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Cutworms Continue Their Move Across Illinois

April 24, 2003

With help from last week's southerly winds, black cutworm moths are now being found in pheromone traps more frequently across the state. Reports of intense captures are increasing, and moths are being observed for the first time in many counties. Counties reporting moth catches for the first time include Dekalb, Grundy, Lee, Ogle, Piatt, Stephenson, Whiteside, Will, and Winnebago. Intense captures (nine or more moths captured in a 1- to 2-day period) have been reported in several counties:

Matt Montgomery, Sangamon/Menard Extension unit educator in crop systems, is working with several cooperators in the Sangamon/Menard area who reported intense captures last week, including Jeff Harbour (20 moths on April 16), Jeff Libe (12 moths on April 16 and 17), Matt Dambacher (12 moths on April 17), and Vito Stallone (10 moths on April 19).

Sean Evans, Extension educator in crop systems at the Macomb Extension Center, captured 9 and 15 moths on April 17 and 18, respectively, in his trap in Macomb.

Marc Rigg, with the Good Hope Pioneer Production Plant, captured 9 and 12 moths on April 16-17 and April 20, respectively, in Mason County.

Mike Christenson, Stephenson Service Company, reported 11 moths on April 20.

Jeff Staley, Wabash Valley FS, continues to report intense captures in Gallatin County. Ron Hines reports in the Hines Report ( the presence of black cutworm moths in Massac, Pope, and Pulaski counties, although no intense captures occurred last week.

We can predict the first dates of larval cutting activity based on the dates of intense captures (Table 1). These dates suggest when cutting may begin to occur and should be used as a guideline for scouting cornfields.

When scouting for cutworms, check fields for leaf feeding, cutting, wilting, and missing plants every 3 to 4 days. Feeding damage may be missed if you scout only once per week, especially if temperatures have been high and cutworm larvae are developing rapidly. Examine a minimum of 250 plants (50 plants in each of 5 locations) in a field. When injured plants are found, dig around the bases of the plants to look for live cutworms. Several species of cutworms feed on corn. Pictures of their larvae may be found in Integrated Crop Management with photos by Marlin Rice (Extension entomologist at Iowa State University); ( 2000/5-8-2000 cutworm2000. html).

Pin hole feeding caused by early instar black cutworm.

When you find cutworms, determine the average instar (stage of larval development) of a sample to estimate how much longer the larvae will feed. For example, if most of the cutworms are fourth instars, the larvae will feed for approximately 25 days if the average temperature is 70°F. The best way to determine the instar of a black cutworm larva is to use a head-capsule gauge. The width of a cutworm's head capsule increases as it molts from one instar to the next. To use the head-capsule gauge, grasp a cutworm larva tightly behind the head and squeeze (gently) to force the head forward. Hold the head flat against the gauge, first at the top of the scale (fourth instar). Move the head down the scale until the width of the head matches the width of the bar. The number corresponding to that bar is the instar of the cutworm. Based on the instar, you can determine the approximate days left to feed and the potential number of 1-leaf, 2-leaf, or 4-leaf plants that will be cut. Cutworm larvae will cut more 1-leaf-stage plants than 4-leaf-stage plants.

Head capsule gauge and black cutworm larvae.

Black cutworm larvae next to cut corn plant.

As the corn planting season continues, black cutworm season will also get under way. Please keep us posted on any findings.--Kelly Cook

Author: Kelly Cook

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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