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Corn rootworm resistance in Ont. fields

Corn rootworm resistance in Ont. fields

Producers growing continuous corn should watch for adult beetles, lodging and root clipping

By Jackie Clark
Staff Writer

Pest specialists are concerned about Bt resistance in Ontario fields due to increased corn rootworm populations.

“Last year, we started seeing numbers building. It’s just been ideal conditions for (corn rootworms) to do well,” Tracey Baute, field crop entomologist for the Ontario Ministry of Agriculture, Food and Rural Affairs, told

The corn rootworm “is a beetle that lays it’s eggs in the soil this time of year. Those eggs overwinter and come out as larvae in late spring or early summer. The larvae feed almost exclusively on corn roots.”

The larvae “clip the roots and the plant can’t stabilize as well. It (reduces movement of) nutrients to the plant, and (corn stalks) can even fall down. Especially if you have wind events, you can see significant lodging,” Baute explained.

Farmers have tried several options to protect their corn from this damage.  

“Historically, we used to protect (our crops) using granular insecticides, which were tiny beads of insecticides that would be delivered into the soil right at the root zone,” Baute said. “But (this insecticide) would only exclusively protect the roots, it wouldn’t knock the (corn rootworm) population back. Then we moved to seed treatments, but they really only work on a low to moderate dose.”

Then, farmers started using Bt (Bacillus thuringiensis) technology. It was similarly effective at the same low to moderate dose. So “there was some (rootworm) survival and clipping is expected … but at least we protected (our corn) as best as we could,” she added.

However, that survival, as well as ideal weather conditions for the proliferation of this pest, may have led to a buildup of resistance in corn rootworm populations.

Ontario has had “mild winters, (so rootworm eggs) can overwinter well, (and) decent springs where the larvae aren’t really challenged when they’re hatching,” Baute explained. “We’re planting (corn) quite early too, which gives the larvae good time to feed.”

Spring and early summer heat helps the larvae emerge and grow quickly. Producers, especially those who have fields with continuous corn, should scout for corn rootworm.

“In general, the adult beetles are very abundant this year in any field,” Baute said. Growers generally don’t need to spray for the adults, but they indicate that many eggs will be laid this year.

“In a lot of cases, the same fields have used the same Bt trait (repeatedly) year after year. So, in those scenarios, you’re seeing a lot of adults,” she explained.

Corn reps or extension officers can collect beetles to send to the lab to test for resistance, Baute added. However, this testing takes awhile to complete and get results.

An additional challenge is that “there are not a lot of easy ways to exchange information across companies,” she said. So, addressing corn rootworm “has to be done just assuming that resistance likely exists.”

For the best chances at reducing the population, “we really need to try and take away corn in that system – not just within the field of concern but nearby fields too, because these beetles can fly,” Baute said. “The beetles tend to lay their eggs where corn grows year after year. … Without the corn, the larvae won’t survive and populations significantly drop.”

This need to shift crop rotations presents a challenge for livestock producers with high corn needs.

“We’re working hard to try to come up with the best options that can help knock down corn rootworm populations but still give (farmers) alternative feed options,” Baute said.

If the industry does not address this resistance, Bt will no longer be effective against corn rootworm and producers will constantly have to battle the pest.

“We can look to the U.S.,” Baute said. Corn growers south of the border have battled corn rootworm Bt resistance since about 2009.

“They’ve been really challenged with trying to find solutions. … We have an opportunity here in Canada to do our best and not end up with the same results as in the United States,” she added.

Zoran Zeremski\iStock\Getty Image Plus photo


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