Time for 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 lodged or appeared "goosenecked" often point to corn rootworm damage earlier in the season. 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 caused by larvae feeding on and within the roots of the corn plant 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 occurs in the fall. The eggs overwinter and hatching begins in early June the next year, 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, an unintentional selection was made for a new variant of western corn rootworm (WCR). The variant population 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 rootworm insecticide or use of rootworm Bt corn in first-year corn. Monitoring for the variant population usually involves the use of Pherocon® AM yellow sticky traps to determine the presence and density of western corn rootworms adults in soybeans fields during the month of August.

WCR Variant Not Detected Yet In Ontario

Low populations of CRW adults can nearly always be found in soybean fields adjacent to corn fields. However, to date in Ontario, field monitoring has failed to detect CRW variant at economic thresholds. After investigating a few isolated cases of lodging in first year corn fields, Dr. Art Schaafsma, of the University of Guelph, Ridgetown, states that "there is no evidence that the western corn rootworm variant is present at economic levels in Ontario". In the absence of the CRW variant, crop rotation remains our best control strategy for corn rootworm.

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