University of Illinois

No. 14/June 27, 1997

European Corn Borer Activity Increases

Many of our readers have answered our request for information about European corn borer activity. We have received numerous calls from folks who have been scouting corn for the presence of corn borers. The reports have included information about moth flights in light traps and pheromone traps, moth activity in "action sites," numbers of egg masses on plants, and initial signs of pinholes or small shot-holes as the larvae initiate their whorl-feeding activity. In central and northern Illinois, scouting for European corn borers should be a primary activity right now.

Our most frequent reports have been about the activity of the adults. Several individuals who are monitoring light traps and pheromone traps noticed an increase in captures of adults during the week of June 16. That trend continued through the weekend and the first part of the week of June 23. For example, Dave Mowers, owner of Mowers Soil Testing Plus, Inc., in Toulon (Stark County) reported that during the week of June 16, captures in their light traps averaged about 15 moths per night. During the weekend and into the first part of the week (June 21 to 23), nightly captures of adult corn borers were 155, 25, and 330, respectively. Trina Seary with Seneca Foods in LaSalle County, a cooperator working with Rick Weinzierl, observed a significant increase in the numbers of moths captured in her blacklight trap on June 11. However, she began capturing consistent large numbers of moths on June 18, 19, and 20 (152, 222, and 123 moths, respectively).

The most difficult question to answer concerning flights of adult European corn borers is "Have we already observed peak moth flight?" Often the only way to determine when peak moth flight occurred is to look at the data after the fact, when it's easier to discern an increase, a peak, and then a decrease of moth captures. If you review the information in the preceding paragraph, you'll notice dips in moth captures followed by significant increases the next night. Nevertheless, the odds are pretty good that peak flight of adults throughout most central and northern counties will have occurred by the end of the week of June 23, or at least by the weekend.

Reports from field scouting provide some insight about egg-laying and larval-feeding activities. Maria Venditti, my graduate student who is sampling cornfields in Sangamon County, easily found egg masses on leaves and small larvae just beginning to feed in the whorls. However, she and her crew members found only an average of 7 to 8 egg masses per 100 plants. The maximum amount of whorlfeeding injury she observed was about 10 percent. Rick Kesler with FMC visited a field in Menard County on June 25 and determined an average 40 percent infestation, with as many as 3 larvae per plant. All larvae he found were first and second instars. Dave Mowers has observed pinhole feeding in several fields, with the maximum infestation at slightly greater than half of the plants. The highest levels of infestation he and his scouts observed were in Knox County.

Although these reports don't suggest a level of infestation that approaches the outbreak we experienced last year, all these observers noted one common thread: Corn borer moths were still plentiful in the cornfields and in "action sites" near the cornfields. We suspect that egg laying is not complete, so this may complicate making control decisions.

Because of many folks' experience with corn borers last year, people are somewhat anxious about the potential for economic infestations this year. I have heard from at least one individual who wanted to treat his field based upon his observations of egg masses; he had seen no sign of whorl-feeding injury at the time he scouted his field. The reason our thresholds are not based upon the numbers of egg masses is because percentage mortality of eggs and young instars usually is quite high. Natural enemies (pathogens, parasitoids, and predators) take their toll, but unpleasant environmental conditions also cause some mortality. The hot, dry weather that has predominated in some areas of Illinois is causing corn leaves to roll, so some egg masses could be dislodged. The environmental extremes, from hot and dry to unnaturally wet, usually lower the survival rates of corn borer larvae.

Let's remain rational about management of corn borers. Because our thresholds and decision-making guidelines are based upon the percent of plants with whorl-feeding injury and the number of small larvae per plant (these have been standard for years), we suggest you wait until females have concluded their egg laying before making a decision too soon. Remember, whorl-feeding injury is not economic, and the larvae remain in the whorl through their third instar of development. Consequently, the "window of opportunity" for insecticide application, if necessary, is probably 2 weeks, even during the type of hot spells we are experiencing right now. So keep your eyes on the fields in which corn borers seem most numerous, but don't pull the trigger too soon.

Scouting techniques, corn borer larval measurements, the management worksheet, and suggested insecticides for control of corn borers were published in last week's issue (no. 13, June 20) of this Bulletin. Keep your eyes on the whorls!

Kevin Steffey and Mike Gray, Extension Entomology, (217)333-6652