Armyworms

Beet armyworm egg masses have a distinctive cottony appearance. The pale green, early instars of the beet armyworm often feed gregariously for the first few days. Larger larvae range from green to black and often have a broad stripe along each side. The larger larvae feed singly. Although newly emerged adults can be found throughout the year in central and southern California, these armyworms usually reach their highest population levels from spring through late summer.

Armyworm larvae are variable in color, but are usually dark green or gray with three thick stripes on each side. First instar larvae loop as they move, but older larvae move the same way as other armyworms. This insect overwinters as larvae in the soil or under crop debris and emerges in spring to feed and then pupate in the soil. Adult female moths lay tiny white eggs in clusters or rows (up to 500) on leaves and fold the leaf over the eggs, fastening it with a sticky secretion.

Larvae of the yellowstriped armyworm are almost black with two prominent and many fine bright yellow stripes on the side. At maturity, the yellowstriped armyworm larva is about 1.5 to 2 inches long. Eggs are laid in clusters and covered with a gray, cottony material. This pest may be abundant any time from June to early September.

In perennial cropping systems, beet armyworm occurs during summer when the adults move into artichokes from other crops. Heavy feeding damage to the growing point of the plant causes it to produce callus tissue where new young leaves and flower stalks are normally formed. Plants thus affected may have normal-appearing leaves on the outside but only a ball of undifferentiated tissue in the center of the plant near the base. This distorted tissue will not form new leaves or artichoke buds. The damage to the fall production can be 3 to 5%. On older plants, larvae feed on leaves and bracts of floral heads. Feeding on leaves is usually not economically damaging but feeding on bracts will result in culled heads.

On annual artichokes grown on the southern coast of California and in the southern California desert, a complex of armyworms (beet armyworm, the armyworm, yellowstripedarmyworm) occurs. In this cropping system, larval feeding on the young seedlings soon after transplanting results in stand loss. Rarely is the damage serious enough to consider replanting. Once the seedlings are established and start pushing new growth, the crop can tolerate armyworm damage. Armyworm causes negligible bud damage in annual artichokes.

Weed control in and around the field can help reduce an important source of infestation. Monitor adult activity and the crop to detect developing populations.

Disc fields immediately following harvest to remove the food source for any remaining larvae. Some pupae may be killed by discing as well.

Cultural control and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically certified crops.

Beet armyworm eggs and larvae are often easier to find on weeds in and near the field than on the artichoke plant. Chenopodium species (e.g., lambsquarters, goosefoot) appear to be particularly attractive to larvae. Populations can build rapidly, so check fields twice a week. Monitor adults with pheromone traps placed along the edges of fields. This is a particularly good technique for detecting large emergences or migrations. Inspect the artichoke plants by gently pushing the central leaves apart and inspecting the growing point for frass (larval excrement). Treat when fresh frass is observed in a significant number of plants in a perennial crop. Make treatments when larvae are small; large larvae are more difficult to kill with compounds such as Bacillus thuringiensis. Because larvae become active at dusk and sunlight degrades many pesticides, the best time for insecticide treatment is in the twilight evening hours.