Georgia farmers are never surprised to see fall armyworms munching on their stands of corn, sorghum and forage hay crops – they just hope for a low number of armyworms.
UGA Extension Entomologist, Will Hudson, describes fall armyworms as the “larval or caterpillar stage of a nondescript, small gray moth which overwinters in Florida and the tropics.” Each year, storms bring the adult moths north.
When it comes to pastures, the fall armyworms feed on most all forage grasses, including bahiagrass, bermudagrass, fescue and mixes. Armyworm caterpillars also prefer turfgrasses and feed above the ground, primarily eating foliage and tender stems.
Adult fall armyworm moths are active at night. Females lay between 50 and several hundred eggs in masses that hatch in just a few days. The life cycle from egg to moth takes about 28 days in the warm weather of August and September. The larvae take several weeks to develop. When they are small, they don’t eat much, but when they molt to the last stage, they can eat up an entire pasture in four to five days.
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 one 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 (1/2 oz. dishwashing soap/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, and then don’t mow for three days after the treatment.
For more information or if you need assistance identifying armyworms, contact your UGA Cooperative Extension office.