Armyworm

Armyworm larvae vary in color but are usually dark green or gray, with three thick stripes running down each side. First instar larvae move by "looping", but older larvae move the same way as other caterpillars. Adult moths fly at night, have a wingspan of about 1.5 inches (about 35–45 mm), and a single white spot in the middle of their buff-colored forewing.

Armyworms occasionally mass migrate from grasses when their food source is being depleted. They can damage lettuce seedlings by feeding in the crown and chewing away the midrib and growing point. Once heads form, these caterpillars can bore into them, making the heads unmarketable.

Many natural enemies attack armyworms. Among the most common are the parasitoid wasps (Hyposoter exiguae, Chelonus insularis, Meteorus autographae, Cotesia marginiventris, and Trichogramma spp.), the tachinid fly, Lespesia archippivora, predaceous bugs (minute pirate, bigeyed, damsel, assassin), spiders (Araneae: Lycosidae and Phalangiidae), and beetles e.g., ground beetles (Carabidae). Viral diseases also kill significant numbers.

Disc fields immediately following harvest to kill larvae and pupae. Destroy weeds along field borders.

Use biological and cultural controls and sprays of Bacillus thuringiensis and the Entrust formulation of spinosad on organically certified crops. Note that spinosad is detrimental to beneficial syrphid flies and parasitoids.

Begin monitoring before seedlings emerge by checking for egg masses and young larvae in surrounding weeds. If numbers are high on weeds, watch carefully for larvae on seedlings.

• From germination to head formation, check at least 25 plants in each quadrant of a 40- to 80-acre field twice a week. In fields where the crop is heading, stop at five different locations in each quadrant and sample five plants at each location. Fields smaller than 40 acres may require fewer samples.
• Check for egg masses and young larvae. Loopers, cabbageworms, armyworms, corn earworms, tobacco budworms, cutworms, and other caterpillars that feed on leaves and heads of lettuce can be assessed together, but species identification is important for selecting the appropriate insecticide.

Most insecticides are more effective against young larvae than against eggs, so wait until the majority of eggs have hatched before treating. Apply insecticides if you find

• an average of one second or third instar larva per 10 young plants.
• more than one larva on every two older plants (after thinning and before heading).

Apply insecticides just before heading if this threshold is exceeded. For armyworm control, the best time to apply insecticide is at dawn or dusk (twilight hours).