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Second-generation European corn borers (image) are rearing their ugly heads once again in area corn fields. As a result, both stalk breakage and ear drops are common sights in some fields. The damage is somewhat surprising given the lack of accumulated heat units through August. However, it appears that there was an extended egg laying period (rather than a peak) and the warm weather in September and October gave the little critters plenty of time to do their damage. Unlike 1995 when we saw record numbers of second generation corn borers, much of the damage this year occurred relatively late and probably will have minimal yield impact from a plant physiology standpoint but will cause problems and yield loss from stalk breakage and ear drop.
Corn borer life cycle
The insect overwinters in old corn stalks as larvae (the “worm stage”), pupate in the spring, and the emerged moths lay their eggs on the underside of corn leaves around mid-June. They are usually most attracted to the tallest, early planted corn fields. This first generation of eggs hatch and the larva move to the plant whorl where they begin to feed. The feeding results in “shotholes” across leaf blades as they emerge from the whorl. Control measures are usually warranted when damage is noted on 50 percent of the plants. The larvae then pupate again and emerge as moths. The second generation moth flight usually peaks around mid-August. Eggs are again laid on the underside of leaves in about the middle third of the plant. Late planted fields are most attractive to second generation moths and treatment thresholds are generally reached when there is one or more egg mass found per two plants..
After egg hatch, larvae move to the stalk, ear shank, and ear to begin feeding. The entire cycle is driven by heat units. Plant yield losses average 5 percent per borer for first generation and 3 percent for second generation larvae. Losses from ear drops would be in addition to these figures.
What can be done now?
We are now to the point of cutting losses. Plants are already breaking off and ears are dropping. Many stalks have lost all integrity from stalk rot disease organisms. Harvesting corn as early as possible is now the only recourse. Producers will need to balance the cost of higher drying expenses with an earlier harvest to higher field losses by allowing the corn to dry down.
Chopping corn stalks or moldboard plowing this fall will have little impact on reducing infestations for next year. Although larvae overwinter in the crop residue, moths can fly long distances. Thus, all corn stalks would have to chopped or plowed under within a large geographic radius to have any significant impact. In many years, the fall corn borer larvae population is drastically reduced by our winter weather or parasitism. Additionally, egg laying conditions may not be favorable the next year.
Is Bt corn the answer?
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