Beet armyworm and cabbage looper are destructive pests of lettuce and other desert vegetable crops grown around Yuma County, Ariz., according to University of Arizona entomologists.
The cabbage looper will feed on cole crops, leafy greens, melons, and tomatoes.
Populations are especially a problem in the fall, when newly-planted winter vegetables are emerging.
Cabbage looper moths lay single, dome-shaped eggs on the under side of older leaves. A single female may lay 275 to 350 eggs. Eggs will darken as they age, and will hatch in 2 to 5 days. The larvae are light green in color and have a distinctive white stripe along each side of the body.
The larvae have two sets of legs in the front of the body and three sets of fatter, unjointed prolegs at the rear. They move in a “looping” manner, arching the middle portion of the body as they move forward. Two to four weeks are required for full development to a 5th instar larva. Cabbage looper pupae appear as greenish to brown pupas wrapped in a delicate white cocoon of fine threads usually attached to the underside of the leaf. Pupation usually takes 10 to 16 days. The moth is mottled brown in color, and has a small silvery spot near the middle of its front wing. Cabbage loopers may have 3 to 5 generations per year.
Loopers damage plants by eating ragged holes in leaves, and sometimes working their way into heads. They also cause damage by contaminating marketable portions with their bodies and frass. High populations can chew seedlings severely enough to kill them or slow growth enough to inhibit uniform maturing of the crop, but most economic damage occurs after heading. Young plants between thinning and heading can tolerate substantial feeding by loopers and other caterpillars without loss of yield or quality. Heads contaminated with loopers, or tunneled into by loopers are not marketable.
Monitoring for cabbage looper and other lepidopterous pests should involve sampling plants twice a week once seedling emergence begins. When populations appear to be increasing, check more often.
On lettuce, monitor for eggs and larvae of loopers while checking for other caterpillar pests that feed on leaves and heads. Action thresholds are similar to those of beet armyworm: treat seedlings or small plants when populations of small loopers are large enough to stunt growth. If other lepidopterous species are present, also include them in this total. Between thinning and heading, treat if the worm population reaches one larva per 50 plants. During head formation, treat if sample counts exceed one larva per 25 plants. Cabbage loopers are especially sensitive to B.ts. Including a B.t. with insecticide applications targeting beet armyworms will usually control any cabbage loopers present.
Beet armyworm is also most prevalent from August through November on fall-planted lettuce. However, when temperatures are warm, this pest can be a problem season-long, particularly if alfalfa is nearby. The larvae feed on many field crops and often migrate from these crops onto lettuce in the fall. Several summer annual weeds also serve as hosts.
Eggs are light green in color and are laid in irregular clumps or masses, usually on the under surface of leaves. One female will lay on average 500 to 600 eggs over a 4 to 10 day period. The female moth covers the eggs with white scales from her body, giving the egg masses a cottony appearance. Eggs will darken as they near hatching, and will hatch in 2 to 5 days. The young larvae will feed in groups and spin webs over the underside of the foliage where they are feeding.
Larvae vary in color, but are usually olive green with light-colored stripes down the back and a broader stripe along each side. Beet armyworms usually have a dark spot on the side of the body above the second true leg. Mature larvae vary in size will generally pass through five instars.
Armyworm larvae disperse as they get older and move toward the center of the plant. Large larvae are quite mobile, and a single larva may attack several plants. Larvae reach maturity in about 2 to 3 weeks in warm weather and pupate in the soil. The moth has grayish-brown forewings with a pale spot in the mid-front margin, and the hindwings are white with a dark anterior margin. The wingspan of an adult is approximately 1 1/4 inches. The entire life cycle from egg to adult requires approximately 36 days at 80 degrees.
Hatching larvae begin feeding on the leaf and may completely consume seedlings. Beet armyworms may severely stunt or kill seedling lettuce plants. Damage to lettuce is usually not economically damaging between thinning and cupping stages unless populations are high. However, once cupping begins, larvae may feed on the head, rendering it unmarketable. Armyworm larvae enter heads from the bottom working their way inward while feeding along the leaf margins. Often the damage cannot be seen without removing frame leaves and dissecting the head.
Cultural controls can help suppress armyworm populations. Disk fields immediately following harvest to kill larvae and pupae. Sanitation along field borders is important; armyworms often migrate from weedy field edges into newly planted fields.
Monitoring for beet armyworm on lettuce should begin before seedlings emerge. (Use these same guidelines for monitoring cabbage looper).
Control of beet armyworms on seedling lettuce is essential for stand establishment. Check weeds on ditch banks and field borders for larvae and egg masses as fields are being seeded. Once seedlings emerge, sample at least 25 plants in each quadrant of the field twice a week for armyworm egg masses and young larvae. The action threshold for all lepidopterous larvae in fall lettuce between thinning and heading is one first or second instar larva for every 50 plants.
Good spray coverage and insecticide resistance management tactics should be practiced. Target small larvae which are easier to control with insecticides. Timing insecticide applications at peak egg hatch will improve control. Addition of a B.t. to conventional insecticides will usually increase control and aid in resistance management. Just before and after heading, treat if lepidopterous larvae reach one worm per 25 plants.