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Monday, August 12, 2013

Bollworm or Headworm


 
        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 =
 

Cost of control in dollars per acre x 9754
Grain value in dollars per cwt x number
of heads per acre x 0.19
 


                                                   
     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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