In the relatively short time since the European corn borer reached the concentrated corn-growing areas of Illinois and Iowa, the population increase and rate of spread have been greatly accelerated. Its advance across Illinois and deep into Iowa has been from 50 to 100 miles a year. The spread of the borer after its discovery in Clinton County in 1942 is shown in fig. 1.

The last few years, which have been exceptionally good corn-growing years, likewise have been good for the borer. Thus, it appears that weather, soil conditions, farming practices and other agricultural factors most favorable for growing large acreages of high-yielding corn are also quite favorable to the borer. As the borer spread into Iowa during 1942 and 1943, its buildup has been most rapid in the counties of more intensive corn production. In general, the borer population is heaviest from Clinton and Scott Counties westward toward the central part of the state. Conversely, the population rise has been much less rapid in those infested counties along the southern and northern borders, where corn-growing conditions are less favorable and the corn acreage much smaller.