Just got off the phone with Marie, a seed dealer and DEC-certified pesticide applicator. I asked her if she, or any of her customers, had encountered any fall armyworm (Spodoptera frugiperda) damage in their crops. She said in early July two farmers had reported such damage in their field corn and small grains. These crop people live in northern Oneida County and southern Jefferson County. To a large extent we haven’t had to worry about S. frugiperda in the Northeast for quite a while. But I contend that understanding a potential enemy is always a good practice.
As far as a host is concerned, this species will feed on a very wide variety of greenery, but prefers plants in the grass family. Most turf and pasture grasses are subject to infestation by fall armyworms in late summer and fall. Large numbers can consume all above-ground plant parts, and they are capable of killing or severely retarding the growth of grasses. This species may migrate in large numbers in search of new areas in which to feed. The way they cover ground en masse almost resembles a military formation — hence their common name. The moths (mature S. frugiperda) are attracted to lights and may lay masses of eggs on non-host plants, walls, clothes on lines, etc. Armyworms particularly like to hide out in volunteering crops, such as sweet clover, wheat, and barley.
Fall armyworms do not overwinter in northern areas boasting frigid temperatures. Such northern tier states — like New England and those bordering the Great Lakes — are re-infested each year by moths that migrate northward from Texas or Mexico. They usually reach our region by early July. Each female lays about 1,000 eggs in clusters of 50 to several hundred. After feeding for 2 to 3 weeks, the larvae dig into the soil to pupate. A new generation of moths emerges about 2 weeks later. Some years overlapping generations may extend into late October.
Appearance-wise, male moths have dark gray front wings mottled with darker and lighter splotches. There is a prominent pale, diagonal marking near the center of the front half of each wing and a prominent white spot at the extreme tip. The front wings of female moths are dull gray brown with only small, inconspicuous markings. The hind wings of both sexes are white with a slight purplish sheen. The wingspread is about 1 1/2 inches. The eggs are pale gray, laid in clusters, covered with grayish, fuzzy scales from the body of the female moth. Mature larvae may be green, brown, or almost black and are about 1 1/2 inches long. There are black and reddish-brown stripes on each side of the body and four small, black spots on the dorsal side of each abdominal segment. The head capsule is mostly black and is marked with a pale, inverted “Y” on the front. Overall color intensity may range from faintly colored to those which are extremely dark.
The very name “armyworm” conveys images of defoliated cornfields. While this pest is hardly living up to this image in the Northeast during 2018, crop growers in Wisconsin are less fortunate. In that state damage ranges from barely threshold to near complete defoliation in isolated fields. Armyworms do not overwinter in Wisconsin, like New York. Migrating adults usually arrive early spring; this migration is usually not a single event but rather a sequence of arrivals over time.
Typically, armyworms like grassy areas to lay eggs, which explains some of the infestations. Wheat and other small grains are at risk until harvest. Before considering an insecticide application check with your entomology cooperative extension agent to make sure that any insecticide used fits in with your harvest plans. Many insecticides have a long required pre-harvest interval, which may prevent timely harvest. Pastures should also be monitored. If larvae run out of a food source, they move to adjacent crops and/or lawns. They may also move from adjacent marshes into fields.
Feeding in cornfields above the ear zone is a major economic concern. If you find signs of armyworm feeding, check five sets of 20 plants at random. Record the number of damaged plants and the number of worms per plant. Spot treat, if possible, when you find two or more armyworms (0.75-1.0 inch or smaller) per plant on 25 percent of the plants or one per plant on 75 percent of the plants. When making a treatment decision think about damage you can prevent — not worrying about how much damage already exists. Large larvae feed for a much shorter period of time. Adjacent fields may show different-sized larvae, thus complicating control decisions. Do not assume corn planted with an above ground Bt trait will not have damage. Trait packages vary in their insect control spectrum and may not provide adequate control under heavy infestations.
Here’s a real-life armyworm story. About two decades ago, I was advising a young couple who rented a dairy farm a few miles southeast of Middleburgh. They had a rental contract with the landowner. The landowner’s nephew wanted to take over his uncle’s farm. The owner agreed, but the contract wouldn’t run out for quite a while — and the couple couldn’t be convinced to void out the agreement.
So the owner took advantage of a clause in the lease agreement which allowed for the tenants to be evicted, if they fell behind three months in rent payments. The owner refused to deposit three consecutive rent checks and proceeded to evict the family for non-payment. The family and their cows (along with equipment) moved to her parents’ farm, also in Schoharie County. The nephew came home and took over the farm. Within days two things happened: one, the family welcomed a bew healthy baby; two, an epic proportion armyworm invasion obliterated any standing crops which the nephew was planning to harvest. This disaster bore strong resemblance to the locust plague mentioned in Exodus 12 in the Old Testament.