The beet armyworm, Spodoptera exigua, has a wide host range, occurring as a serious pest of vegetable, field, and flower crops. Among susceptible vegetable crops are asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, chickpea, corn, cowpea, eggplant, lettuce, onion, pea, pepper, potato, radish, spinach, sweet potato, tomato, and turnip. Field crops damaged include alfalfa, corn, cotton, peanut, safflower, sorghum, soybean, sugar beet, and tobacco. Weeds also are suitable for larval development, including such common plants as lambsquarters, mullein, pigweed, purslane, Russian thistle, and parthenium.
Eggs are laid in clusters of 50 to 150 eggs per mass. Normal egg production is about 300 to 600 per female. Eggs are deposited on the lower surface of the leaf, often near blossoms and the tips of branches. The individual eggs are circular when viewed from above. When examined from the side, the egg is slightly peaked, tapering to a point. The eggs are greenish to white and covered with a layer of whitish scales that gives the egg mass a fuzzy or cottony appearance. Eggs hatch in two to three days during warm weather.
There normally are five larval instars. The larvae are pale green or yellow during early instars, but acquire a dark lateral stripe during the fourth instar. Pupation occurs in the soil. The pupa is light brown and measures about 15 to 20 millimeters (mm) in length. Duration of the pupal stage is six to seven days during warm weather.
The moths are moderately sized, the wing span measuring 25 to 30 mm. The forewings are mottled gray and brown, normally with an irregular banding pattern and a light colored bean-shaped spot. The hind wings are a more uniform gray or white color and trimmed with a dark line at the margin.
Survival And Spread
Seasonal activity varies considerably according to temperature. In Florida, all stages can be found throughout the year, although development rate and overall abundance are reduced during the winter months. The life cycle can be completed in as few as 24 days, and six generations have been reared during five months of summer weather in Florida.
Larvae feed on both foliage and fruit. They feed gregariously and skeletonize foliage. As they mature, larvae become solitary and eat large irregular holes in foliage. They also burrow into the crown or center of the head on lettuce, or on the buds of cole crops.
Pheromone traps can be used to detect the presence of adult beet armyworm. Regular monitoring of crops, probably about twice per week, is recommended because adults frequently invade from surrounding crops or weeds. Pheromones also can be used to disrupt mating and inhibit or eliminate reproduction. Due to insecticide resistance concerns, growers should rotate between different modes of action.
Various Bt formulations have long provided a standby for management efforts and should be a starting point for IPM programs. Lep materials such as Avaunt (indoxacarb, DuPont Crop Protection), Coragen (rynaxpyr, DuPont Crop Protection), Confirm (tebufenozide, Gowan Co.), Intrepid (methoxyfenozide, Dow AgroSciences), Proclaim (emamectin benzoate, Syngenta), Radiant (spinetoram, Dow AgroSciences), and many others are now on the market so growers have a number of options available and should use rotations of these materials for optimal control.
Gene McAvoy is Regional Vegetable Extension Agent IV with the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)