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Newly-hatched
European Corn Borer larvae often feed deep inside the whorl and do not
chew completely through the leaf but only remove a shallow layer of plant cells.
This early feeding creates an opaque "window" of leaf tissue. As
larvae grow larger, they then can chew through the leaf. Leaves emerged from the
whorl will have small holes either randomly scattered or arranged in a repeating
pattern across the leaf. Holes are not large enough to sever the leaf from the
plant. Injured leaves are not wrinkled. Holes are not bordered by wide halos of
brown or yellow. European corn borers seldom feed on plants shorter than 17-21
inches (extended leaf height).
Transgenic
hybrids (Bt hybrids) are available for control of European corn borers. These
hybrids that have been genetically transformed to include a gene from a
naturally occurring soil bacterium Bacillus thuringiensis (Bt). The gene
produces protein crystals (Cry proteins) that are toxic to corn borer larvae.
Once ingested, the Cry proteins break down and produce a toxin that ruptures the
lining of the insect's gut. This causes the borers to stop feeding and die
within a few days. Bt hybrids offer season long protection against the European
corn borer.
The
decision to use insecticides to manage corn borers is often decided with the use
of management worksheets. Timing of insecticide applications is difficult. Once
larvae begin boring into the stalk, insecticide applications are ineffective.
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