- Pest Type: Insect
- Crops Affected: Cotton, Rice
- Scientific Name: Spodoptera praefica
- Pest Order: Lepidoptera
Larvae are usually green to brown caterpillars, commonly 1-1.5 inches in length. Adults are moths with a wingspan of about 1-1.5 inches. The beet armyworm larvae are green or black, with a dark head, 5 pairs of prolegs, and on the second segment behind the head, there is a small black spot on each side of the body. Fall armyworm larvae have a longitudinal, dark colored stripe along each side of the body and a wider, yellowish-gray strip runs down the back. The head of the fall armyworm larva is often marked with a pale, prominent inverted Y. When less than 0.25 inch, fall armyworm and bollworm are very similar in appearance. Western yellowstiped armyworm larvae are dark colored with a broad yellow stripe on each side, they have a black spot on the side of the first abdominal segment, and they also have an inverted Y, white in this case, on the front of the head. Finally, the yellow-striped armyworm larvae have a pair of dorsal, triangular, black spots on most of the segments; and they have 3 lines on the back - an outer bright orange stripe on each side of a median yellowish white line.
Called armyworms because during the damaging larval stage, they move in large numbers from field to field in the manner of armies. Damage is caused by the larval, or caterpillar, stage which feeds on the foliage. The undersides of leaves and terminal growth are preferred feeding sites of young larvae. When populations reach high levels, armyworms can completely defoliate a field. Feeding can also result in severe stunting or death of seedlings. Armyworms feed on most crops and several weed species. In cotton, most damage occurs when larvae feed on developing bolls.
In warm climates (i.e., cotton growing areas), armyworms are present most of the year, with 4 or more generations per year. Under ideal, warm conditions, an entire generation can develop within only 3 weeks, but more frequently in 4-5 weeks. Where winters are more severe, there are fewer generations per year. Female armyworm moths lay eggs on plant foliage and those hatch to form small caterpillars, which immediately begin to feed. Larvae pass through 5 or more instars (molts) before pupating in the soil. Pupae hatch, usually within a few days, into the adult moths which mate and begin the life cycle again. The adult moths are most active at night.
Various insecticides, including bt in some cases, are registered for control of armyworm larvae. Such applications need to be targeted against small larvae (0.5 inch long or less) for best results. Armyworm populations may be difficult to control, and repeat applications may be necessary based on monitoring data. If you have an armyworm infestation, consult your local adviser for recommendations in your area.