Steve Spangler
Entomologist
Penn State
Since about 1920, com
growers in the East have been
dealing with yield losses from
the European com borer.
This insect takes a bite out of
our com production because it
eats into the stems (see photo)
and shanks of the ears, causing
stalk and ear breakage, eats
holes into the leaves in June
(“shothole” damage), and eats
the ear kernels. And if this is not
enough for us, this damage can
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create entrance for plant dis
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xin levels.
On the average, this insect
takes about 3 to 6 percent of our
yields, with some fields show
ing no economic losses but
some showing much more than
this average amount of loss.
The most obvious evidence
of com borer at this time of year
is stalk and ear droppage and
holes In the stems with “frass”
(sawdust-looking material).
To look for this damage, take
a kitchen knife (please be care-
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full), cut the plant off. and cut
lengthwise down the stalk.
Look for stalk tunnels and lar
vae. Also, take off the ear
leaves and look for damage,
and also on the shank.
Are you seeing significant
stalk tunneling and larvae?
Remember, each larva tunnel
ing through a plant and making
it to full size takes about 3-7
percent of your com grain
yield.
How many of you have actu
ally done anything to control
com borer? Probably very few.
Estimates in Pennsylvania indi-
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An example of a corn stalk showing European corn
borer larvae and pupae and the tunneling caused by
this Insect.
cate that only about 1 percent of
our com fields have used an
insecticide to control com borer
and probably for good eco
nomic reasons. It costs about
$l5 per acre to apply an insecti
cide (that doesn’t, include the
costs of scouting for the insect)
and you will only acheive an
average of about 70 percent kill
of com borer. Pul these two fac
tors together and most growers
have been making good eco
nomic decisions in not controll
ing com borer. Also, in past
years we have been dealing
with grain prices of about
$2.50/bushel.
Bt com might be changing
this picture. We now have
plants (“Bt com”) which,
because of biotechnology, have
a new gene in them which pro
duces a toxin that kills com bor
er. This toxin is present in
tissues, particularly the green
tissues, and kills the young lar
vae or “worms” when they
chew into the plant.
The effect on com borer is
quite dramatic; 95-100 percent
of the young larvae are killed,
compared to about 70 percent
with normal insecticides. The
other factor that this new tech
nology has changed is the cost
of control. Seed companies are
stating that they will charge
about a $7 per acre premium on
a bag of Bt seed over a non-Bt
seed. Compare this with the $ 15
or so cost of applying a conven
tional insecticide to control
com borer.
The result of these differ
ences is that now, the use of Bt
com will pay off in 50-80 per
cent of the com fields in the
U.S. as opposed to less than 10
percent of the fields justifying a
conventional pesticide treat
ment that we have had to date.
Also, don’t forget the
$4.50/bushel price we are see
ing now—that may make both
Bt com and normal com borer
control more economical to
use. '
But there is an important
assumption to this analysis
which states that Bt com will
pay off in 50-80 percent of the
com fields in the U.S. The Bt
hybrids have equal yield poten
tial of the non-Bt hybrids. If the
Bt gene is put into a hybrid that
has superior yield potential to
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