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Captures of Black Cutworm Moths Increase Dramatically

April 19, 2002
The weather front that brought significant rains into Illinois during the weekend of April 12-14 also brought in a horde of black cutworm adults. Many people reported dramatic increases in their captures of black cutworm males in pheromone traps. Higher numbers of black cutworm moths captured in pheromone traps do not always result in subsequent heavy infestations of black cutworm larvae. However, the arrival of the moths in Illinois before much of the corn has been planted should place people on alert for potential problems when corn begins to emerge. We should be able to offer some predictions about the first signs of cutting activity in next week's issue of the Bulletin.

Ron Hines, senior research specialist at the Dixon Springs Agricultural Center, reported the following numbers of black cutworm males captured in traps at his four sites in southern Illinois:

· Massac County--18 moths captured on April 14

· Pope County (upland site)--4 moths captured on April 13, 20 moths captured on April 14

· Pope County (bottomland site)--8 moths captured on April 14

· Pulaski County--22 moths captured on April 13, 13 moths captured on April 14

Matt Montgomery, Sangamon/Menard Extension unit educator in crop systems, reported that Tom Harms and Jerry Harbour, with Lincoln Land FS, had captured very high numbers of black cutworm moths. A handful of phone callers from southern and central counties also phoned in "intense captures" (nine or more moths captured over a 1- to 2-day period) but did not provide any numbers.

Traps in northern Illinois also began to capture black cutworm moths, although the numbers were not high. Kevin Foreman, with Crop Production Services in Galesburg, captured two black cutworm males in his trap near Fairview in Fulton County on April 12. Jim Morrison, Extension educator in crop systems at the Rockford Extension Center, captured his first black cutworm moth on April 15 in Stephenson County.

So, black cutworm adults have been captured, from extreme southern to extreme northern Illinois, and corn planting has barely begun. Keep in mind that corn has been planted very early during the past couple of years, so the arrival of black cutworm moths before corn planting occurs is not unusual. But further delays of corn planting, coupled with continued captures of black cutworm moths, will begin to make people nervous.

In 2001, we projected that cutting of corn seedlings could have begun as early as April 20 in southern Illinois and the first week of May in central Illinois. The projection for southern Illinois last year was based on an intense capture on April 3. Intense captures of black cutworm moths this year occurred as early as March 30 and 31. Overall the dates of intense captures in 2001 and 2002 are equivalent. However, the patterns of temperatures on the days after the captures in 2001 and 2002 have been different, so projected cutting dates may not be equivalent. So, all we can suggest right now is that any grower who has planted corn already should watch emerging seedlings carefully for early signs of cutworm feeding (pinholes in the leaves) and for plants that have been cut off by larger larvae.

If you are planning to scout for cutworm larvae, you should focus first on "high-risk" fields, primarily fields or areas of fields in which early-season weeds were growing at the time moths flew into the area. If tillage or herbicides eliminate weeds 1 to 2 weeks before planting, any black cutworms that had been present probably starve to death. The presence of weeds only a few days before planting increases the likelihood of cutworm damage if larvae are present in the field.

Check fields for leaf feeding, cutting, wilting, and missing plants every 3 to 4 days when "cutworm season" begins. You may miss significant feeding damage if you scout only once per week, especially if temperatures have been high and cutworm larvae are developing rapidly. You should plan to examine a minimum of 250 plants (50 plants in each of five locations) in a field. When injured plants are found, dig around the bases of the plants to look for live cutworms. When you find cutworms, determine the average instar (stage of larval development) of a sample of cutworms 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 deg 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. Figure 1 shows head-capsule widths for fourth-, fifth-, sixth-, and seventh-instar cutworms, as well as approximate days left to feed and potential number of plants that may be cut. To use the head-capsule gauge, grasp a cutworm larva tightly behind the head and squeeze to force the head forward. (Squeeze gently, folks; you don't want to pop the head off like a champagne cork.) 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 two black cutworm larvae.

Rescue treatments for control of black cutworms were listed in Table 3 in last week's Bulletin (issue no. 3, April 12, 2002). We will provide some suggestions about soil conditions and control of cutworms in a future issue of the Bulletin. In the meantime, growers can concentrate on getting corn in the ground first, and worrying about cutworms after.--Kevin Steffey

Author: Kevin Steffey

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

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