ONTARIO GRAIN FARMER AGRONOMY 39 Smith says the challenge is that some growers continue to grow cornon-corn, especially in livestock-producing areas. And that leads to ongoing corn rootworm pressure. “In years like 2025, when we had ideal weather conditions for heavy corn rootworm populations, the pest was spreading to surrounding fields, leading to new infections and potential problems for this year’s corn fields,” she says. Unfortunately, Bt corn, the other common go-to tool for managing corn rootworm, is quickly losing efficacy, says Smith. She notes that 60 to 70 per cent of Ontario corn acres are planted with Bt hybrids to control rootworm. However, few of these fields actually require the trait to manage the pest because it’s either not present, or the pest pressure isn’t high enough. “So, besides overusing corn rootworm traits and contributing to the rising rate of Bt resistance, growers are also paying for something they often don’t need,” Smith says. Soil-applied insecticides, both liquid and granular, are also effective for controlling corn rootworm and can be used in rotation with Bt corn, or in situations where resistance is an issue. STAY AHEAD OF CORN ROOTWORM “Integrated Pest Management strategies can sometimes take a back seat to higher yielding practices and hybrid selection,” says Tracey Baute, entomologist, field crops, Ontario Ministry of Agriculture, Food and Agribusiness. “Corn rootworm management can start with seed selection, but it should be a long-term strategy that incorporates tools like crop rotation and soil insecticides when needed.” Livestock-dense counties, including Huron, Perth, Middlesex and Oxford, are among those dealing with the heaviest corn rootworm pressure, but Bt resistance can be found throughout the province. Baute has been tracking the pest for five years, as part of a monitoring program with the Canadian Corn Pest Coalition and Corn Rootworm Trap Network. Using a network of sticky traps, she’s able to count and monitor the activity of adult corn rootworm beetles in fields in Ontario and compare it with populations across North America. Through this network, she’s discovered that Ontario corn rootworm populations are now equivalent to those in Iowa, where the pest is a significant economic issue for growers. Baute notes that resistant corn rootworm has spread to Quebec and the Maritimes in recent years too. “Resistance doesn’t go away,” she says. Growers dealing with Bt resistant populations need to change their pest management strategy, Baute says. Growers trying to understand the risk level for next year’s crop and determine if a control measure will be needed should consider scouting. Baute says fields should be scouted once a week in August to count the number of adults present in the ear zone (one leaf below and above the ear node). If the total adult count is on average less than one western corn rootworm or two northern corn rootworm adults per plant, then the field will not need protection from rootworm the following spring. Photo: Eric Burkness, Bugwood.org Baute also recommends trapping, and using the sticky traps, to monitor adult activity in continuous corn fields to determine potential resistance development and to assess the effectiveness of a control measure. “Scouting can help inform crop management decisions, and check if they are actually working,” she says. NEW TOOLS, SAME STRATEGY And while no silver bullet exists to control corn rootworm, RNA interference (RNAi) technology released in Ontario in 2025 as a new mode of action for protection, offers some hope. Smith predicts this new tool will slowly disrupt pest pressure, but warns growers against overusing this new technology. “RNAi is another tool that can be used in rotation with our other existing solutions, but corn rootworm could still develop resistance to this too, and we have to protect it,” she says. The use of entomopathogenic nematodes (naturally occurring and beneficial nematodes) as biological controls are also being tested in Smith’s lab. These non-plant parasitic nematodes feed on the host pest and can be effective in controlling corn rootworm. Research into using these nematodes as a biological control is still underway, but Smith says results look promising. "Corn rootworm is notorious for developing resistance to whatever we use against it," Smith warns. "There are new tools available, but the most effective strategy will always be rotating control measures, planning ahead, and avoid relying on a single approach." •
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