Armyworms – An Annual Challenge for Pastures and Hay Fields

by | Oct 16, 2020 | forage, Forage & Pasture, hay, Insects, pasture, Pest Management

Evan Anderson, Walton County Horticulture Agent

With a population that never seems to quite disappear in North Florida, the armyworm is a caterpillar pest of many plants. There are four types found in our area: fall, southern, yellow-striped, and beet armyworms. The caterpillars pupate into moths, which breed roughly from April to December, but warmer weather may lead to them being sighted even in the middle of winter. Adult moths, which are relatively nondescript and gray-brown in appearance, lay masses of 100-200 eggs on plants. Young armyworms are tiny and very difficult to detect, often hiding during the brightest parts of the day.

The armyworm grows to a length of about 1 ¼ inches, taking about two weeks to reach its full size. The caterpillar builds a cocoon in the soil and pupates into a moth, living another one to three weeks. With its ability to eat a broad range of plants, this can lead to large populations of these insects in a relatively short time.

Fall Armyworm head close up showing inverted Y

Head capsule of fall armyworm, showing the light-colored inverted “Y” on front of head. Credit: Lyle Buss, UF/IFAS

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Armyworms seem to prefer eating grasses, but will chew on almost anything in their path. They have been observed eating citrus trees, tobacco, cotton, strawberries, and even weeds like pigweed and nutsedge. A hungry armyworm might even turn on its fellow caterpillars, eating them as well if they are too close. When large numbers of these pests are present, the damage they do to surrounding foliage can be stark. Young caterpillars may skeletonize leaves, while older larvae will make holes in leaves or defoliate plants fully.

Years with an abundance of wet or humid weather seem to help armyworms reproduce. Fertilization late in the summer can also increase caterpillar populations. For those with pastures, their presence may be of special concern. While they seldom do enough damage to grasses to kill the plants, a lost cutting of hay can hurt. Identifying an infestation early is key to controlling it; younger caterpillars are more susceptible to control measures and spot treatment can sometimes control a patch before the insects get out of hand. Detection is more likely during cooler hours of the day, or when it is overcast or rainy.

If 2-3 armyworms per square foot are detected, it may be time to consider a control option. If a field can be cut for hay when the caterpillars are found, it may be possible to simply mow and bale the forage and avoid using an insecticide. If chemical control is necessary, there are numerous options. Products containing Bacillus thuringiensis, malathion, diflubenzuron, spinosad, methomyl, and carbaryl have all been labeled for use on armyworms.

For more information, see our EDIS publication on fall armyworm, and for control options, Alabama Extension’s publication, Management of Fall Armyworm in Pastures and Hayfields, or contact your local Extension Office.

Evan Anderson
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