Recently, I have received some calls from homeowners and farmers regarding the destructive fall armyworm.
UGA Extension Entomologist Will Hudson describes fall armyworms as the “larval or caterpillar stage of a nondescript, small gray moth that overwinters in Florida and the tropics.” Each year, storms bring the adult moths north.
Fall armyworms feed on bermudagrass and other turfgrass species. Damage to established turf is most often aesthetic, but newly planted sod or sprigs can be severely damaged or even killed by fall armyworm feeding.
The female moth, the mature stage of this insect, can lay up to 1,000 eggs over several nights. Within a few days, the eggs will hatch and the caterpillars begin feeding.
The caterpillars will molt six times before maturing and increase in size each time they molt. A generation is completed between 18 to 28 days, depending on temperature.
Newly hatched fall armyworms are white, yellow or light green but darken as they mature. The mature fall armyworms are about 1.5 inches in length with a body color that ranges from green to brown or black.
They have been described as having a greasy or oily appearance and are distinguished by the prominent inverted white “y” on their head capsule.
Small larvae easily go unnoticed. They do not eat through the leaf tissue; rather they scrape off all of the green tissue and leave a clear membrane that gives the leaf a lacey or a skeletonized appearance. On the other hand, large larvae can quickly denude a turf or forage canopy.
Targeting smaller caterpillars of a half-inch or less is important for two reasons. First, the caterpillars do not cause severe damage until they reach a size of 1 inch in length. Second, as with many pests, smaller larvae are much more susceptible to insecticide control than larger ones.
You don’t want to treat a lawn or a field if it is not needed, but homeowners and landowners need to be scouting for the presence of fall armyworms regularly and frequently. It is important to catch an infestation before the armyworms cause major damage, and the bigger they are, the more damage they cause.
Preventive insecticide treatments are not practical because outbreaks of fall armyworms tend to be random and mortality caused by their natural enemies is usually quite good. Unnecessary insecticide applications can eliminate those natural enemies, and ultimately lead to a worse armyworm problem following treatment.
Scouting options for homeowners include close examination of the turf, possibly in tandem with the use of a soapy water flush. When you pour soapy water over a patch of grass (half-ounce dishwashing soap to a gallon water), the solution will irritate the larvae, which will drive them up from the soil surface very quickly. Heavily infested turf will also have visible greenish-black fecal pellets, or “frass,” on the soil surface.
Other indicators of armyworm infestations may include high numbers of birds or even paper wasps that use the fall armyworms as food. If present, the caterpillars will become visible in about 60 seconds, as they are irritated by the flush and will leave their hiding places in the thatch to escape it.
In addition to the birds and paper wasps mentioned above, a number of other insects feed on armyworms, including tiger beetles and other ground beetles. Fall armyworms, like many other turf infesting caterpillars, can also be heavily parasitized by tiny wasps that kill the caterpillars and cause no harm to humans or pets. These natural enemies can be conserved by spot rather than blanket spraying and properly timing control efforts.
Armyworms rarely kill grass, but some lawns may be severely weakened. Feeding damage, coupled with an already stressed lawn, may justify applying insecticides.
In turf or pastures, finding five caterpillars per square foot is a signal to start treating for fall armyworms. Carbaryl (Sevin), pyrethroids and other recommended insecticides are effective caterpillar killers.
Products containing BT (Bacillus thuringiensis) are effective only on little (a half-inch or smaller) worms. Irrigate before treating to move the caterpillars out of the thatch. Treat in late afternoon, when the caterpillars are likely to begin feeding. If possible, mow before you treat, then don’t mow for three days after the treatment.
For more information or assistance identifying armyworms, contact the Extension office in Liberty County at 912-876-2133 or email at firstname.lastname@example.org.