AgriGold

Stalk Borer Damage Becomes Evident

by Jacob Bates, CCA, August 16, 2010

As you drive down the road this August, you may notice that some of the plants in your corn field don't look quite like they should.  You will most likely see a few random plants, primarily along field edges, with a reddish-purple coloring.  Symptoms like this tend to remind us of 2nd generation European Corn Borer (ECB) damage from years past.  However, the culprit this year is due to the common stalk borer.

 Even traited corn may see some stalk borer damage this year.  Unfortunately, most in-plant insect protection on the market today is only labeled for suppression of stalk borer.  However, Smartstax and VT3Pro, with 2 modes of action, should control the pest with limited plant feeding.

 Biology & Life Cycle

Young stalk borer larvae are brownish-purple and have three prominent longitudinal white stripes at the front and rear ends of the body.  The stripes are interrupted at the mid-body by a solid purple area, sometimes called a "purple saddle".  Fully grown larvae lose some of these markings and become a dirty gray.  All larvae have a black stripe on each side of the head.

 The common stalk borer has only one generation per year.  The eggs hatch every spring from grassy areas or weedy fields where they overwintered.  Fields with ragweed, wirestem muhly, quack-grass and giant foxtail could see a higher occurrence of stalk borer.  As larvae, the damaging stage, they feed on non-crop hosts until they become too large for the host.  Once they increase in size they begin their migration to corn fields where they bore into the stalk. 

  Damage & Control Practices

The worst damage is inflicted to the outer perimeter of the field, next to fence rows or in areas with past weed problems.  Stalk borer larvae attack corn by entering the stalk at ground level and tunneling upward or entering higher up the plant and tunneling downward.    In both instances, the larvae will eat the heart of the corn plant resulting in what is termed "dead heart".  This feeding pattern essentially cuts off the main piping system of sugars and photosynthate for the plant and as a result plants will turn reddish-purple and eventually wilt.  Larvae progress through 7-10 instars and feed between 8-10 weeks.  The larvae will mature and pupate inside the corn stalk.

 There are no effective treatment options once the larvae have entered the stalk.  Insecticide applications must coincide with movement of the larvae from one host plant to another to be effective.  Any weed control method that helps eliminate grasses and other weed species will reduce the potential number of egg-laying sites and hopefully reduce stalk borer damage next year. 

 Remember this is primarily an issue you will see along fence rows, field edges, grass terraces and waterways.  Yield losses are expected to be minimal due to the randomness of plant injury.

Categories: Insect Control

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