University of Illinois

No. 11/June 6, 1997

What Are Corn Borers Doing?

For those of you waiting patiently for European corn borers to "show their hand," here are some reminders about their life cycle. Remember that corn borers overwinter as mature, fifth-instar larvae. In the spring when the temperature exceeds 50 degrees F, the larvae break diapause and pupate. After about a week or so, moths emerge from the pupal case and seek shelter in "action sites," usually areas of weedy vegetation that provide cover and moisture. Males and females mate, and the females begin laying eggs in corn that is tall enough to support larval development (usually 16 inches or taller). Consequently, if you are looking for corn borers right now, depending upon your location in the state, you could find moths and egg masses, maybe even some small larvae in southern Illinois; red-brown pupae in old corn stalks, moths, and maybe some egg masses in central Illinois; and overwintering larvae or developing pupae in old cornstalks, and probably moths, in northern Illinois.

Table 2 shows historical first reports of corn borer moths in this Bulletin from 1988 through 1996, providing some indication of when moths might be expected in certain areas of Illinois. As has been the case for most of our insects this spring, corn borer development probably is delayed a bit this year. However, in areas where moths are emerging already, if corn is not tall enough, corn borer infestations may not amount to much. I guess we could all use a break after last year.

Table 2. First reports of corn borer moths in the spring, from 1988 through 1996 issues of this Bulletin.

YearSouthern IllinoisCentral IllinoisStatewide
1988---a---May 1724
1989------June 2
1990---June 8---
1991May 6May 2228June 7
1992May 12---May 22
1993May 21---June 4
1994May 13---May 27
1995May 22---May 23
1996May 20---May 20

aNot reported in the Bulletin.

If you are running a light trap or using pheromone traps to capture European corn borer moths, you might be interested in degree-day accumulations. The best "biofix" to initiate degree-day accumulations is moth capture. Table 3 shows accumulated degreedays (above a developmental threshold temperature of 50 degrees F) from initial capture of moths in the spring to the first occurrence of life stages and activity of first-generation corn borers. This information was taken from European Corn Borer: Ecology and Management, North Central Regional Extension Publication no. 327, Iowa State University, Ames. Remember, these are guidelines based upon research data gathered over years and states. Consequently, some slight variations may occur among areas and years. Use the information for planning purposes, but don't view these guidelines as inflexible. When we begin to receive confirmation of dates of moth captures, we will start publishing degree-day accumulations for the state, as we have been doing for several other insects. If you are trapping corn borer moths or finding moths as you walk through "action sites" to scout fields, give us a call or send us an e-mail message.

Table 3. 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).

Accumulated degree-daysFirst occurrence of stage or eventDays to first occurrenceaGeneral activity
0First spring moth
212Egg hatch (first instar)16.3Pinhole leaf feeding
318Second instar6.6Shot-hole leaf feeding
435Third instar6.5Midrib and stalk boring
567Fourth instar6.6Stalk boring
792Fifth instar10.2Stalk boring
1,002Pupa7.6Changing to adult

aAverage number of days of development to reach the first occurrence of the stage or event since initiation of the previous stage listed. For example, it takes approximately 16 days from first moth capture to egg hatch; first instars require approximately 6.6 days to develop to second instars; etc. The number of days will vary if temperatures are cooler or warmer than average.

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