Scouting corn fields for insect infestation is good practice. Caught early increases the chances of saving the crop.
This is the second article in a series of Strategic Farming 2023 webinars hosted by the University of Minnesota Extension. Next: Let’s talk about climate factors and nitrogen management.
During the second webinar in a series hosted by the University of Minnesota Extension, Integrated Pest Management (IPM) Instructor Anthony Hanson and Extension IPM Specialist Bruce Potter spoke about corn insects.
Hanson said while the European corn borer hasn’t been too much of a problem in recent decades, the insect remains a problem for corn producers. It’s also an insect whose existence the University of Minnesota Extension monitors by conducting surveys every fall — a practice that has continued for more than 20 years.
The study will be conducted by collecting samples from two randomly selected fields of known non-Bacillus thuringiensis (Bt) in each county from September to October.
“Basically we go through the fields and take at least 10 plants per field, do stick dissections to see if we can see any feeding damage from the larvae in there,” Hanson said.
In addition, Hanson says, they would also look at the number of larvae found per stalk and the length of the tunnels they bore.
Looking at historical data from wintering corn borers, Hanson said numbers were fairly high in the 1990s and peaked in the mid-1990s at nearly 400 larvae per 100 plants. However, in 1996, when corn and cotton producers became available to insect-resistant crops containing genes from the soil bacterium Bt, the number of overwintering corn borers declined.
Hanson said the 2022 data received for 2021 showed little corn borer.
“We’ve definitely had trouble finding corn borers, which is a good thing for growers, but we know they’re out there in the background. We can only do a limited amount of surveying at any given time,” he said.
During the webinar, Hanson showed a variety of slides produced by Potter. One of the slides showed European corn borer population data based on stalk cuttings of field corn in Minnesota in 2021. While most of Minnesota has been shown to have no known presence of the noxious insect, a low number of about one to ten tunnels per 100 plants have been found. Affected counties included Crow Wing, Morrison, Benton, Stearns, Todd, Pope, Douglas, Wadena, Otter Tail, Becker and Clearwater. Some counties were hit harder than others. These levels are also generally treatable, Hanson said.
“We only get an average of what we expect for corn borers with these colors. It’s not an indication that you need to go for treatment next year, but these do indicate that you are near an area where corn borers have been found. It’s definitely worth going on Scout next year,” said Hanson.
Survey of known non-Bt fields in 2022 showed the presence of corn borers in two centralized areas. Areas in Morrison, Todd, Wadena, Becker, Otter Tail, Benton, Stearns, Sherburne, Wright, Meeker, Kandiyohi, Chippewa, Swift, Pope and Douglas counties had levels of one to ten tunnels per 100 plants. Some areas had 10-25 tunnels per 100 plants.
The other corn borer area was found in southern Minnesota. These counties included Nobles, Jackson, Martin, Faribault, Freeborn, Murry, Cottonwood, Watonwan, Blue Earth, Brown and Nicollet counties. While some areas reflected 1 to 10 tunnels per 100 plants, most of the area reflected levels of 10-25 tunnels per 100 plants.
Hanson said while they have access to multiple non-Bt fields to sample, the expansion is always looking for more.
The corn borer has several generations each year. The first generation scouting window will be corn planted early.
“In that case, your insecticide may be your best option to control it, and you may have the greatest potential for damage,” Hanson said.
The same goes for the second generation when corn was planted late, Hanson.
Later in the season, growers run the risk of encountering the univoltin biotype, which occurs closer to pollination. It may also overlap with the second generation, Hanson said.
What makes corn borer control difficult, Hanson said, is that the pest resides in the corn stalks. What type of insecticide is used to attack the insect makes a difference.
“If you’re using insecticides, you need to be able to either hit larvae, or if you’re looking for egg masses, they’re outside of the plant. Once those larvae move up the stem, you basically have no efficiency with your insecticides in that case,” Hanson said.
In addition to discussing the European corn borer, Hanson and Potter talked about the real armyworm.
“That was a problem last year, especially if someone had a cover crop with rye,” Hanson said.
While true armyworms cannot overwinter in Minnesota, the moths migrate to the state each spring. Because they prefer rich grasses where they lay their eggs, growers are encouraged to specifically explore their fields with small grains as well as their grass field boundaries where corn and soybeans are planted, Hanson said.
“What happens is, if you have grasses, particularly dense, or if you have a cover crop of rye, for example, they fly in to lay their eggs, the larvae hatch and start eating, and if you sometimes get your rye a little late finish , they have bigger stages, basically more mature larvae, they can survive. When that rye died, they are now looking for something else to eat. I’ve seen cases where in Stearns County we’ve had entire fields of soybeans that were essentially dead from feeding,” Hanson said.
Though he hasn’t seen entire crop fields die from true armyworms recently, Hanson warns growers who use rye as a cover crop.
“Just remember, if you use rye you can be susceptible to armyworm infestations and whatever else is grown out there. Keep your eyes peeled, look for it,” he said.
Hanson said the earlier an infestation is detected, the greater the chance of saving the crop.
Another corn insect breeder of concern is the northern corn rootworm and western corn rootworm. As the name suggests, Hanson said the insect can affect certain areas more than others.
“That’s a problem with continuous and twisted corn,” Hanson said.
To some extent, northern corn rootworm populations appear to be increasing in northwestern Minnesota. It is also more mobile than the western corn rootworm. In addition, Hanson said, the eggs tolerate cold weather, have demonstrated resistance to Bt, and have prolonged diapause.
While the western corn rootworm is a threat to continuous corn, the variant has not been documented in Minnesota, nor is it an extended diapause. It is also resistant to insecticides, Bt traits and Bt pyramids.
Hanson said that as the insect’s name suggests, the larvae feed on the roots and ultimately damage the plant. Injured roots also affect the plants’ ability to take up water and nutrients, Potter said.
“So if you have a root injury we tend to see those yield impacts worse when it’s dry when you have nutrient and water uptake issues. I should also mention that both phosphorus and nitrogen are quite important for this corn crop in this early season growth,” Potter said.
Some risk factors for resistant corn rootworm are high beetle populations, long-term continuous corn, concentration of continuous corn fields, early or late silking, including voluntary corn, and long-term use of the same management practice in area fields, Hanson said.
