Armyworms

Adult beet armyworms are small, mottled gray- or dusky-winged moths. Females deposit pale greenish or pinkish striated eggs on leaves in small or large masses covered with white cottony material. Eggs hatch in a few days and tiny caterpillars begin feeding on the plant. When caterpillars are full grown in about 2 to 3 weeks, they are about 1.25 inches long. The color down the middle of the back may be olive green to almost black with a yellow stripe on each side of the body. There is a dark spot on each side of the thorax just above the middle leg. Beet armyworms may become abundant and cause severe injury in summer and fall.

Western yellowstriped armyworm may be abundant in fields in the Central Valley any time from June to early September. The caterpillar is usually black, with two prominent stripes and many narrow bright ones on each side. At maturity it is approximately 1.5 to 2 inches long. Eggs are laid in clusters and covered with a gray, cottony material.

Armyworms skeletonize leaves, leaving the veins largely intact. In severe infestations, as food becomes scarce they will consume the veins, petioles, and will even feed on the exposed portions of the beet root. If infestations occur very early in the crop, particularly during cotyledon stages of fall-planted beets, caterpillars can consume the entire plant and cause reductions in stand. During mid-season, severe defoliation can cause reductions in root size. During the latter parts of the season, regrowth that occurs to compensate for skeletonized leaves can reduce percentage sucrose in the harvested root.

Because of their ability to reach high numbers and cause severe defoliation, armyworms need to be monitored closely, particularly during the mid- and late summer. Control is attained through a combination of beneficial insects and a viral disease coupled with periodic insecticide applications.

Armyworm larvae are attacked by a parasitic wasp Hyposoter exiguae. Parasitized larvae can be identified by removing the head and rolling the internal contents of the worm out and looking for pale green parasite larva. Several other parasites also attack armyworm.

Armyworm eggs and small larvae are also preyed upon by numerous predators such as lacewings, minute pirate bugs, damsel bugs, and big-eyed bugs. Assassin bugs will feed on a range of sizes of larvae.

Virus and bacterial diseases of armyworms, the most common of which is nuclear polyhedrosis virus, provide some level of natural control. Diseased caterpillars first appear yellowish and limp, and after death hang from the plant as shapeless, dark tubes from which the disintegrated body contents ooze. Virus spreads as healthy caterpillars feed on leaf tissue containing virions on their surface. The level of virus infection in caterpillars is lowest early in the season and increases throughout the summer. The greatest impacts of virus are seen when caterpillar populations are allowed to be high, as increased pest density facilitates spread of the disease. In such cases caterpillar populations are often observed to crash quickly and remain that way for the remainder of the season.

Biological control and sprays of Bt and the Entrust formulation of spinosad are acceptable for use on organically certified sugarbeets.

Monitor for armyworm larvae by taking sweep samples in a couple of locations of each field weekly. Beet armyworm adults can be monitored using pheromone traps. Pheromone traps are used primarily to determine when each of the three to four flights occurs and for timing applications of insecticides, such as Bacillus thuringiensis, that target young larvae.

Economic thresholds based on worm populations have not been established for beet armyworm. Sugarbeets can compensate for considerable amounts of defoliation in the middle and latter parts of the season without reductions of yield or sucrose percentage. Treat only if natural (biological) pest suppression fails to bring the populations under control. Once the crop has neared irrigation shutoff, even severe defoliation will not effect yield, but may cause reductions in percentage sucrose. Treat only if there is sufficient defoliation to cause the plant to try to regrow foliage instead of storing energy in the taproot during the period from a few weeks before to just after cutting the water.