB populations have been observed within Minnesota, including a 2024 case in Clearwater County, where approximately 90% of corn plants showed damage from ECB.In response, Dr. Yang collected larvae from the affected field to study whether this population exhibited resistance to any Bt traits targeting ECB. The collected larvae were reared, and the resulting adults were mated. Their offspring were then exposed to each Bt protein. The study found that when larvae were exposed to Cry1 proteins, no survivors emerged, indicating that Cry1 resistance is low in the field. However, the study also revealed high resistance to the Cry2Ab2 protein. This mirrors findings from other populations in Minnesota and Wisconsin, where ECB larvae show low mortality even when exposed to very high concentrations of Cry2Ab2. While the overall proportion of ECB populations resistant to Bt remains low, the risk of resistant populations spreading still exists. Despite this, Cry1 proteins continue to be an effective tool for controlling ECB.
New corn pests discovered in Minnesota in 2024
Corn leafhopper and cornsilk fly
The corn leafhopper was first detected in September 2024 on the UMN campus in St. Paul, MN. It was discovered in a trial plot where late-planted corn was being studied for frost-resistant hybrids. Significant infestations of both adults and nymphs were found on corn plants at the V4 to V5 growth stages. The primary damage caused by corn leafhoppers is through their role as vectors for pathogens, including viruses and bacteria, which they can transmit to corn. This can result in stunting diseases, with red leaves being a common indicator of infection. Management strategies from regions with high corn leafhopper populations, such as Brazil and the southern U.S., suggest that seed treatments are effective in controlling these pests. Combining seed treatments with targeted insecticide applications can enhance control effectiveness.Cornsilk fly, a pest primarily affecting sweet corn rather than field corn, was also first discovered in September 2024 in Rosemount, MN. The larvae were found in a sweet corn plot that was being monitored for corn earworm and ECB resistance to Bt corn. Cornsilk fly can cause significant damage to sweet corn, as evidenced by previous outbreaks in Florida. While both the corn leafhopper and cornsilk fly are new and currently present in low numbers, there is minimal concern at this time. The University of Minnesota Extension will continue to closely monitor these emerging insect pests.
Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program!
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