Post-Season Monitoring of Corn Rootworm

Corn rootworm damage can shave precious bushels off your corn yields and turn harvest into a nightmare. There is still time to walk through your fields to check for rootworm damage to corn plants.

Corn plants that are lodged or appear to be "goosenecked" often point to corn rootworm damage. Pull out some of these plants and look for pruned and tunnelled roots to confirmed rootworm damage. Roots from harvested fields can also be pulled and examined. Checking for "goosenecking" and root damage does not provide you with threshold numbers, but will give you the opportunity to assess your fields for potential rootworm damage next year.

Life Cycle

Corn rootworm damage is cause by larvae feeding on and within the roots from mid-June to mid-July. The adults then emerge from the soil in late-July and deposit eggs in the soil until a killing frost in the fall. The eggs overwinter and hatching begins in early June, continuing the cycle.

Fields At Risk

Corn fields at risk of corn rootworm damage next year are fields which:

• were in corn this year,
• are a heavy soil type (clay),
• had more than one rootworm beetle per corn plant in late August, and/or
• had "goosenecked" plants with pruned and tunnelled roots.

Damage In First Year Corn

Rootworm damage has been observed in first year cornfields in some U.S. States, including parts of Michigan and Ohio. Beginning in the early 1990’s, in areas where a strict corn-soybean rotation was followed, a selection was made for a new variant of western corn rootworm (WCR) that lays at least a portion of its eggs in soybean fields. With the strict corn-soybean rotation, larvae hatching from eggs laid in soybean fields emerge in first year corn fields, where they can cause root injury, lodging, and yield reduction. In affected areas, monitoring procedures have been developed for WCR in soybean fields to determine the need for a soil insecticide in first-year corn. The monitoring usually involves the use of Pherocon® AM yellow sticky traps to determine the presence and density of western corn rootworms in soybeans fields during the month of August.

WCR Variant Not Detected Yet In Ontario

To date, similar monitoring in parts Ontario has not detected the presence of the new western corn rootworm variant. After investigating a few isolated cases of lodging in first year cornfields, Dr. Art Schaafsma, of the University of Guelph, Ridgetown, states that "there is no convincing evidence that the western corn rootworm variant is in Ontario". With the absence of the variant, crop rotation remains our best control strategy for corn rootworm.

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