European Corn Borer
Contact: Eric Day, Manager, Insect Identification Laboratory
Publication 444-232, August 1996
European Corn Borer
Description of Damage
European corn borer (ECB) is a major pest of corn grown for grain
in Virginia. This pest is found throughout the commonwealth, but
its population density fluctuates from year to year in a given
locality. Typical damage to corn plants caused by this insect
are reduced plant vigor leading to subsequent ear drop and stalk
lodging.
Identification
When fully grown, ECB larvae are 3/4 to 1 inch in length and
creamy-white to pink in color. The larval head capsule is dark
brown and, on top of each abdominal ring or segment, there are
several small dark brown or black spots (Fig. 1). Pupae vary
from 1/2 to 3/4 of an inch in length, are torpedo in shape, and
range in color from reddish-brown to dark-brown. The moth has a
wingspan of about one-inch, with the female being slightly larger
than the male. The wings are dusky-yellow in color and bear
transverse, irregular, olive-green bands (Fig. 2). Although
similar in appearance, male wings are often darker in color.
Adult females typically lay 15 to 30 eggs in masses and may lay
up to 500 eggs over their lifetime. Egg masses usually are laid
on the underside of corn leaves and are about 3/4 of an inch in
diameter (Fig. 3). Egg coloration changes from white to a creamy
translucence during development. Immediately before hatching,
the dark brown head capsules of the young larvae become visible
through the shell (Fig. 4).
Life History
ECB overwinters as a full-grown larva either in tunnels of old
corn stalks or in the junction formed by a leaf and stalk. In
addition, ECB larvae will sometimes overwinter inside the stems
of certain weeds. Development begins in spring when temperatures
exceed 50 degrees F. Larvae pupate in late spring and emerge as
adults after about two weeks. These first generation adults
usually appear from mid-May to early June in Virginia with the
exact date depending on both location and weather conditions.
Moths become active in the evening seeking mates and laying eggs
but spend the daylight hours hiding in grassy fence rows and
other protected areas. First generation moths seek out the most
mature (earliest planted) corn on which to lay their eggs. In
addition to corn, ECB has been reported to feed on over 300
different plants, including green peppers, edible beans, and
gladiolus.
Weather conditions play an important role in governing the
survival of ECB. Strong winds and beating rains during peak
flight activity may dramatically reduce both egg laying and egg
survival. In contrast, curling and uncurling of corn leaves
during periods of drought can help dislodge the eggs from a leaf.
Under ideal conditions, egg hatch of first generation ECB takes
place within three to seven days after the eggs are laid. The
tiny caterpillars feed on leaves in the whorl causing a "shot-
hole" appearance after the leaves have uncurled (Fig. 5). Older
caterpillars tunnel in leaf midribs, but eventually bore into the
stalk where they will complete their development in about three
to four weeks. Full grown caterpillars pupate inside the corn
stalks. In Virginia, second generation moths emerge and begin
laying eggs from late June to mid-July. This second generation
is considered to be the most damaging to corn grown for grain
despite the fact that heavy infestations of first generation ECB
can cause extensive visual damage.
Damage to corn by first-generation caterpillars is primarily
physiological. Specifically, yield loss results from
interference with transport of nutrients and water to the stalk
and leaves. The extent of nutrient and water loss depends on
weather conditions, soil type, plant variety, plant maturity, and
level of infestation. USDA researchers in Iowa have shown yield
reductions of about 8 bushels per acre from low ECB infestations
and about 22 bushels per acre from large infestations.
Second generation damage consists mainly of stalk breakage (Fig.
6), ear drop, and ear feeding, and is more physical and
potentially much more serious than first generation damage. In
general, heavy infestations of second generation ECB can reduce
yields by as much as 30 bushels per acre. Stalk rot diseases
also tend to be more prevalent in areas infested by second
generation ECB.
Non-Chemical Control
The most effective, non-chemical method for controlling ECB on
corn is to plant resistant varieties selected on the basis of
their ability to withstand attack. To determine the best variety
for your location, consult your county extension agent or local
seed distributor.
Other factors that can help minimize ECB damage are time of
planting and spring plowing. Whenever possible, avoid planting
corn too early or too late. Corn that has been planted very
early can serve as an unintended "trap crop" for newly emerged
first generation moths, resulting in heavy infestations and
subsequent damage. Similarly, late-planted corn can suffer even
greater ECB damage because, as younger plants, they are less able
to withstand the higher population pressure of the second
generation. Spring plowing helps reduce the severity of ECB
infestations by burying the previous season's corn stalks and
weed residues. Also, there is some evidence that chopping stalks
after harvest with a rotary mower will destroy ECB caterpillars.
It is important to realize, however, that because ECB moths are
strong fliers, and thus, highly mobile, plowing and destruction
of crop residues must be practiced on a large scale to achieve an
area wide reduction in population size.
Scouting and Chemical Control
Although second generation ECB are more likely to attack corn
that has been planted late, all corn grown for grain should be
scouted. To assess the potential for economic damage, begin
checking for egg masses when second generation moths emerge and
begin egg laying. The presence of ECB moths in areas bordering a
field can be used to indicate a likely infestation. To survey
for these moths, walk along the grassy sides of the field and
look for large numbers of moths flying as a result of being
disturbed. However, if no other information is available, it is
suggested that scouting for egg masses be initiated the last week
of June and continued at two to three day intervals through the
third week of July. To do this, randomly select five sets of 20
consecutive plants from throughout the field. Count the number
of egg masses found on each plant. ECB moths usually lay their
eggs in masses on the underside of leaves, two or three leaves
below the ear; however, because they can be laid anywhere, it is
best to look for egg masses on every leaf.
To reduce the potential for economic losses from second
generation ECB in grain corn, an insecticide treatment is
warranted if 35 to 50 percent (or more) of the plants in the pre-
to post-tasseling stage of development have at least one egg mass
per plant. Consult the 1992 Pest Management Guide for Field
Crops (publication number 456-016) for specific information on
postemer- gence insecticides available for control of ECB on
field corn.
Prepared by Roger R. Youngman and Eric R. Day, Department of
Entomology, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0319.