No. 4 Article 3/April 21, 2006

Intense Black Cutworm Flights in Central and Northern Illinois

Black cutworm flights have continued across the state this week. While slowing down in southern Illinois, flights picked up in both central and northern Illinois late last week and early this week. Over the weekend, intense captures of black cutworm moths occurred (nine or more cumulative moths caught over two consecutive days) in both Crawford and Piatt counties. Ogle and Whiteside counties both reported intense captures early this week on Monday; Coles and Shelby counties joined the list on Wednesday.

After an intense capture is recorded, we can calculate degree-days to project when black cutworm injury, specifically cutting of corn plants, will occur. Larvae are expected to begin feeding on and cutting corn plants with the accumulation of approximately 300 degree-days (base 50°F) after an intense capture. Dates of significant moth flights and projected cutting dates are listed in Table 1. If you are monitoring a pheromone trap, you can predict cutting dates in your area with the Degree-Day Calculator found on the IPM and WARM Web sites.

As planting season gets under way, we encourage growers to scout emerging cornfields in their area for black cutworm injury. Small larvae will feed on corn plants, causing small, irregular holes in the leaves. Larvae begin cutting plants when they reach the fourth instar. Feeding primarily at night, larvae move up the row as they feed. Large sections of fields may be quickly destroyed before the injury is noticed. On average, one larva may cut an average of three to four plants in its lifetime.

For years, the rule-of-thumb treatment guideline for black cutworm injury has been 3% to 5% cutting. No more recent data have been gathered to suggest that this threshold should be changed. However, you can judge the severity of damage and the possible need for an insecticide based on the location of the cut (above or below the growing point), the size of the cutworms, and the soil conditions (moist or dry). Plants cut above ground will recover, to a certain extent; plants cut below the growing point will not recover. Younger cutworms (third and fourth instars) will cut more plants over time than older cutworms (fifth and sixth instars). For more information on black cutworm injury, scouting, and management, please see our black cutworm fact sheet (Adobe PDF).--Kelly Cook

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