Corn borers: How fast do they grow?

A common question the past week has been, "How long does it take before European corn borers tunnel into the stalk?" The answer depends on the influence of ambient temperatures in the cornfield. When temperatures are cooler than average, such as during the week of July 7-13, larval development slows down and larvae stay in each stage a little longer. The eggs require approximately 100 degree days to hatch. The larvae need another 300 degree days to reach the fourth instar, which is the stage when most of the larvae tunnel into the stalk. With daytime temperatures of 85°F and nighttime temperatures of 60°F, the corn borer is accumulating 22.5 degree days per 24 hours. At this rate it takes approximately 13 days from egg hatch to stalk tunneling by fourth instar larvae.

Corn borers hatching from egg mass.

You can determine degree days for corn borers in your area by using this formula:

  1. add the daily maximum and minimum temperatures together and then divide by 2 to get a mean daily temperature (but do not use a maximum above 86 or a minimum below 50);
  2. subtract 50 from the mean daily temperature to obtain the heat units for one day;
  3. sum the heat units to get the accumulated degree days for a known time period.

This article originally appeared on page 136 of the IC-476(19) -- July 22, 1996 issue.

Updated 07/21/1996 - 1:00pm