Broken leaf midribs, tassels with saw-dust-like frass at the breaks, and holes in the stalks and ears with bunches of frass at the tunnel openings are common signs of the European corn borer. This damage can lead to reduced yield, lodging, and dropped ears, and the borings may provide an entryway for stalk and ear rots.
The European corn borer over-winters as a fully grown larva in cornstalks and stubble in fields and in ears stored in cribs. Moths emerge from late May to early July and again in late July to early September in those areas where two generations occur.
The moths lay their eggs on the underside of corn leaves. The eggs are flat, about the size of a pinhead, and are arranged in masses of 5 to 50, overlapping like shingles. They are white when first laid but have a black spot in the center of each just before hatching.
The eggs hatch in a few days, and the resulting larvae damage the crop. When small, the larvae feed on the leaf surface and in the whorl of the plant. As they mature, they tunnel in all parts of the plant and, at this point, are out of the reach of insecticides. Proper timing of treatments, when called for, is therefore essential; the insecticide must reach the young larvae before they bore into the plant. The fully grown larvae are 3/4 to 1 inch in length, flesh colored, and marked with small, round brown spots. When mature, these larvae either cease activity and overwinter in the stalk or pupate in the tunnel to produce the moths of the second generation.
Economic populations of corn borer remain a localized problem and are seldom predictable. Problems have been further reduced with the widespread use of GMO-BT corn borer corn. However, if GMO-BT corn borer corn is planted, refuge requirements need to be adhered to. The usual refuge requirement is a structure refuge, 20% of the size of the GMO field and planted within ¼ mile of the GMO field.
On many farms, the corn borer populations are low and the planting of GMO-BT corn borer corn is unnecessary. In these situations, select a variety that is well adapted to your area, is rated highly for standability, and has done well for your neighbors. Avoid early-maturing varieties and planting very early if the corn borer has been a problem in the past. When harvesting, cut stalks as close to the ground and as early as possible.
Chopping the crop for silage or fodder will kill any borers in the stalks. If soil erosion is not a problem on your farm, clean plowing (leaving no crop residue on the soil surface) in the fall or before May 1 is effective in reducing the corn borer population overwintering in that field. This practice should be avoided where soil erosion is likely to occur. Consult your local office of the USDA’s Natural Resources Conservation Service for advice.
Conserve natural enemies by avoiding unnecessary use of insecticides. Planting-time treatments with soil insecticides are not recommended for European corn borer control. Chemical treatments must be timed properly to be effective and are not generally recommended in NYS. Fields with chronic corn borer problems can be planted to the BT-corn borer corn varieties for control. However, if the BT-corn varieties are planted, growers must plant a BT-free refuge according to the label requirements. The widespread planting of BT-Cornborer corn has dramatically reduced the corn borer populations across NYS. Since the older BT-Cornborer events are not effective on Western bean cutworm, that insect has invaded and become established in the nitch previously occupied by corn borer in our corn production systems.