European corn borers: How long before they tunnel into corn?

by Marlin Rice, extension entomologist, Department of Entomology

European corn borer larvae are hatching in south central Iowa according to Mike White, extension field specialist-crops. Also, I saw moths flying in Ames on the night of May 30. By the end of this week (June 10-11) females should be laying eggs throughout the state once the warm evening temperatures return.



Enlarge		 Newly-hatched larvae are very small and may be hard to find in the corn whorls.

A common concern every year is after European corn borers hatch, how long does it take before they tunnel into the stalk? The reason for this question is that many people know these larvae feed inside the whorl (where they can be killed with an insecticide) before they tunnel into the stalk (where they can't be killed). The answer depends on the influence of ambient temperatures in the cornfield. Insects are cold blooded, so their growth is a function of temperature over a period of time. When temperatures are cooler than average, development of the larvae slows down and they stay in each larval stage a little longer. The eggs require approximately 100 degree days for larvae to hatch. The larvae need approximately another 225 degree days to reach the third instar, which is the developmental stage when some of the larvae start to tunnel into the stalk. With daytime temperatures of 85°F and nighttime temperatures of 60°F, the European corn borer accumulates 22.5 degree days per 24 hours. At this rate it takes approximately 10 days from hatching to the first stalk tunneling. Sixteen days after the first larvae hatch, they will be in the fourth instar and most of them will be tunneling into the stalk at this time. However, if the average daily temperatures are cooler, there will be more days between hatch and stalk tunneling. The table provides more details and the average number of degree days to reach each stage. The graph shows the approximate size of each of the five larval stages.

Accumulated degree days (developmental threshold of 50°F) from initial capture of adult European corn borer moths in the spring for first occurrence of life stages.*

Accumulated

degree days		 First occurrence

of stage or event		 Days to

first occurrence		 General activity
0 first spring moth — mating and egg laying
212 1st instar 16.3 pinhole leaf feeding in whorl
318 2nd instar 6.6 shothole leaf feeding in whorl
435 3rd instar 6.5 midrib and stalk boring
567 4th instar 6.6 stalk boring
792 5th instar 10.2 stalk boring
1,002 pupa 8.8 changing to adult moth
1,192 adult moth 7.6 mating and egg laying

*From European Corn Borer - Ecology and Management. Publication NCR-327, page 7. Available from ISU Extension Distribution Center (515-294-5247).

This article originally appeared on page 101 of the IC-484(13) -- June 12, 2000 issue.

Updated 06/11/2000 - 1:00pm