University of Illinois

No. 18/July 24, 1998

Where Have All the Corn Borers Gone?

The excessive rain and late planting this spring must have taken a heavy toll among European corn borer populations throughout most of the state. The most frequently asked question recently has been "Where are the corn borer moths?" Virtually everyone trapping, looking for, or expecting to find adult corn borers has been surprised by their relative absence. Captures of adults in pheromone and blacklight traps have been low statewide, and car windshields have remained relatively clean during evening drives this July.

The lack of significant numbers of European corn borer adults thus far has translated into a scarcity of reports of scouts finding many egg masses on corn leaves. Maria Venditti, my graduate student, has been monitoring four corn fields weekly in Sangamon County since early June. On July 15, her crew of six people found a total of 12 egg masses in the four fields (2,400 plants examined), resulting in a density of 0.005 egg mass per plant. That's below the economic threshold, don't you think?

European corn borer egg mass.

All evidence suggests that the second generation of European corn borers will not amount to much in Illinois this year. However, survival of corn borers in some small regions in the state could have been good, so the vigil for second-generation borers should continue. Remember, moths laying eggs for the second generation will preferentially lay more eggs in late-planted and late-maturing fields (fresh pollen and silks). With the variety of corn developmental stages to choose from this year, some corn borers may find a purchase in some fields in some areas. For those of you who continue to monitor moth flights with pheromone or blacklight traps, Table 2 provides predictions of selected corn borer activity based on degree-day accumulations. As indicated in the footnote to the table, cooler-than-normal or warmer-than-normal temperatures will slow down or accelerate corn borer development, respectively. Consequently, you should use the "days to first occurrence" with consideration for recent weather conditions. Undoubtedly, corn borers, the few that are out there, havedeveloped more quickly during the past couple of weeks than they would have if the temperatures had been "normal" (average of 75 to 77 degrees F).

Table 2. 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 European corn borers (from European Corn Borer: Ecology and Management, NCR Publication no. 327, Iowa State University, Ames).

of stage or
Days to first
General activity
0First spring
Mating and
egg laying
1,192Adult mothsMating and egg
laying to begin
second generation
1,404Egg hatch
(first instar)
8.2Pollen and leaf-axil
1,510Second instar4.1Leaf-axil feeding
1,627Third instar4.3Sheath, collar, and
midrib boring
1,759Fourth instar5.1Stalk boring
1,984Fifth instar9.0Stalkboring

a Average number of days of development to reach the first occurrence of the stage or event since initiation of the previous stage listed. For example, first instars of the second generation require about 4.1 days to develop to second instars; second instars require about 4.3 days to develop to third instars; etc. The number of days varies if temperatures are cooler or warmer than average.

We will keep you updated regarding reports of European corn borers, or lack thereof. Additionally, we will offer guidelines for scouting and management suggestions in the next issue of the Bulletin.

Kevin Steffey (, Extension Entomology, (217)333-6652