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No. 15/July 3, 1997

First-Generation European Corn Borers Are Busy in Some Locations

Although I doubt that the levels of infestation of first-generation European corn borers this year in Illinois will rival the widespread outbreaks we experienced in 1996, folks in some areas have encountered some healthy degrees of corn borer injury. Insecticide applications for corn borer control are being made to a fair number of fields in some western and northwestern counties. Dave Mowers with Mowers Soil Testing Plus in Toulon (Stark County) indicated that several fields have been treated in fields northwest of Peoria. Mike Roegge, crop systems Extension educator in Quincy, reported that several fields in Brown County have been treated, but very few in Adams County have been treated. Bill Brink, crop systems Extension educator in Springfield, reported that several seed-corn fields in and around Sangamon County have been treated for control of corn borers. These hit-and-miss reports suggest that although economic infestations are intense (more than 70 to 80 percent of the plants infested) in localized areas, the "outbreak" is not widespread.

Numerous explanations for these spotty infestations are plausible. It is likely that the survival of corn borer larvae in some areas has been reduced by the microsporidian disease Nosema pyrausta (refer to issue no. 3, April 11, of this Bulletin). In addition, although corn got off to a slow start this year and we speculated that the plants might not be tall enough when corn borer adults began laying eggs, emergence of corn borer adults also was delayed. Consequently, egg-laying females had plenty of choices when they began seeking egg-laying sites. Some dilution of the population may have occurred in some areas.

Where the infestation of corn borers is heavy (50 percent or more of the plants infested is common), most of the larvae are third instars, so tunneling in stalks will begin soon. Observers have found as many as two to three larvae per infested plant, suggesting that survival of larvae has been relatively good. This type of information is required for the management worksheets that were published in issue no. 13 (June 20) of this Bulletin.

Regarding the height of corn, most people realize that "short corn" (less than about 16 inches tall) has a high concentration of DIMBOA, a plant aglucone that interferes with corn borer larval behavior in such a way that the borers do not survive. However, some people in southern Illinois have observed healthy corn borer larvae on short corn this summer, and they are puzzled by this occurrence. Although the corn was planted early enough to have some size on it by now, the cause for the corn being short has mystified several specialists. However, survival of corn borers on this short corn may have a logical explanation. Although we link DIMBOA concentration to the height of corn plants, it may be more appropriate to link it with the age of the plants. In the situation I just described, although the corn was short (because it was unable to grow properly), the plants had aged, so the concentration of DIMBOA may have declined over time. Another possible explanation is that, because the short plants were not photosynthesizing properly, they did not produce much DIMBOA. Consequently, the unusual circumstances surrounding the poor growth of these short plants may have allowed for survival of corn borer larvae.

What can we expect from corn borers now? Our best guess is that peak flight of adults has occurred in most areas of Illinois. Dave Mowers reported that captures of adults in one of their light traps were 320, 450, 159, 43, 70, 95, and 27 moths per night from June 23 through 29, respectively. The trend in numbers of adults being captured in other light traps placed at other locations has been similar. Consequently, most egg laying is complete, and larvae are feeding in whorls. Insecticide applications, if needed, should probably cease soon. By July 4, most corn borer larvae will be old enough (fourth instars) to begin tunneling into the stalks where they cannot be controlled with insecticides.
Kevin Steffey, Extension Entomology, (217)333-6652