Skip to main content

AgAlert! Fall Armyworms

Monitor lawns and fields through September to be proactive against fall armyworms

Fall armyworm larva

Fall armyworms have been reported causing significant damage on forage grasses, turfgrasses and pipeline vegetation cover in West Virginia. Fall armyworms feed on over 80 plants species, but prefers grasses, including rye and wheat. This insect also frequently damages field crops, including alfalfa, barley, bermudagrass, buckwheat, clover, corn, oats, millet, sorghum, sugar beets, sudangrass and soybeans. Occasionally, fall armyworm injures apple and peach trees, grapevines, and strawberry plants.

Late in June, fall armyworms arrive in West Virginia from the south. Shortly after, eggs are laid in masses on the undersides of plant leaves, tree trunks, undersides of tree limbs and other structures near suitable host plants, such as fences, bleachers and light posts. Outbreaks, or large numbers, of armyworm occur every few years. 

Fall armyworm larvae feed as a group and can devour an entire lawn or hay field in a matter of a few days. Damage is caused by larvae (caterpillars) when chewing plant tissue. The grass may seem to thin out and develop brown spots, which look burned or browned out. This appearance is the result of grass plants rapidly dehydrating after fall armyworm larvae damage. For this reason, fall armyworm damage often resembles drought damage. Feeding of all armyworm is reduced by cold temperatures in fall and the insects will die in the first frost.

Fall armyworm damage

Farmers and turfgrass managers should scout their fields every three to four days to monitor for caterpillars in July, August and September. Treatment is recommended when more than three worms of 1/4 inch in length or greater are found in a 1-square-foot area. Control will depend on good coverage. Because tall grass is more difficult to treat than shorter grass, it is recommended to mow prior to treatment. Grazing or harvesting infested areas can reduce losses, but treatment will still be required to reduce insect populations.

If the hay or alfalfa is mature enough, harvest for hay or baleage. Wrapping bales will help contain the insects. If bales are not wrapped in plastic, store away from and do not transport over other fields to avoid spreading the insect. If more growth is required before harvesting hay, or you are stockpiling forage, treatment is recommended as soon as conditions allow. See label directions for weather and harvest/grazing intervals.  

Several insecticides with different active ingredients kill fall armyworm. Spinosad, chlorantraniliprole, azadirachtin, pyrethrins, neem oil, methoxyfenozide are reduced risk insecticides that are labeled against this pest. Products with the active ingredient Bacillus thuringiensis can be used to control the younger larval instars. Other active ingredients, such as cyfluthrin, bifenthrin, carbaryl, malathion and lambda-cyhalothrin, also kill fall armyworm. However, these are broad-spectrum insecticides that can be harmful to beneficial organisms and can occasionally cause spider mite outbreaks. Read and follow label directions.  

For more information, contact your local WVU Extension Service office

This alert impacts all West Virginia counties

Find other insect pest resources here

Contact

Carlos Quesada
WVU Extension Specialist
Entomology
304-293-8835
Email Carlos

Trade or brand names used in this publication are for educational purposes only. The use of such product names does not imply endorsement by the WVU Extension Service to the exclusion of other products that may be equally suitable.

Content was written by Carlos Quesada, WVU Extension Service Entomology Specialist, and Karen Cox, WVU Extension Service Agent - Ohio County. Photos provided by Elizabeth Rowan (Figure A) and Brad Smith (Figure B).