Integrated Crop Management is published by the Department of Entomology , Iowa State University , Ames, Iowa. 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. Enlarge Newly-hatched larvae are very small and may be hard to find in the corn whorls. 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. 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.