ARTHROPOD MUSEUM NOTES
Number 103 • May 9, 2013 • Jeffrey K. Barnes
Genus and species: Spodoptera exigua (Hübner)
The beet armyworm is a serious pest of a wide range of crops, including various table vegetables cotton, cereals, flowers, and tobacco. It also consumes many weed species. Young larvae feed gregariously on leaf parenchyma, leaving behind only the epidermis and veins. Larger larvae are solitary and eat holes in foliage. They also have a habit of burrowing into thick areas of plants - into a head of lettuce, a maturing tomato, or an onion leaf, for example. They feed on both foliage and fruit. The caterpillars consume buds and new growth, preventing flowers, leaves, and vegetables from developing.
Although coloration can be quite variable, the caterpillars are typically pale green with darker, lateral, longitudinal stripes, and they often have a diagnostic small dark spot behind the head laterally, above the second pair of true legs. Unlike fall armyworms, they lack stout, black setae on the body. Mature caterpillars are about 1 ¼ inches long. Adults are small, drab, mottled brown and grey moths. They have a wingspan of about one inch, but living adults at rest hold their wings draped over the sides of the body in a tent-like fashion.
Females deposit eggs in scaly clusters of 50–150, usually on undersides of leaves. Eggs hatch in 2–3 days. Larvae develop in about 10–20 days and burrow in the ground to pupate. Adults emerge in 6–7 days in warm weather and live for only about 10 days. In Arkansas, several generations occur annually.
Beet armyworm can winter in warm areas, such as Florida, Texas, and Arizona, where all life stages can be found year round. However, it does not survive the winter in areas where its host plants freeze. Those areas are reinvaded by adult moths in the spring. The reinvasion reaches as far north as the line from Maryland to Colorado to northern California. Beet armyworm is rarely a pest in the most northerly parts of its range, but it is a serious pest in southern states, including Arkansas.
In Arkansas, beet armyworms tend to be abundant under drought conditions. They tend to develop large populations in soybean fields late in the season, feeding on flowers, beans, and leaves. They are a pest in cotton fields, with young larvae skeletonizing leaves and older larvae feeding in foliage, squares, flowers, and bolls. The gregarious larvae often produce spotty damage only in certain areas of a field. In recent years, beet armyworms have become an increasing problem to Arkansas spinach producers. This might be due to use of minimal tillage in nearby soybean fields. Pigweed in particular is more abundant than formerly, and this weed is a favored host of the beet armyworm.