Armyworms

Pest Management Fact Sheet #5040

Pseudaletia unipuncta (Haworth)
Order: Lepidoptera
Family: Noctuidae

James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician

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Biology

The adult armyworm (Mythimna unipuncta; formerly Pseudaletia unipuncta), is a moth captured in Maine in light traps in late April and early May. The female moth mates and begins laying eggs in mid-May. The eggs hatch in approximately 10 days, depending on temperature. Larvae feed for three to four weeks and then pupate in the soil, emerging as adult moths after about two to three more weeks. This process is repeated again, producing a third generation. In September, this third generation of nearly full-grown caterpillars spend the winter in the soil.

In the spring, the caterpillars finish growing and pupate from mid-April to early May. Two to three weeks later, the moths emerge, mate, and lay first-generation eggs. One female moth can lay several hundred eggs.

: Adult

: Larva

Description and Feeding Habits

When full-grown, the caterpillars can be almost 1.5 inches long. The caterpillars are usually greenish or brownish, but can also be nearly black. The sides and back of the caterpillar have light-colored stripes running along the body.

The caterpillars normally feed at night, and much damage can occur before they mature. The preferred foods are grasses, including corn, grains and timothy. They will feed on other plants if grasses are unavailable. Once the grass has been devoured, the caterpillars will move in a mass into other fields of suitable hosts; hence the name ‘armyworm.’

The first generation of caterpillars–those feeding in June–are usually the most destructive. Rarely are there outbreaks with the second or third generation of caterpillars.

The moths are tan or light brown in color with a white dot in the center of each front wing and a dark area on the edge of each hind wing. The moths feed on nectar and possibly on overripe fruit. The females lay their light-green eggs at night on the underside of grasses. Many moths congregate in a local area and lay their eggs, which may lead to the first-generation outbreaks.

Management

By the time damage is seen (destroyed grain, corn, hay, or lawns) it is often too late to control armyworm caterpillars, and by that point they are almost fully grown so will not feed much more. However, that is not always the situation. If a spotty infestation is found in a field, a 30 to 40-ft wide perimeter spray around the infested area may prevent further infestation. Since the caterpillars usually feed at night, the best time to spray is late in the day.

To prevent the caterpillars from moving to another field, apply a 40-ft border spray around the non-infested field. Residual pyrethroids (e.g. Warrior II®, Asana®, Pounce®, etc.) may offer some control.

For home use, carbaryl (Sevin®) may have some effect. Organic controls include pyrethrum, pyrethrins, or a combination of pyrethrins and spinosad. However, the pyrethrum/pyrethrins do not provide residual control. Formulations of Bt-k (Bacillus thuringiensis, kurstaki strain) are effective in controlling smaller caterpillars. Be sure to read and follow the label instructions and make sure the product is registered for the intended use.

Other management strategies include keeping lawns mowed short to make them less attractive to the caterpillars. If a lawn is infested, mowing it short will kill many of the caterpillars and reduce their food source. Hayfields can be mowed to salvage the crop, but many of the “worms” will survive and move elsewhere, looking for a new food source. Therefore, neighboring grasses should be protected before mowing.

Usually when the armyworms are on the move they are fairly mature and robust, making them harder to kill with insecticides.

Armyworm Damage

: A field in South Waldoboro, Maine, showing armyworm damage (week of July 4, 2001).

: Mature or nearly mature armyworms

: Corn field damage from armyworms in Solon, Maine. Photo by Rick Kersbergen.

When Using Pesticides

ALWAYS FOLLOW LABEL DIRECTIONS!

Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
17 Godfrey Drive, Orono, ME 04473
1.800.287.0279 (in Maine)

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