Corn Borer Injury Reported in Western Illinois

June 4, 1999
On June 1, Mike Roegge, Crop Systems Unit Educator, Adams and Brown counties, discovered corn borer larvae in the whorls of corn plants in a field located in Brown County. The cornfield had been planted in late March. Survival of the 1/4-inch borers appeared to be good in the field of knee-high corn. Although it may seem a bit early to find larvae, recall that European corn borer moths were observed in some southern Illinois locations in early May. Table 1 provides projections for seasonal corn borer events based on the initial capture of spring moths and subsequent heat-unit accumulations (base 50°F). If you have records of when corn borer moths were first observed in your area of the state, you should be able to predict with reasonable accuracy when corn borer larvae are most likely to make their presence known.

Producers that planted corn in March or early April should especially monitor fields that, in essence, will serve as trap crops for egg-laying corn borer moths. 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), which 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. The following questions concerning European corn borer management are frequently asked.

Corn borer larvae.

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 pm to 1 am) 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.

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.

Scouting whorl-stage corn for first-generation European corn borer injury.

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 (Figure 1). If a rescue treatment is warranted, the granular formulation of an insecticide is more effective than the liquid formulation when applied aerially (Table 2).

Checking whorl leaves for first-generation European corn borer.

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 being a corn borer. From 20 to 70 percent of newly hatched larvae fail to establish 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 are not 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 injury, it is crucial to pull whorl leaves from infested plants to find out why borers aren't being found in your field.

Shot-hole injury caused by first-generation European corn borer.

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), they begin boring into stalk tissue (Figure 2). 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 3.

Mid-rib injury caused by first-generation European corn borer.

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 corn
field, 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

Author: Mike Gray