Armyworms

Beet Armyworm

Larvae are usually dull green and have wavy, light-colored stripes running lengthwise down the back and broader stripes on each side. Eggs are laid in a mass covered with hairlike scales.

Western Yellowstriped Armyworm

Females lay eggs in clusters covered with a gray, cottony material. Larvae measure about 1.5 inches (3.8 cm) long when fully grown. They are usually black with one prominent stripe over many narrow bright ones on each side of its body. The head is brown with a netted pattern. There is also a large black spot above the first abdominal spiracle.

Yellowstriped Armyworm

Larvae of the yellowstriped armyworm are almost black, with two prominent and many fine, bright yellow stripes on the side.

Primarily a foliage feeder, armyworms will also attack fruit, creating single or closely grouped circular or irregular holes on the surface. In many cases, feeding is superficial and little loss would result if not for decay organisms that enter wounds and rot fruit. The beet armyworm caterpillars occasionally develop inside the fruit. Yellowstriped armyworm does not enter the fruit and infestations are most severe from July to mid-September.

Yellowstriped armyworm is not a serious pest every year but is very destructive on occasion. Armyworms tend to build up in alfalfa and weedy areas around the field and migrate from these areas when cut. Armyworms only need to be controlled if it is feeding on the crop. Keep crop residue and weeds in field and surrounding areas to a minimum to lessen the attraction of the field.

Many natural enemies attack armyworms. Among the most common parasites are the wasps, Hyposoter exiguae and Chelonus insularis, and the tachinid fly, Lespesia archippivora. Armyworms can easily be checked for the presence of Hyposoter exiguae by pulling the larva apart and looking for the parasite larvae. Viral diseases also kill significant numbers.

Sanitation in the field and surrounding areas along with biological control and sprays of Bacillus thuringiensis or the Entrust formulation of spinosad are acceptable to use in an organically certified crop.

Beet armyworm may be present in and around the field feeding on bindweed and little mallow (malva).

• Start monitoring weeds and crop foliage for larvae just after transplanting or when seedlings emerge.
• If young instars are found, consider treating with low-impact product such as Bacillus thuringiensis, methoxyfenozide (Intrepid), or spinosad (Entrust). For yellowstriped armyworm, the older stages can be treated with methomyl. Treat the crop only if there is severe defoliation prior to fruiting. No thresholds have been established.