There’s another reason not to grow corn on corn in Manitoba: corn rootworm.
The insect, which as its name implies, feeds on corn roots and is a major pest in the American Corn Belt, is showing up in greater numbers in Manitoba. But Manitoba Agriculture entomologist John Gavloski says there’s an easy fix: crop rotation.
“Even one year (without corn) should kill them,” Gavloski said in an interview Sept. 22. “When they are a small larva they need to feed within a few days. If there are no corn roots there, they are dead within a few days.”
The Manitoba Corn Growers Association’s most recent newsletter warns farmers to be on the lookout for the pest.
“The yield penalty you can get from infestations is greater than any producer wants to deal with and it is such an easy pest to scout for it’s just not worth overlooking,” wrote MCGA field agronomist Morgan Cott.
Last year corn rootworms were found in a continuous cornfield near Souris.
“This year we started looking a little bit harder and we did find some near Morden and near Winkler and in fields where corn has been growing several years in a row,” Gavloski said. “I think we probably weren’t noticing it previously. Maybe they were here, but we just weren’t picking it up. But now that we have been looking for it specifically you can, in some of these fields, find them quite easily.”
The best time to scout for corn rootworm is when corn is silking in late summer, Gavloski said. Adult corn rootworm beetles feed on corn pollen and can be seen on cobs and leaves as well.
“Adult rootworm beetles will be jumping all over the place once they sense your movement… ” Cott wrote. “… we don’t have any economic thresholds in Manitoba since this is such a new pest to us.”
It’s not the adult, which can vary in colour from lime green to yellow, that damages corn plants, it’s the larva.
Corn rootworms overwinter in fields as eggs. They hatch in spring and larvae feed on corn roots. The feeding can affect nutrient uptake, but the main issue is with fewer roots corn is more susceptible to lodging, especially if there are strong winds.
“The (lodged) plants will continue to grow and the plant will straighten itself,” Gavloski said. “You get what we call this ‘goosenecking’ effect. You do eventually get a straight cornstalk but closer to the ground you get a curve in it. When you get a lot of plants like that it can really make it difficult for harvesting.”
There is a variety of silage corm that’s resistant to corn rootworm thanks to a Bt (Bacillus thuringiensis) gene, but it’s different than varieties with the Bt to control European corn borers. Some varieties have both traits.
“But every time you add technology into the genetics you are adding extra cost,” Gavloski said. “It is probably overkill for Manitoba.”
And with repeated use, resistance genes can be overcome.
“It hasn’t been a really serious problem here in Manitoba,” Gavloski said. “Throughout the Corn Belt of the U.S. it is one of the major problems. In other areas in North America it is a real significant issue. Other than that one instance in Manitoba where they were having trouble harvesting, it hasn’t been a huge issue, but we know it is established. That is a good reason to rotate.”
There are lots of other good reasons to rotate, including reducing the risk of other pests invading the crop, Gavloski said.