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First Flight of European Corn Borer Moths Well Under Way in Central Illinois: Egg Masses Detected in Southern Illinois

June 7, 2002
Mike Roegge, Crop Systems Educator, Adams and Brown Extension Unit, reported that the first flight of European corn borers is well under way in his area of the state. According to Mike, at least one local fertilizer dealer observed that so many moths were present in one field that it appeared as if it were "raining." Todd Nelson, IPM Director with Alvey Laboratory, reported on June 5 that egg masses could be found in some cornfields across southern Illinois. On Monday and Tuesday of this week (June 3-4) I spent some time in Adams County. During my trip across central and western Illinois, I noted that quite a few fields were at least knee high. Fields meeting this description are very susceptible to egg laying by this first flight of European corn borers. In addition to these fields, I observed many fields that were in the 3- to 4-leaf stage of development. Fields that were planted late due to excessive precipitation are at less risk for oviposition by corn borer moths this spring. Producers, consultants, and dealers should pay most of their attention to the early-planted fields for signs of first-generation injury later this spring. This article answers questions concerning the first generation of this perennial pest.

How did the overwintering population of corn borers stack up in 2001 compared with previous years?

Although the 2001 fall survey of European corn borers revealed that overwintering densities of larvae were below the overall long-term average (approximately 1.5 borers per plant), they were larger than the two previous years (Figure 1). In fact, the 2001 population was roughly twice as large as the densities in 1999 and 2000. So last fall we began to alert producers that the first flight of moths would most likely be larger than they had experienced in several years. For some areas of the state, this prediction is proving accurate.

What areas of the state are most at risk of economic infestations of European corn borers this spring?

The 2001 fall survey revealed that the largest densities of overwintering borers were located in northeastern, northwestern, and east-central counties (Figure 2). Surprisingly, populations of larvae were relatively low in western Illinois, an area of the state where impressive flights have been observed this spring. However, more than 50 percent of corn plants were infested with borers last year in western Illinois counties (Figure 3). As we move through the early weeks of June, producers in northern Illinois should remain increasingly vigilant for large flights of moths and potentially significant oviposition into the earliest-planted fields. Because of the planting delays experienced by many producers in east-central counties this spring, moths most likely struggled to find suitable egg-laying sites. Consequently, first-generation corn borer populations in east-central Illinois may crash this spring. As Figures 2 and 3 reveal, populations of corn borers across much of southern Illinois were very low in 2001. Because of the low overwintering densities in southern counties and the very delayed planting efforts, European corn borer first-generation populations are expected to remain low this spring.

Why are early-planted cornfields most at risk to the first generation of borers?

Corn plants that are small (less than 18 inches, extended leaf height) are less susceptible to corn borer injury. Corn borers that feed on corn less than 18 inches in height typically fail to establish. The explanation for this response is the presence of a plant compound commonly referred to as DIMBOA (2-4 dihydroxy-7-methoxy-1, 4-benzoxazin-3-one) that prevents larvae from establishing. As plants mature, the concentration of DIMBOA decreases. Larval survival is much better on corn plants that are in mid- to late-whorl stage of development.

What are corn borer "action sites"?

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, moths 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 "homes" for large numbers of moths.

Is treating an action site with an insecticide a good idea to prevent corn borers from causing problems in my cornfields?

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 assisted by mild breezes. Treating action sites on a farm-to-farm basis won't take the place of scouting individual fields and making management decisions based on observations.

What's the best approach to scout a field for first-generation corn borers?

When corn plants have reached a height of 18 inches (extended leaf height), scouting should begin right away. If possible, at least 20 consecutive plants should be examined 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. Walk at least 100 feet into a field before checking plants for corn borers. If more than one corn variety 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 percentage 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. If you were able to pinpoint the beginning date for the first flight of moths this spring, information contained within Table 2 should be of help in predicting other important corn borer events (such as stalk penetration) this season. If a rescue treatment is warranted, the granular formulation of an insecticide is more effective than the liquid formulation when applied aerially (Table 3).

Many plants within my field have whorl-feeding injury, but I can't find any borers. Should I treat anyway?

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 for small borers to establish. From 20 to 70 percent of newly hatched larvae fail to "take hold" in plants due to weather-related variables such as high temperatures and related moisture stress. Even the larvae that make it into whorls 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 also may 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 injury, it is crucial to pull whorl leaves from infested plants to find out why borers aren't being found in your field.

When will corn borers begin to tunnel into 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 4). 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 corn borer biology, please refer to Table 4.

If I find European corn borer larvae within the whorls of Bt plants, does this mean that the Bt corn is not working?

No. Don't jump to that conclusion. Representatives with seed corn companies indicate that the expression of the Bt endotoxin may not occur within every single plant across an entire cornfield. As you scout your Bt cornfield, finding a few plants with injury should not be alarming. However, if injury and live larvae are found, the seed company should be contacted. Company representatives should be able to determine whether the plants on which 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). We urge anyone who has planted Bt corn to scout fields in order to assess the pest management performance of these transgenic hybrids.--Mike Gray

European corn borer moth flight is under way, as evidenced by captures on a car radiator.

European corn borer moths (from left): female, male, and female.

European corn borer egg mass.

First-instar larvae hatching from egg mass.

Scouting for first-generation European corn borers.

First-generation corn borer whorl-feeding injury.

Checking whorl leaves for corn borer larvae.

Author: Mike Gray

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

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