Beet Armyworm

Beet armyworm larvae are 1 inch long when fully grown. They are usually dull green but color can vary from pale to dark green with wavy, light-colored stripes running down the back and a broader pale stripe along each side. They usually have a dark spot on each side of the body above the second pair of true legs. Eggs are laid in clusters. These clusters are covered with dirty white, hairlike scales, forming a cottony covering. Adult beet armyworms are mottled gray and brown moths with a wingspan of a little over 1 inch. There are three to five generations a year. The pupa is the overwintering stage, but all stages may be present throughout the year in the low desert agricultural production valleys of southeastern California.

Beet armyworm destroys seedlings, terminals of young plants, and squares and small bolls during early July. Early season infestations may develop on weeds and move to cotton when weeds are controlled, destroying seedling cotton or the terminals of older plants. As cotton plants grow, young larvae skeletonize leaves and bracts, often spinning loose webbing over the feeding site. Older larvae chew irregular holes in leaves and also feed on squares, flowers, and bolls. Square damage by the beet armyworm differs from bollworm damage in that the surrounding bracts and foliage are often damaged by the beet armyworm but not by bollworm. The loss of a majority of squares and bolls during July or August may reduce yield or delay maturity by delaying fruit set. Severe defoliation may cause crop loss as well.

In addition to cotton, beet armyworms feed on alfalfa, vegetables, sugarbeets, beans, and weeds such as pigweeds (Amaranthus spp.) and nettleleaf goosefoot (Chenopodium murale). In occasional years, there may be widespread outbreaks when favorable weather allows exceptionally large populations to build up early in the season on alternate hosts. Damaging populations may also occur where insecticides applied for other pests reduce natural enemy populations. Watch for beet armyworm on adjacent crops and on weeds in and around the field. If many larvae are present on weeds while cotton plants are small, it may be worthwhile to use an insecticide to kill them before destroying the weeds. Otherwise, they could move to the seedlings and cause stand loss. Treatment of a limited area, such as a strip at the edge of the field, is usually successful. When selecting an insecticide from a group of effective products, always select the insecticide that is least harmful to natural enemies.

Many predators and parasites combine to substantially maintain armyworm populations at low levels. Predators include assassin bugs, bigeyed bugs, spiders, minute pirate bugs, damsel bugs, and lacewings. The parasitic wasp, Hyposoter exiguae, is believed to be the most important of at least 10 parasites attacking this pest; other parasitic wasps include Trichogramma spp. and Cotesia marginiventris. Virus and bacterial diseases can also be important. Insecticide sprays for other pests may disrupt natural control.

A recently developed transgenic cotton, Bollguard II, offers suppression of a broader range of caterpillars, such as beet armyworms, cotton bollworm, pink bollworm, and tobacco budworm, than earlier Bt varieties (Bollguard I).

Biological control and applications of Bacillus thuringiensis and the Entrust formulation of spinosad are acceptable for use on organically grown cotton.

To manage insecticide resistance in beet armyworm, limit the total number of sprays of each insecticide. The best way to do this is to practice the basic principles of IPM:

• Monitor pests and maximize the use of biological and cultural controls.
• Spray only when pest numbers warrant an application.
• Use the most selective insecticides first to conserve natural enemy populations so they help with the control of pests.
• Do not use insecticides with the same mode of action number on successive beet armyworm generations.

The following table provides insecticide resistance management guidelines.

Early in the season, plants can sustain up to 50% loss of leaf surface without affecting yield. During the fruiting period, only 20 to 25% of the leaf surface can be lost without yield loss. After this period, up to 50% loss of leaf surface can again be tolerated. If beet armyworms build up on weeds in areas adjacent to the crop, consider treating a strip at the edge of the field to prevent entry into cotton.

When taking sweep net samples for lygus bug, also look for beet armyworm egg masses. The egg masses are covered with grayish white, hairlike scales and are laid on upper leaf surfaces in the upper plant canopy, but below the terminal area. Also watch for clusters of small, greenish caterpillars that feed in groups in leaf folds that are webbed together. To determine the actual number of caterpillars present, lay a 40-inch square piece of canvas between the rows and vigorously shake an arm's length of plants from one row onto the canvas. Count the number of armyworms on the canvas. There is no set treatment threshold for beet armyworm; it is up to the grower, based on past history and overall crop conditions, to determine if beet armyworm is causing significant economic losses to justify a treatment.