Life Cycle of Spodoptera

Armyworms eat beet greens and other broad-leafed plants.

Moths from the genus Spodoptera are named for their larva which are commonly called armyworms. Armyworms are common in the southern United States, throughout Central and South America and through Australia and New Zealand. Voracious eaters, they can wreak havoc on crops and pastureland. The armyworm's very short four-stage life cycle, as little as three weeks in warm temperatures, allows several generations to be born during a tropical summer. Uncontrolled, armyworm numbers can increase quickly.

Egg

Eggs are usually laid in batches of 30 to 300 on the lower surface of leaves. The eggs are covered with hairs shed from the female moth. Within two to five days, nearly all of the eggs will produce larvae which will begin feeding on the host plant.

Larva

Larvae start off as green caterpillars but, molting six times throughout the 18-day stage, mature into large, fat dark gray worms with black and white markings and a yellow line down each side of their backs. Armyworms feed throughout the day and night. They do well in warm, humid weather while hot dry conditions can reduce their numbers. The digestive system of the armyworm larva adapts to the scarcity or abundance of food. If food is scarce, the armyworm will efficiently extract and retain nutrients. If food is plentiful, the armyworm's digestion will work rapidly and the larva will eat more than it needs for growth and survival. Armyworms' tendency to overeat contributes to their destructiveness.

Pupa

The armyworm pupa is dark reddish brown. The larvae dig a shallow burrow in the ground to pupate. Pupae like warm soil but will die if the soil is too hot. The pupal stage lasts about five days in summer and can stretch to as many as 31 days in winter.

Adult

Adult armyworm moths typically emerge at night and are active from dusk to dawn. Some species will migrate several miles while others remain closer to home and spread through increasing population. Females mate within about a day of emerging and may mate a second time during their lifetimes. Males mate as many as six times. Two days after mating, females lay an average of 1,000 eggs each. The lifespan of an adult moth can be as short as two days or as long as three weeks.

Favorite Foods

Larvae feed on the foliage of beets, beans, cabbage, celery, tobacco, clover and broad-leafed weeds. Most species leave grass alone. Larvae prefer young, tender portions of the plant, favoring new leaves, buds and flowers.

References

• HortFACT: Tropical Armyworm
• USDA: New Pest Response Guidelines--Spodoptera