European Corn Borer Moth Captures Up;
Reports of Larvae in Southern Illinois

European corn borer moth captures are now commonplace throughout the state. Trina Searcy with Seneca Foods, a cooperator of Rick Weinzierl, Extension entomology specialist for fruit and vegetable insects, reported initial moth captures using a light trap near Manito, Mason County, on May 14. On May 26, 27, and 28, captures were 257, 463, and 546, respectively, per evening. Ron Hines, Dixon Springs Agricultural Center, indicated the first significant moth flight for southern Illinois started about May 15.

What makes this year's moth flight most interesting so far is the uniform flush of moths that occurred nearly statewide. Unlike many years in which a wave of moths begins in southern Illinois and moves northward, this insect season appears a little more compressed statewide. Besides the common moth captures, European corn borer larvae are being found in plants. Randy McElroy, a regional agronomist with DeKalb Genetics Corporation, found a cornfield near Grayville, Edwards County, with 90 percent of the plants infested with first instars. Larry Martin, also with DeKalb, reported a similar infestation of first- and second-instar borers in Washington County on corn (34 inches) planted in mid-April. Ron Hines has already found third-instar larvae feeding on corn plants 8 to 10 inches tall.

By using heat-unit accumulations (base 50 degrees F) from the first significant moth flight of the spring, May 15 to 22, for most of the state, a range of about 200 to 300 heat units has accumulated for northern and southern Illinois counties, respectively, through June 1. Compare Figure 2 in issue no. 9 of this Bulletin with Figure 1 of this week's issue. As Table 2 suggests, when 300 to 400 heat units have accumulated since the first capture of moths, second and third instars should be present. Take a look at Table 3 to make a rough comparison of how this year's moth captures contrast with those from previous years.

Figure 1. Actual heat-unit accumulation (base 50 degrees F), January 1 to June 1, 1998.

Table 2. Accumulated degree-days (developmental threshold of 50 degrees F) from initial capture of moths in the spring to first occurrence of life stage or activity of first-generation European corn borers (from European Corn Borer: Ecology and Management, NCR Publication No. 327, Iowa State University, Ames)

Table 3. First reports of corn borer moths in the spring, from 1988 to 1998 issues of this Bulletin

The bottom line is that the 1998 European corn borer season is well under way, and producers should begin scouting for this potentially destructive pest. This recommendation holds even for fields planted with Bt-hybrids. The following questions concerning the biology and management of the European corn borer are commonly asked by growers.

Are certain cornfields more likely to be damaged by first-generation European corn borers? Although this season will be remembered by folks in several areas of the state for significant delays in planting, and replanting, fields that were planted early relative to nearby fields are at greater risk to injury from the first generation of borers. Producers that were able to plant in late April or early May should especially monitor fields that in essence may act as a trap crop for egg-laying corn borer moths. Corn that is small, less than 18 inches in extended leaf height, is not very susceptible to corn borer injury. Larvae that feed on corn less than 18 inches tall generally fail to establish and simply wander off the plants and die. The reason for this response is the presence of a plant compound commonly referred to as DIMBOA that prevents larvae from establishing. As plants mature, the concentration of DIMBOA decreases. Larval survival is much better on corn plants that are in the mid- to late-whorl stage of development.

What exactly are "action sites?" My neighbors have heard that treating these areas with an insecticide or mowing them may prevent corn borers from damaging my corn. Is there any truth to this? Action sites are areas of dense vegetation--usually grasses--where moths spend most of their time, especially during the day. Areas that frequently serve as action sites include grassy ditch banks, fencerows, and grass waterways. As soon as moths emerge from cornfield residue during the spring, they fly to action sites because the microclimate is suitable (morning dew, necessary for drinking, is heavier). There they mate and rest. Female moths emit a sex attractant (pheromone) during the late evening hours (10 p.m. to 1 a.m.) to which male moths are very attracted. Due to the favorable microclimate and the emission of pheromone by females in these grassy areas, action sites provide a home for large numbers of moths. Treating action sites with an insecticide is not recommended. European corn borer moths are very mobile; just because you treat the grassy areas around your cornfield doesn't mean your neighbors are doing the same thing. Moths can easily fly for several miles during an evening, especially if helped along by gentle winds. Treating action sites on a farm-to-farm basis won't take the place of scouting individual fields and making management decisions based upon observations.

What are the scouting procedures for first-generation corn borer, and how many plants do I need to look at? When corn plants have an extended leaf height of 18 inches, scouting should begin in earnest. If possible, growers should examine at least 20 consecutive plants in each of five random areas for every 40 to 50 acres within a field. In very large fields, it will be more practical (although less precise) to examine 25 consecutive plants in each of five random areas for every 80 acres. It's a good idea to walk at least 100 feet into the field before sampling. If more than one variety of corn is being grown or if different planting dates occurred within the same field, it is important to consider each section as a separate field. Plants should be checked for fresh whorl-feeding damage, and the percent of infested plants calculated. For every 20 to 25 plants examined, remove the whorl leaves from two plants and check for live borers. This will enable you to estimate the average number of borers per infested plant. After the field has been scouted, you should fill out a management worksheet and make the appropriate decision (Figure 2). If a rescue treatment is warranted, the granular formulation of an insecticide is more effective than the liquid formulation when applied aerially (Table 4).

Figure 2. Management worksheet for first-generation European corn borer.

Table 4. Suggested insecticides for first-generation European corn borer in field corn

Over half the plants in my field have whorl-feeding damage, but I can't find any borers. Why is this, and should I treat the field? Whorl-feeding injury doesn't always mean that an economic loss will occur. Even when eggs are laid on corn plants at the optimal stage for larval survival (mid to late whorl), it's not easy being a corn borer. From 20 to 70 percent of newly hatched larvae fail to establish in plants due to weather-related variables such ashigh temperatures and related moisture stress. Even the larvae that make it into the whorl area are susceptible to drowning following heavy downbursts of rain or to predators, parasitoids, and diseases. If corn borers aren't found even though the leaves are shot-holed, it may mean that larval survival and establishment were very poor. It may also mean that borers have already penetrated the stalk tissue of the plant and rescue treatments are no longer a viable option. So, even if a good share of your plants have whorl-feeding damage, it is crucial to pull whorl leaves from infested plants to find out why borers aren't being found in your field.

At what stage of development do corn borers begin to penetrate into the stalk tissue? By the time corn borers reach the third-larval instar (there are five larval instars altogether), they begin boring into stalk tissue (Figure 3). At this point, rescue treatments--if needed--are no longer effective. If plants within a field have many leaves snapped or broken over at right angles along the mid-ribs, this usually signals that larvae are beginning to burrow into stalks. The larvae tunnel up and down the mid-ribs and eventually leave an exit hole that weakens the leaf at that point. For a quick overview of the biology of the corn borer, please refer to Table 5.

Figure 3. Body lengths and prothoracic shield widths for larval instars of the European corn borer.

Table 5. Biology of the European corn borer.

As folks walk through Bt-corn, maybe to assess its performance, finding a few injured plants should not be alarming. However, if live larvae are found, the seed company should be contacted. Company representatives should be able to determine whether the plant(s) on which the borers survived are expressing the endotoxin. When we have tested plants on which borers survived in Bt cornfields, we have found that plants were negative (not expressing the endotoxin).

Thus far, the Extension entomologists at U of I have been very impressed with the control provided. However, we urge anyone who has planted Bt-corn to scout and assess the performance. Let us know what you find.

Mike Gray (m-gray4@uiuc.edu) and Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217)333-6652