Source: Canadian Biodiversity Information Facility
In Arkansas, there are two types of armyworms–true armyworms and fall armyworms. Armyworms are caterpillars, the larval form of moths, according to Dr. David Fernandez, Extension livestock specialist and interim dean of graduate studies for the University of Arkansas at Pine Bluff.
“True armyworms are most often a spring-time pest, especially in northern Arkansas on cool season grasses. They can and do reach problem status in southern Arkansas from time to time,” Dr. Fernandez said. “Fall armyworms are more common in southern Arkansas and can start appearing in mid-summer, especially in bermudagrass pastures.”
Armyworms start out very small and they can easily avoid detection by the casual observer, he said. This is why armyworm damage seems to appear overnight. Large numbers of small armyworms don’t do much damage, but during their last two stages of growth, they can eat tremendous amounts of grass. True armyworms eat at night, so their damage is especially noticeable in the morning.
“To scout for armyworms, you need to look where they might be and when they are active. Look for early signs of armyworm damage. Examine areas that look like they have been frosted or areas where you find them stuck on your truck or tractor tires after you have driven through,” Dr. Fernandez said. “Birds feed voraciously on armyworms. If you see a large concentration of birds feeding in a part of your pastures, you may also find armyworms there.”
The best time to check for true armyworms is at night, and for fall armyworms, early in the morning or in the late afternoon when they feed, he said. Otherwise, you will have to get down on your hands and knees and look under the thatch to find them where they rest during the day.
Armyworm damage looks like brown drought damage because of dehydration from the leaves of the damaged grass. You may also find frass, which are small dark seed-like caterpillar droppings.
“Just because you find signs of armyworms doesn’t mean you need to spray. Armyworms have many predators and diseases that kill them,” he said. “It is not uncommon to scout your pastures one day and find many caterpillars, then be unable to find any a couple of days later. Deciding when to spray requires sampling.”
Dr. Fernandez recommends making a one square foot sampling device from PVC pipe and PVC elbows.
“Take at least 10 random samples from across your pastures by throwing it (not too far so you can find it easily) on the ground. Get down on the ground and carefully count the number of caterpillars you can find. Remember to look under the leaves and thatch. If you find three or more caterpillars one half inch long, you should treat your pastures,” he said. “You may not need to treat if the caterpillars are smaller, their natural enemies may control them for you.”
If your hay is ready to cut and bale, you can avoid spraying by baling, Dr. Fernandez said. Larger caterpillars are hard to kill, and caterpillars over one- and one-half inches have already done most of the damage they will do, so spraying won’t help and will cost you money.
For more on controlling armyworms in your pastures, refer to Extension fact sheet FSA7083, Managing Armyworms in Pastures and Hayfields, at (https://www.uaex.uada.edu/publications/PDF/FSA-7083.pdf) or contact Dr. Fernandez at 870-575-8316 or fernandezd@uapb.edu.
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