Friday, May 30, 2014

Is it an Armyworm or a "Look-a-Like"?



Missouri recently cautioned growers to watch for armyworms in it's Southern region, and Illinois has reported some armyworm in wheat as well.   It is therefore, time to conduct a little armyworm review:

The True Armyworm, Pseudaletia unipunctata, can actually be a pest in corn, pasture, and wheat.  Female armyworm moths do not overwinter in Illinois, Indiana, Iowa, Missouri, or Wisconsin (the Burrus/Hughes sales footprint).  Instead, moths migrate back into our area each growing season from the Southern United States.  Moths arrive in the Midwest and then deposit numerous eggs in small clumps on the lower portion of grasses.  The eggs hatch around early-mid May, and the resulting larvae tend to feed at night or during overcast days.  When not feeding on the plant, larvae hide in leaf sheaths and beneath crop debris.  When found on the soil surface, they are usually curled up.  The larvae tend to prefer grasses, but at high enough populations they will feed on broadleaf plants.  A generation of true armyworm larvae tends to appear in May/June, and in July/August (The Handbook of Corn Insects notes a possible third generation in some areas). 

 
Picture of a True Armyworm.


 True armyworm larvae have three pair of true legs near the head, and five pair of false legs (prolegs) toward the tail.  Each false leg has a dark diagonal smudge-like marking (an especially useful feature often used to identify the pest).

Correct identification is important because some look-a-like insects may be found in the field.  If those look-a-likes are assumed to be true armyworms, the grower might overestimate the population – assume an economic threshold has been hit – and spend dollars on insecticide that is not needed.  

A recent example of look-a-like found in Missouri was the white-lined sphinx moth caterpillar.
White lined sphinx caterpillar.
Threshold:
The armyworm scouting message is the same as it is for many pests – scout and scout frequently (at least weekly).  If one fails to do so, they just might miss the opportunity to catch armyworms, while they are still a minor issue. Six armyworms per foot of row is the type of population requiring an insecticide in wheat.  Two to three larvae per square foot likely require control in grass pasture.  In corn, damage to twenty-five percent of plants and/or some plant death is considered significant. Corn near grassy areas, corn near small grains, or corn planted into a cover crop tends to be slightly more at risk than corn elsewhere.


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