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Beet Armyworm
Classification and Description — Beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), eggs are laid in a mass of 25-75 eggs covered loosely by white scales from the female moth’s body. Moth forewings 
are mottled grayish-brown, and hind wings are silvery white. Moths have a wing span of about 1 - 1 ¼ inches. The larvae are typically green, but a variety of color phases are possible. A characteristic small black dot is generally present on either side of larvae on the second segment behind the head. Like most other caterpillars found in cotton, except loopers, larvae have five pairs of prolegs. Larvae reach a maximum size of about 1¼ inch in length.
Hosts and Distribution — The beet armyworm is widely distributed across the southern United States during the summer and fall. Like the fall armyworm, this pest is subtropical in origin. It is incapable of overwintering in Tennessee, and moths migrate into Tennessee from southern latitudes on an annual basis. Besides cotton, the beet armyworm is an occasional pest of soybeans and a number of vegetable crops. It also feeds on a variety of wild hosts including pigweed, which is a preferred host.
Life History — Beet armyworm eggs hatch in 2-4 days, and after hatching, small beet armyworm larvae usually feed gregariously for about five days. The larvae then disperse. Larvae go through 5-6 instars before pupating in the soil. It takes 25-30 days for development from the egg to adult stage at normal summer temperatures. Four or more generations per year are possible.
Pest Status and Injury — The beet armyworm is an occasional pest of cotton, particularly during dry years. Economic infestations are much more likely when broad-spectrum insecticides have reduced populations of natural enemies. This pest can completely destroy cotton fields when population levels are high. Larvae are primarily defoliators but will also feed on squares and bolls. The window-pane feeding of small larvae (pictured above) is characteristic of their feeding. In cotton, these feeding sites are called “hits” (= actively feeding group of larvae on leaf or plant). Beet armyworms are capable of causing economic damage to the original Bt cotton technology (i.e., Bollgard cotton).
Management Considerations — Beneficial insects are important in preventing beet armyworm outbreaks. Besides general predators, a parasitic wasp (Cotesia marginiventris) will attack both beet and fall armyworm larvae. This species makes a characteristic cocoon on the plants after it emerges from a parasitized larva. Beet armyworm treatment thresholds are based on the number of feeding hits found in a given number of row feet. These thresholds and recommended insecticides are listed in the Insect Control Recommendations for Field Crops (PB1768). Historically, beet armyworm larvae have been difficult to control with insecticides, but newer insecticides such as Tracer, Steward and Intrepid are more effective. Second-generation Bt cottons (e.g., Bollgard II and WideStrike) provide better control than the original Bt technology.
Print-Ready PDF Publication (Beet Armyworm W029)