Armyworms in Tulsa
Armyworms are caterpillars that eat the entire green part of the grass plant and can consume an entire lawn in just a day or two. They tend to eat as a group (thus the name ‘armyworm’,) eating the grass in roughly circular patches. Armyworms are especially threatening to agriculture as they can wipe out a crop in short order!
As far as we know, there is no environmentally sound method of preventing armyworms, and one cannot treat for them when they are in the moth stage. The trick is to detect them as soon as possible. If caterpillars are visible on a lawn, the lawn should be treated immediately with a liquid insecticide labeled for armyworms. (Granular insecticide is not effective.) Always read and follow label directions. If you hear of armyworms in your area, you might want to buy some liquid insecticide just to have it on hand.
A more natural alternative to insecticide is Bt (Bacillus thuringiensis) spray which is a biological control containing a bacteria harmful to caterpillars when they eat it. Retail names are Dipel or Thuricide. Bt won’t kill most beneficial insects, but might be harmful to butterfly caterpillars. It only works on leaf-eating caterpillars, so it may not work on older caterpillars that are not feeding anymore. Photo, right, courtesy of OSU.
Note: GreenGrass will treat its full-program customers’ lawns at no charge if they are invaded by armyworms.
Armyworm damage usually occurs in the spring and early summer and is actually rare. However, in 2000, eastern Oklahoma sustained substantial armyworm damage--practically an invasion--in mid to late summer. In 2012, the armyworms seemed to miss Oklahoma, thankfully, but destroyed many crop fields in the Midwest and even in New York.
Therefore, it is safe to say that one cannot predict an armyworm invasion. But if you see them, spraying immediately would be your best defense. They aren’t that hard to kill, if you notice them in time.
What to look for:
Armyworms (one word, not "army worms") are hairless, striped caterpillars, so named because they crawl from field to field (or lawn to lawn) when they have exhausted their food supply. There are two species of armyworms that can infest turf areas – the true armyworm, and the fall armyworm. (Actually, there is a third, but it's only a problem in Hawaii.)
Armyworm damage can look exactly like drought damage. Photo on left is mild armyworm damage just beginning. Whenever you see a brown area in your lawn for no apparent reason, we urge you to go look at it closely and see if you can spot any caterpillars or other insect activity. A flock of birds pecking on your lawn can be a warning sign also!
Fall armyworms (Spodoptera frugiperda)
Fall armyworms are the species most likely to create havoc on Tulsa lawns. However, fall armyworms don’t actually overwinter in Oklahoma. We are re-infested each year with moths from Texas or Mexico that migrate here and lay their eggs. Drought seems to precipitate heavy armyworm infestations, possibly due to fewer numbers of predators that would normally feed on the armyworms.
Armyworm cycle: Fall armyworm moths are a mottled dark brown and gray (male) to a dull greyish-brown (female.) Each female lays up to 1000 eggs in masses of about 50 to several hundred and covers them with grey fuzz from her body. The eggs hatch within 2-4 days.
The tiny larva (light-covered with black heads) lower themselves to the ground on silken webs and start eating. As they grow, they develop darker bodies (light green to almost black) with white stripes, and a light-colored inverted Y appears on their heads. From egg to full-grown larva takes about 2-3 weeks and results in a caterpillar about 1 ½ inches long. The more they grow, the more they eat.
When they are full-grown, they burrow into the ground and form pupae. A pupa is a non-feeding stage between larva and adult, where the pupa undergoes a complete transformation within a cocoon – like a butterfly. The moths emerge in 10-14 days, and the cycle starts all over again.
Unlike the true armyworms, fall armyworm caterpillars have distinct teeth on their mandibles, and they feed during the day as well as at night. If disturbed, they will curl into a tight ball and drop to the ground, but will soon climb back up on the grass and resume feeding. Moth and caterpillar photos courtesy of BugGuide. Armyworm head photo courtesy of OSU.
True armyworms (Mythimna unipuncta or pseudaletia unipuncta): Tulsa rarely sees damage from true armyworms, which are usually a problem east of the Rocky Mountains and north up to Canada. But true armyworms cause the exact same kind of damage as fall armyworms, so it doesn’t much matter in the long run!
But, for the record, true armyworms look similar to fall armyworms, but they crawl by looping (like an inchworm) until about half-grown. Full-grown, they’re about 1-1/2 inches long, of grayish to greenish-brown color, with two pale-orange stripes along each side of the body and another pale-colored, broken stripe down the middle of the back. The head is brown with dark lines, and they have no teeth (unlike fall armyworms which do have teeth.) Large true armyworms will often curl into a tight ball when disturbed. They hide in the soil or thatch in the day, and only feed at night (fall armyworms usually feed all day, with most activity in the morning and late afternoon.)
Below is an example of serious armyworm damage. Scary! Photo courtesy of Superintendent Magazine.