Fields decimated by ravenous insects might bring to mind images of biblical plagues of locusts, but across eastern Nebraska, another pest is leaving a similar trail of destruction; the fall armyworm.
Fall armyworms, (Spodoptera frugiperda) are a species from the southern U.S. that does not overwinter in Nebraska. Bob Wright-Nebraska Extension Entomologist, Nathan Mueller- Nebraska Extension Crops Educator, and Melissa Bartels-Nebraska Extension Crops Educator share that; as populations build up during the summer, moths fly north often reaching the Midwest later in the summer or early fall — hence their common name.
Reports of damage from these caterpillars have been coming in from southeast Nebraska for a few weeks, but recently have started to spike in the northeast part of the state. A fairly indiscriminate feeder, army worms can devastate many crops ranging from alfalfa stands, newly seeded cover crops of wheat, triticale, and rye, bromegrass pastures, and have even been reported damaging lawns.
Newly seeded alfalfa is especially sensitive to these pests. Populations as low as 1-2 worms per square foot can easily take out new seedlings. In established stands, 10-15 worms per square foot can easily strip alfalfa plants up to 14 inches in height.
Bartels shares that insecticides can be effective in controlling infestations, but will be most beneficial when a majority of worms are under ¾ of an inch in length. Over this length, the caterpillars are reaching the end of their time as a worm. This is when most damage is done as they do most of their feeding in the last 4 days of their larvae development. Therefore, it is very important to scout fields regularly in the early morning and late afternoon when caterpillars are most active to spot these forage offenders when they are small.
Fall armyworm caterpillars vary in color from light tan, green to nearly black with a darker head. Some key features of the fall armyworm are a predominantly white, inverted Y shape on its head, and four spots in the shape of a square on the end of its abdomen.
If identified, insecticide options include products with active ingredients including the pyrethroids, Alpha-cypermethrin, Beta-cyfluthrin, cyfluthrin, Gamma-cyhalothrin, Lambda-cyhalothrin, permethrin and Zeta-cypermethrin, organophosphates, chloropyrifos, and carbamates, carbaryl and methomyl. Keep an eye on label restrictions to harvest and grazing when considering your options. For hay fields, a longer restricted use period may not be an issue this time of year, but producers who may want to dormant graze any regrowth that may occur in a pasture need to consider their options carefully.
This late in the year, armyworm infestations are especially worrying. In alfalfa fields, heavy damage is essentially a late season cutting, interrupting the winterization process. Keeping an eye on fields and treating before damage gets too far along could prevent a field from exposure to additional winterkill risk later on.
In brome pastures, armyworms are also a concern beyond the loss of forage they are causing. Stripping plants this close to winter puts extra stress on the stand. If left untreated, worms may continue to consume any regrowth as it appears stressing the plant even more. A weakened stand will result in lower production next year and open the door to weedy plants establishing themselves and causing problems for years to come.
While armyworm feeding should slow down with cooler temperatures, since we don’t know exactly when cold weather will force them south, the amount of stress and damage that may occur is up in the air. For now scout, treat when necessary, and keep an eye on the forecast for cool weather to send this pest marching south.
-Ben Beckman is a beef systems Extension Educator serving the counties of Antelope, Cedar, Knox, Madison and Pierce. He is based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: firstname.lastname@example.org