Eric Sideman's Pest Report

7/9/01

Army worm

The critter is the army worm. Luckily there are many natural enemies of the army worm and it is rarely a major problem two years in a row in any location because as the pest populations build up so do its natural enemies. There are many natural enemies of the army worm including ground beetles, parasitic wasps, and birds. The most effective enemy is the red-tailed tachina-fly. The army worm is related to the cut worm and somewhat has that same greasy appearance. They are gray to black with a narrow yellow stripe on their back and a wider yellow stripe on each side. There is some question as to whether the army worm can over winter in Maine. Where it does, it over winters as a hibernating, partially grown caterpillar. The young larvae feed in the spring, not usually doing much damage, pupate in May, and the moths of the first generation appear in June. The females lay about seven hundred eggs each that hatch in about ten days. The moths tend to cluster when egg laying and so you see patches of very dense populations of caterpillars. The caterpillars from these eggs are the destructive worms that you see now feeding. The worms feed mostly at night when their enemies are sleeping. Leaves of grass are their favorite food and many fields of hay and small grains have been destroyed already. As the worm finishes the food in an area they march along spreading out from the patch and devouring grass along the way, hence the name army worm.

It takes about three to four weeks for the worms to reach their full grown size of about 1.5 to 2 inches, at which time they burrow into the soil and pupate. New moths emerge in about two weeks. These lay eggs for another generation of army worms but this will not be nearly as destructive as what is happening now.

Controls may be late now. In hay crops the best bet is to harvest the crop if close to ready. The best pesticide available to organic growers will be a Bt. Dipel Df (2 lb/A) or Xenteri (1.5 lb/A) are labeled for army worm. I tried Condor yesterday and saw good results with it. However, the larger stages of the caterpillar are much harder to kill. The Condor killed caterpillars that were an inch or so. I am not sure whether it is cost effective to spray hay ground, but if the army worm is in your sweet corn you better spray.

In Dwight Sanderson's 1912 copy of Insect Pests of Farm, Garden and Orchard he talks about building a dust furrow as a barrier to their progress. He says to make a dust furrow plow a deep furrow around the field to be protected and thoroughly pulverize the soil by dragging a heavy log back and forth in the furrow making the side towards the protected crop as steep as possible. In attempting to climb this barrier, the dust will slide from under the insect and large numbers will collect in the bottom of the furrow and die.