By Abby Whitaker, Henderson/Buncombe County Livestock Agent

Fall Armyworms have been confirmed in our area and it’s important for you to be on the lookout!

A full article from Mississippi State can be read here.

Read this NC State article for current recommendations for insect pest control in forages. Be sure to take note of any precautions, remarks and restricted use products.

The Highlights of “What to Know”:
Information is from the Mississippi publication linked above.

Identification

Fall armyworm caterpillars vary in color depending on their stage of development and diet. Most are green or tan, but some can be dark brown to almost black, especially late in the year when numbers are high (Figure 1). The body is punctuated with dark spots. Most notably, there are four spots at the rear of the body that form a square. Mature caterpillars are about 11/2 inch long and are mostly slick-bodied with a few small, stiff hairs visible up close. The head capsule ranges from light brown to dark brown, and, on larger specimens, there is a distinct white inverted Y shape on the head. This Y and the square of four spots on the rear are two of the best ways to identify fall armyworm caterpillars.

Warning Signs

Newly hatched caterpillars are too small to feed all the way through a leaf blade. They begin by feeding on the underside of the leaf but leave the clear upper epidermis intact. This creates tiny “windowpanes” in the grass blades that appear silver or white (Figure 4). When large numbers of small larvae are just beginning to damage a field, the grass often has a subtle “frosted” appearance because of these windowpanes. Recognizing this early sign of infestation can help save a cutting of hay.

Scouting and Treatment Threshold

The best way to avoid losing a cutting of hay to fall armyworms is to visit fields every 3 or 4 days and check for fall armyworms. Get out of the truck, get down on your hands and knees, and look closely. Scout for fall armyworms by vigorously ruffling the grass with your fingers and carefully counting the larvae that have fallen to the ground in a 1-square-foot area. Do this at several locations in the field and average your results. Treatment is recommended when counts exceed three caterpillars that are one-fourth inch or longer per square foot (Figure 5). Be sure to look carefully for small caterpillars. You want to find and treat them when they are small because small caterpillars are easier to control and, more importantly, have not eaten nearly as much as they are going to eat.

Like most caterpillars, fall armyworms do about 80 percent of their eating during their last two to three days as larvae—when they are “teenage” caterpillars. Therefore, if a field has reached threshold on Friday but is not scheduled to be cut until the next week, it needs to be treated as soon as possible. A moderate to heavy population of large fall armyworm caterpillars can eat a whole field of grass in just a couple of days (Figure 6). Be sure to pay attention to preharvest intervals when choosing an insecticide to use on a field that is near cutting!

Deciding Whether to Spray or to Cut

One option for dealing with an infestation of fall armyworms in hay fields is to just go ahead and cut the field. This can be a good choice, but only if the field is nearly ready to cut anyway and the forecast promises good hay-curing weather. Be careful with this decision. If there are a lot of big caterpillars, they will “cut the field” for you in another couple of days. If you decide to go ahead and cut the field, you need to do it that day or the next morning. If there are threshold numbers of caterpillars and it will be 2 days or more before you can cut the field, you probably need to go ahead and spray.

When to Stop Scouting for Fall Armyworms

Fall armyworms cannot survive freezing temperatures. Populations usually begin to decline a little before first frost because fall armyworms cannot develop at temperatures below about 50°F. Some years the threat of fall armyworm infestations lasts longer than others. It depends on the weather, and late infestations are more likely during warm, “late” falls.

Insecticides for Fall Armyworms

Read this NC State article for current recommendations for insect pest control in forages. Be sure to take note of any precautions, remarks and restricted use products.

Contact Abby Whitaker with any questions!

abby_whitaker@ncsu.edu

NC State University and N.C. A&T State University work in tandem, along with federal, state and local governments, to form a strategic partnership called N.C. Cooperative Extension.