This is the most visible time to actually see the insects. You will often see them buzzing around your ceiling lights in late July and August. Similar in size, they are yellow with a black strip along their wing covers. The Western corn rootworm is much more damaging to corn than the Northern. In fact, research in the early 80 s in Vermont showed very little damage from Northern corn rootworm even at high populations.
Western cousin is a different beast and requires some serious attention.
So, if you don t know if you have it, do yourself a favor and go out and look. First, look in a field that has been in corn for more than one year. Since larvae feed only on corn roots and no other crop, few problems exist in the first year of corn. Stealth is a key as you move through the corn crop to detect adults. These insects are easily startled and will literally drop off the plant if disturbed.