Corn earworm, sorghum headworm,
and cotton bollworm, no matter what you call it, this insect is a
nuisance. We have found a few bollworms
in cotton fields this summer, but none at threshold yet. As the area’s corn begins to harden, we
should see the next generation of moths moving into the cotton and / or sorghum
to lay eggs.
The
adult bollworm will lay its eggs just about anywhere on the plant these days,
but the old textbook states that they prefer the upper 1/3 of the cotton plant
on the upper side of the leaf surface.
The eggs are small, oval shaped and pearly white in color, with a slight
dent in the tip. If you have eggs in a
field, even huge numbers of eggs, you don’t have to treat yet. The eggs can be parasitized or eaten by a
variety of things and some may even be unviable, so several won’t get to
hatch. Once the caterpillars hatch they
begin to feed on whatever is nearby, but move to the fruit as soon as possible. These worms do live in a world where it is
survival of the fittest. The
caterpillars will often cannibalize each other, if their eggs have been laid
too close together or they attack the same fruit, ensuring that the strongest
larva will survive. These factors
combined are why we usually wait until we find a threshold number of
caterpillars (the smaller you catch them the better) before treating a field. The size of the caterpillar changes the
threshold as well, since during the last two instars, the caterpillars consume
up to 90% of the total amount they consume before pupating.
1We
really start looking for bollworms in cotton when our blooms have started
becoming bolls, but they can infest cotton earlier if their preferred host
(corn) is unavailable. The threshold for
this insect in non-Bt cotton with bolls forming is about 10,000 worms ¼ of an
inch or smaller per acre and 5,000 worms larger than ¼ of an inch per
acre. In Bt cotton, we look for 5,000
worms larger than ¼ of an inch per acre with 5 t o15% damaged fruit on the
plant. Once the insects reach this
threshold, treatment should be considered.
The
threshold for sorghum also varies with the size of the insect and the crop
value. The worry starts when the
caterpillars found are ¼ of an inch or larger.
While they are ¼ to ½ of an inch long, the way to determine the
threshold by considering the economic injury:
Number of larvae per head =
|
When they are larger than ½ of
an inch, the formula to calculate the threshold by considering economic injury
is:
Number of larvae per head =
Cost
of control in dollars per acre x 9754
Grain
value in dollars per cwt x number
of
heads per acre
A sorghum headworm ET
threshold calculator is available for Droid phones from
the Google Play Store at http://goo.gl/8mXvv, or for other
systems on the internet at http://goo.gl/5k7ZtU.
Good Luck! Please call or come by
the office if
you have any questions. Thanks!
Kate
Blayne
References
1 http://amarillo.tamu.edu/files/2010/11/Panhandle-Pest-Update-v4i11-8-24-2012.pdf.
Panhandle Pest Update on August 12, 2013
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