cows in field

Armyworms On The March

Forage & Grains

Added July 16, 2012. The early warm weather has brought an invasion of true armyworm (as opposed to Fall armyworm) to New York and Vermont, causing damage in young corn crops, pasture and hay fields. Armyworm will eat anything grassy in vast quantities, they feed at night so your pasture may look fine this afternoon and be virtually gone by morning. They will also eat alfalfa, and bean, cabbage, carrot, onions, pea, pepper, and radish leaves and other broad-leafed plants when the grasses are exhausted.

They get their name from moving through a field in an "army-like" fashion, devouring plants as they go.

Armyworms do not overwinter in the northeast but adult moths fly north from Southern states in the spring along storm fronts. Moths lay their eggs on weeds and/or grasses along the edge of a field, on leaves of corn, or on small grains. Larvae hatch about a week later and develop over a 3 week period, feeding mostly at night. True armyworm larvae appear smooth, cylindrical, pale green to brownish when they are still small. Mature larvae are smooth and marked with two orange, white-bordered strips on each side. Larvae range in size from 1/8 inch when they first hatch out to 1 1/2 inches long at the end of the 3-week period. Larger larvae eat more, with 80% of the feeding damage happening in the last 7 days before the pupation stage.

When the first armyworm larvae are done feeding they will burrow down into the soil and pupate. In 10 to 14 days they will emerge as adult moths and mate. Females will begin to lay eggs all over again. In past seasons we have not seen any economic damage from this second generation. The natural enemies have traditionally built up their populations and the armyworm population is kept in check. But this has been a very non-traditional year so far! There will certainly be more predators than there were back in June, but there is also the potential for a very large second brood, given the size of the first brood, especially in western NY. The one "good" thing is that most of the grass in pastures and lawns in Western New York are totally dormant in the severe drought, so there won't be as much for the worms to eat this second time around.

The effectiveness of spraying and being able to spray at the correct time makes it marginally economical. To be successful with any spray the larvae have to be young. Spray may be justified if there are 4 or more larvae per square foot in pasture or 3 per leaf in corn. For organic farmers, Entrust and Pyganic, the 2 organically approved materials, are expensive. Dipel is labeled for armyworm but field reports suggest that this particular invasion seems to be fairly immune to Bt perhaps because they have built immunity by eating genetically modified Bt resistant crops. GMO's making life difficult again for organic producers.

It may be best to leave it to the natural predators; larger populations of true rmyworm predators will be present with the second batch of larvae. A major natural enemy to the armyworm is a large parasitic fly that lays a white egg on the back of the armyworm. The emerging maggot burrows into the armyworm, feeding on it and finally killing it. A second natural enemy is a virus that infects the armyworm. The virus causes the armyworm to crawl to the top of the plant and die. The wind spreads the virus to other nearby plants.

To learn more about true armyworm, and its impact in your general area, contact your local extension office. New York Cooperative Extension has put together quite a lot of information regarding spray materials, threshholds, insect life cycle, and more.