Claire LaCanne, Extension Educator - crops, Bob Koch, Extension Soybean Entomologist, Fei Yang, Extension Corn Entomologist
Fei Yang, Extension Corn Entomologist and Bob Koch, Extension Soybean Entomologist joined moderator Liz Stahl, Extension educator - crops, to discuss insect updates and management during the August 6 session of Strategic Farming: Field Notes.
Northern corn rootworm eggs are more tolerant to cold temperatures than Westerns and are predominant in northern Minnesota. Though growers traditionally think of corn rootworm as an issue in continuous corn, a portion of Northern corn rootworm has the extended diapause genetic trait, allowing eggs to delay hatching over two winters, with some eggs exhibiting delayed hatching of three or four winters. This extended diapause trait allows Northerns to be a significant pest in both continuous corn and rotated corn fields.
So far in 2025, the issue of extended diapause appears to have resurfaced in Minnesota, with numerous reports of damage in first-year (rotated) corn fields. Currently, there are no specific data indicating what percentage of Minnesota populations have extended diapause exactly, but based on studies in other states, it is possible that approximately fifty percent of eggs hatch the first year and then the other fifty percent may hatch in following growing seasons over the next two to four years.
The good news is that we haven’t found Northern corn rootworm populations that have resistance to Bt proteins, meaning that Bt technology is still effective for management of Northern corn rootworm. From a management perspective, if a grower has Northern corn rootworm populations with extended diapause and has a one year crop rotation, using Bt traits can help effectively manage risks. Growers that have a longer and more varied crop rotation likely won’t have to rely on Bt traits.
Unfortunately, it’s a different story with Western corn rootworm. While there are not extended diapause issues, there are problems with resistance to some Bt proteins. In Martin County, MN there are populations of Western corn rootworm with resistance to multiple Bt proteins. The Handy Bt trait table provides guidance on which Bt proteins are still effective for various corn insect pests.
Notable damage was observed along the edges of Bt corn fields, with relatively high numbers of larvae present in the stalk of the plants. This unexpected damage on these Bt corn plants may suggest some common stalk borer populations in southern Minnesota may have developed resistance to Bt traits. However, common stalk borers typically hatch and feed on the grass before moving into corn fields. Further investigation is needed to determine whether the survival of larvae in these Bt corn plants is due to large larvae movement or indeed resistance to Bt proteins.
As for resistance in European corn borer, some recent lab research confirmed that Cry2Ab2 Bt resistance alleles in European corn borer populations are present in multiple locations in Minnesota and Wisconsin. This indicates that currently, resistance to this Bt toxin is widespread in the area, but further studies found Minnesota populations are susceptible to other Cry1 Bt proteins. Luckily, almost all the traits in the market have multiple Bt proteins. Again, The Handy Bt trait table provides guidance on which Bt proteins are still effective for various corn insect pests, including European corn borer and stalk borer.
True armyworm adults migrate to Minnesota and lay eggs in grassy areas. Areas with grass weeds or grass cover crops are at particular risk for egg-laying females. Because they are migratory, True army worm populations are difficult to predict.
Armyworm larvae are most active at night and other low-light periods. The larval stage lasts around 30 days depending on temperature. Most vegetation is consumed during the last two instars and as a result, infestations often go unnoticed until it’s too late. Scouting and monitoring for this pest is important, and growers can also see the location of True armyworm captures on this webpage, along with black cutworm monitoring. This monitoring effort is supported by the Minnesota Corn Growers Association.
There is no measurable yield loss at 250 aphids per plant, and this threshold is recommended through R5. It is possible that aphids can cause yield loss in early R6, but that late in the season there are risks that plants are no longer going to benefit from an insecticide application and therefore would not pay off financially. Additionally, there is potential for coming up against pre-harvest intervals at that point in the season.
Another thing to consider on top of wiping out natural enemies is insecticide resistance. The more pesticide applications are made, the more pressure that is applied to aphid populations for the development of resistance. A grower might be tempted to mix an insecticide with a fungicide application, or use an inexpensive insecticide even when low numbers of aphids are present as an insurance spray of sorts. However, several of the cheap insecticides that growers think about using are pyrethroids, and there is widespread resistance to pyrethroids in soybean aphids. In the case of spraying a field that has aphids with pyrethroid resistance, a pyrethroid is unlikely to kill the aphids, but will kill off the natural enemies.
As an example of this, some field experiments have shown that plots treated with pyrethroids, or even some of the mixtures of pyrethroids with neonicotinoids, can end up with more aphids than untreated plots. This is likely a scenario of resurgence of the pest population, where natural enemies were killed and the aphids could then have a free for all. There are some products that are more selective and have fewer effects on natural enemies.
Making management decisions to try to prevent resistance can include thoughtfully using seed treatments as well. Neonicotinoid seed treatments can sometimes provide some benefit for soybean aphid. The question is, when are the aphids coming in, how much of that insecticide remains in the plants? If certain fields are historically colonized early by aphids and the insecticide concentrations are still high in the plants, the insecticide can have an effect on those aphid populations and may provide a return on investment. However, in most cases aphids are coming in to fields after that insecticide concentration has diminished to ineffective levels in the plants. Furthermore, if a soybean field has a neonicotinoid seed treatment and then later receives a foliar application of a neonicotinoid, that can be risky in terms of resistance pressure. Soybean aphids, like all aphids during the growing season, are reproducing clonally. That means that the genetic stock of aphids out in the field is likely getting exposed to the same group of insecticide multiple times, probably consecutively, which is a recipe for the potential development of resistance.
One way to help manage resistance is to vary insecticide groups. Currently, the organophosphate insecticide, chlorpyrifos, is available in some products. There has been a regulatory roller coaster with chlorpyrifos, but some products with chlorpyrifos are available, and those are effective against aphids and spider mites.
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 feeding on corn silks. |
Fei Yang, Extension Corn Entomologist and Bob Koch, Extension Soybean Entomologist joined moderator Liz Stahl, Extension educator - crops, to discuss insect updates and management during the August 6 session of Strategic Farming: Field Notes.
Insect pests in corn
Corn rootworm complex
Both Western and Northern corn rootworms are present in Minnesota and come with unique management challenges. Larvae typically begin to hatch from overwintering eggs during May to June and then go through three instars during which they feed on corn roots for 4-6 weeks. Larvae feed on the roots of corn and adult corn rootworm beetles feed on corn silks and ears.Northern corn rootworm eggs are more tolerant to cold temperatures than Westerns and are predominant in northern Minnesota. Though growers traditionally think of corn rootworm as an issue in continuous corn, a portion of Northern corn rootworm has the extended diapause genetic trait, allowing eggs to delay hatching over two winters, with some eggs exhibiting delayed hatching of three or four winters. This extended diapause trait allows Northerns to be a significant pest in both continuous corn and rotated corn fields.
So far in 2025, the issue of extended diapause appears to have resurfaced in Minnesota, with numerous reports of damage in first-year (rotated) corn fields. Currently, there are no specific data indicating what percentage of Minnesota populations have extended diapause exactly, but based on studies in other states, it is possible that approximately fifty percent of eggs hatch the first year and then the other fifty percent may hatch in following growing seasons over the next two to four years.
The good news is that we haven’t found Northern corn rootworm populations that have resistance to Bt proteins, meaning that Bt technology is still effective for management of Northern corn rootworm. From a management perspective, if a grower has Northern corn rootworm populations with extended diapause and has a one year crop rotation, using Bt traits can help effectively manage risks. Growers that have a longer and more varied crop rotation likely won’t have to rely on Bt traits.
Unfortunately, it’s a different story with Western corn rootworm. While there are not extended diapause issues, there are problems with resistance to some Bt proteins. In Martin County, MN there are populations of Western corn rootworm with resistance to multiple Bt proteins. The Handy Bt trait table provides guidance on which Bt proteins are still effective for various corn insect pests.
European corn borer and common stalk borer
Common stalk borer was observed in several Bt corn fields expressing Cry Bt proteins in southern Minnesota this growing season. At first, European corn borer was suspected, but splitting the stalks revealed that common stalk borer was the culprit. The damage caused by European corn borer and common stalk borer look the same, so growers should be sure to scout thoroughly and split some stalks to determine which insect is causing issues.Notable damage was observed along the edges of Bt corn fields, with relatively high numbers of larvae present in the stalk of the plants. This unexpected damage on these Bt corn plants may suggest some common stalk borer populations in southern Minnesota may have developed resistance to Bt traits. However, common stalk borers typically hatch and feed on the grass before moving into corn fields. Further investigation is needed to determine whether the survival of larvae in these Bt corn plants is due to large larvae movement or indeed resistance to Bt proteins.
As for resistance in European corn borer, some recent lab research confirmed that Cry2Ab2 Bt resistance alleles in European corn borer populations are present in multiple locations in Minnesota and Wisconsin. This indicates that currently, resistance to this Bt toxin is widespread in the area, but further studies found Minnesota populations are susceptible to other Cry1 Bt proteins. Luckily, almost all the traits in the market have multiple Bt proteins. Again, The Handy Bt trait table provides guidance on which Bt proteins are still effective for various corn insect pests, including European corn borer and stalk borer.
True armyworm
Recently, several growers have been experiencing problems with true armyworm, especially in wheat fields in western Minnesota. There appear to have been multiple later flights of true armyworm moths in that area.True armyworm adults migrate to Minnesota and lay eggs in grassy areas. Areas with grass weeds or grass cover crops are at particular risk for egg-laying females. Because they are migratory, True army worm populations are difficult to predict.
Armyworm larvae are most active at night and other low-light periods. The larval stage lasts around 30 days depending on temperature. Most vegetation is consumed during the last two instars and as a result, infestations often go unnoticed until it’s too late. Scouting and monitoring for this pest is important, and growers can also see the location of True armyworm captures on this webpage, along with black cutworm monitoring. This monitoring effort is supported by the Minnesota Corn Growers Association.
Insect pests in soybean
Soybean aphid populations this year
Soybean aphid remains our most important insect pest across Minnesota and in neighboring states. It's been kind of a slow year for soybean aphid (so far), with generally low populations of about ten to twenty aphids per plant on average in most fields. Some fields are getting a little higher with around 100 aphids per plant, which is still below the economic threshold, and those are fairly rare occurrences at this point. The weather has likely played a role in these low aphid populations; strong rainfalls and high temperatures do not favor aphids.Scouting for aphids
Scouting for aphids is important, and it is recommended to continue scouting regularly through August to make management decisions, especially as weather forecasts indicate temperatures in the low to mid-eighties, which will favor aphid reproduction. Aphids can also redistribute within or between fields in this August timeframe, so scouting should definitely continue. The economic threshold for soybean aphid is 250 aphids per plant, with 80% or more of the plants infested and with aphid populations increasing. This threshold is not when damage occurs to the plant, but indicates that the aphids will likely continue to increase and reach economically damaging levels; it is the trigger point when an insecticide application should be lined up.There is no measurable yield loss at 250 aphids per plant, and this threshold is recommended through R5. It is possible that aphids can cause yield loss in early R6, but that late in the season there are risks that plants are no longer going to benefit from an insecticide application and therefore would not pay off financially. Additionally, there is potential for coming up against pre-harvest intervals at that point in the season.
Insecticide applications and natural enemies
At low aphid populations, it is likely that an effective insecticide will take care of the aphids that are there, but trying to control these low-level populations should not be your goal. There are a lot of different factors in the field, including natural enemies like predators, parasitoids, and fungal diseases that can help keep the aphid populations in check, meaning that not all infestations will outbreak and reach damaging levels. Furthermore, many insecticides (especially pyrethroids and organophosphates) will kill off natural enemies which can allow any surviving or newly colonizing aphids to thrive.Another thing to consider on top of wiping out natural enemies is insecticide resistance. The more pesticide applications are made, the more pressure that is applied to aphid populations for the development of resistance. A grower might be tempted to mix an insecticide with a fungicide application, or use an inexpensive insecticide even when low numbers of aphids are present as an insurance spray of sorts. However, several of the cheap insecticides that growers think about using are pyrethroids, and there is widespread resistance to pyrethroids in soybean aphids. In the case of spraying a field that has aphids with pyrethroid resistance, a pyrethroid is unlikely to kill the aphids, but will kill off the natural enemies.
As an example of this, some field experiments have shown that plots treated with pyrethroids, or even some of the mixtures of pyrethroids with neonicotinoids, can end up with more aphids than untreated plots. This is likely a scenario of resurgence of the pest population, where natural enemies were killed and the aphids could then have a free for all. There are some products that are more selective and have fewer effects on natural enemies.
Resistance management
There are insecticide options for resistant soybean aphid, but these tools should be used thoughtfully as well.Making management decisions to try to prevent resistance can include thoughtfully using seed treatments as well. Neonicotinoid seed treatments can sometimes provide some benefit for soybean aphid. The question is, when are the aphids coming in, how much of that insecticide remains in the plants? If certain fields are historically colonized early by aphids and the insecticide concentrations are still high in the plants, the insecticide can have an effect on those aphid populations and may provide a return on investment. However, in most cases aphids are coming in to fields after that insecticide concentration has diminished to ineffective levels in the plants. Furthermore, if a soybean field has a neonicotinoid seed treatment and then later receives a foliar application of a neonicotinoid, that can be risky in terms of resistance pressure. Soybean aphids, like all aphids during the growing season, are reproducing clonally. That means that the genetic stock of aphids out in the field is likely getting exposed to the same group of insecticide multiple times, probably consecutively, which is a recipe for the potential development of resistance.
One way to help manage resistance is to vary insecticide groups. Currently, the organophosphate insecticide, chlorpyrifos, is available in some products. There has been a regulatory roller coaster with chlorpyrifos, but some products with chlorpyrifos are available, and those are effective against aphids and spider mites.
Soybean gall midge updates
Soybean gall midge has been found in a few counties around the Twin Cities where they had not been previously observed; Ramsey County, Chisago County and Dakota County. Growers should be on the lookout for symptoms of soybean gall midge, such as soybean plants wilting and dying on the edges of the field. If those plants have dark lesions present at the base of the stem, peel back those outer stem tissues to look for the bright-orange colored larvae of soybean gall midge. Growers are asked to please contact Bob Koch with new findings of soybean gall midge.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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