By Adam Varenhorst and Philip Rozeboom et.al
European corn borer caterpillars (Figure 1) have been observed in conventional corn at several locations in South Dakota. In these fields, the caterpillars have been observed in the whorl and upper stem. Indicators for the presence of European corn borer caterpillars in corn are a line of straight holes in the upper leaves that is often referred to as shot-hole injury (Figure 2). These holes are an indicator that a European corn borer caterpillar fed on the leaf while it was still curled in the whorl.
In South Dakota, there are populations of European corn borer that have one generation per year in the northern areas of the state, and populations that have two generations per year in the southern areas of the state. The fields that are observing caterpillar defoliation and caterpillars in the upper whorl are in areas where the European corn borer has two generations per year.
Source : sdstate.edu