Beet Armyworm

The beet armyworm adult is a gray and brown moth that lays its masses of round, pale-colored eggs beneath a covering of hairlike fluff collected from their wings. Newly hatched armyworms are often green and feed in groups, skeletonizing the undersides of leaves. Older beet armyworm larvae are green and smooth skinned with light stripes lengthwise along their sides. They commonly have a black spot on their side above the second leg.

Moths from overwintering larvae lay eggs in spring (late winter in Southern California). Young larvae feed on foliage and crowns before attacking berries. Damage most commonly occurs in Southern California and Santa Maria growing areas, but damage can be serious in any region if larvae feed on the crowns of newly transplanted strawberry plants. Feeding at this time can kill the young transplants. Damage also can occur to summer-planted strawberries. Fall populations of adult females often fly into strawberry fields to lay eggs. Newly hatched armyworms feed on foliage, skeletonizing the upper or lower leaf surfaces next to their egg mass. Beet armyworm numbers can become greater in previously infested second-year plantings and damage fruits in spring. Larger armyworms feed directly into the berries, while smaller armyworms often feed on the shoulder of the berry beneath the calyx sepals.

As with lygus and cutworm management, weed control in and around fields is an important aspect of managing armyworms. Insecticides may be necessary in Southern California if beet armyworm numbers are high around the time of transplanting. At other times, consider the level of parasitism and mortality from disease before making the decision to spray for beet armyworm.

Young beet armyworms can be heavily parasitized by the ichneumonid parasitic wasp, Hyposoter exiguae. This parasite can easily be monitored in the armyworm populations by simply pulling young worms apart and looking for the parasite larva inside. In addition, armyworms often become diseased with a virus that can cause high mortality; larvae turn black when killed by the virus. High natural mortality translates to few mature larvae surviving to cause further damage.

Because adult moths are attracted to weeds for egg laying, good weed control helps minimize armyworm numbers.

Use cultural and naturally occurring biological controls, and sprays of Bacillus thuringiensis ssp. aizawai or the Entrust formulation of spinosad on organically certified strawberries.

In Southern California and the Santa Maria growing area, plants are most vulnerable to beet armyworms soon after transplanting when larval feeding in the crown can kill the young transplants. Monitor beet armyworms flights with pheromone traps just before and after transplanting. If moth catches indicate a lot of beet armyworm activity, examine young strawberry plants for egg masses and time treatments to egg hatch.

At other times of the year and in other areas, if large numbers of predators, parasites, or virus are present, delay spraying to determine if armyworms will be controlled by natural enemies.