“Corn rootworm is the poster child of primary concern in continuous corn,” says Kevin Steffey, Extension entomologist at the University of Illinois. It is the main pest to be concerned with. European corn borer is another very likely threat and other corn pests, such as wireworm, seed corn maggot and white grub, thrive in continuous corn fields.
Rotating corn and soybeans was the conventional wisdom for reducing pest pressure until the development of the variant western corn rootworm that also lays eggs in soybeans, Steffey adds.
Keep in mind, however, that western corn rootworm won't be the damaging pest for everyone. Northern corn rootworm is playing a role in Minnesota fields.
.“We're concerned that the increasing acreage of continuous corn is going to exacerbate the problem in rotated corn due to extended diapause,” says Ken Ostlie, Extension entomologist for the University of Minnesota. “Not only would we be getting eggs hatched that were laid two or three years ago, we'll also get eggs hatching from the last year. We'll see the full extent of the damage in continuous corn.”
The key to controlling rootworm and other pests in continuous corn is management. Using transgenic rootworm corn for variants including northern corn rootworm should be sufficient. But, Ostlie says, it's more of an insurance approach and is being applied to more and more acres, which can lead to resistance problems.
“We've got a tendency to go overboard on the transgenics and ignore the refuge requirement,” says Ostlie. “Some growers are exceeding the 80% for transgenics; they're not only increasing their resistance risks, but increasing the risk for their whole area. They need to take a hands-on approach.”
Bt corn for control of western corn rootworm, however, may not be enough, and soil insecticides or seed treatments may need to be used, Steffey suggests.
“More people will be using various types of Bt corn to control rootworm and European corn borer,” says Steffey. “But insecticidal seed and soil treatments may be necessary in some areas to control secondary insects such as white grubs or wireworms.”
Burt Norell, south-central Minnesota farmer, regularly seems to find new pests, or something that has been there but was not a major concern. “As we control major pests, we realize how much damage is done by secondary pests,” he says.
All transgenic corn comes with seed treatments at a low rate, says Ostlie. “In most situations, this will provide adequate protection for secondary pests. Minnesota at this point has seen very few poor performances of rootworm transgenic corn.”
Continuous corn improves the chances for secondary below-ground insects such as white grubs and wireworms to build up over time, says Steffey. “They have very long life cycles and will have the opportunity to cause damage every year instead of every other year. There is little you can do to scout for them, so preventive management is key,” he says.
Though grubs can't necessarily be scouted for, it's still very important to scout your field for pests and insects.
“Scouting is continuous,” says Norell. “Spending time in your fields on a regular basis really helps, along with knowing what problems you've had in the past.”
To scout specifically for rootworm, think a year ahead — preventative scouting.
“Scout for adults one year and manage the larvae the next,” says Steffey. “Scouting for beetles in the field gives an indication of potential for larval damage the next year. With continuous corn, the most practical method is to count the number of beetles on the plants one year to make decisions about how to treat and manage the next year.”
Growers need to be aware of the rootworm pressure in their fields, says Ostlie. “Check the refuge corn to see how badly it's damaged, and check beetle numbers.”
What to take from all of this? Manage and scout your fields; know what's happening in them. Take the necessary steps to control insects you see, and to prevent insects the next year, but don't overtreat. Follow the guidelines and keep an adequate refuge.
“Situations of severe damage to transgenics are rare, but the fact that they are occurring has entomologists troubled,” says Ostlie. “The downside is that an adequate explanation is still lacking as to why some fields are more damaged than others.”