LSU AgCenter Horticulturist
Sod webworms were biblical last summer. Not to be outdone, the armyworm troops have now set up camp. They are out in full force, causing headaches for many homeowners.
If you’ve noticed large brown patches in your lawn, you may have armyworm damage. It is the caterpillars, or larval stage of the adult moth, that feed on turfgrasses such as Bermuda, St. Augustine and Centipede in addition to other agricultural crops.
In fact, that is how armyworms got their name. As infestations of caterpillars devoured agricultural fields, they destroyed large areas in just days, resembling an army troop.
Most homeowners will not notice the damage until large patches of turfgrass begin to die. Once the caterpillars have consumed a large area, they move on to nearby fields and lawns.
It’s a good practice to scout your lawn in the summertime for damage from armyworms, chinch bugs and sod webworms to stay ahead of extensive damage. You might notice the adult moths of armyworms flying around outdoor lights at night when they are active. Moths are gray to light brown in color and are small, with a wingspan of 1 to 2 inches. The upper wings are brown with white spots, and underwings are white.
Like most other insects, the armyworm has four life stages: egg, larvae, pupa and adult. Females lay thousands of eggs on leaves and stems of plants or structures near lawns. Eggs hatch and larvae develop in about two to three weeks. Then they go down into the ground to pupate and emerge as adults in one to two weeks. Unfortunately, there are several generations of the armyworm per year.
With the hard freezes experienced across the state in February, it’s a bit of a surprise to see such an infestation of armyworms because they are typically susceptible to extensive cold weather. Outbreak years like the one we are experiencing do happen from time to time.
Damaged areas of the lawn may resemble drought stress, but you will be able to tell that it is insect damage when a distinct line is seen between the damaged areas versus undamaged. The caterpillars are most active in early morning and late evening hours, and they can be observed flying around outdoor lights.
When scouting for damage on the turfgrass, look for chewed leaves that are translucent on the ends. You may also find green frass (caterpillar poop). Leaves also may look skeletonized. Another great indicator is large groups of feeding birds.
Initial damage is low, but as larvae grow, they consume more plant material and damage becomes more rapid in the later stages of development.
Early detection is important. Large numbers of armyworms can cause extensive damage. Scouting the lawn periodically is important. Take time before you mow to walk around and look for caterpillars and signs of damage.
There are a couple of forms of organic control. Products that contain the active ingredients Bt (Bacillus thuringiensis) or spinosad are considered organic options. For heavy infestations, synthetic chemicals may be necessary. Products with one or more of the following active ingredients are effective in controlling armyworms: acephate, bifenthrin, carbaryl or clothianidin.
Read and follow all label and instructions. Chemicals will need to be repeated in seven to 14 days to treat newly hatched eggs. And remember — when using insecticides, beneficial insects and the animals that feed on them in addition to pollinators could be harmed by the chemicals. Spray very early in the morning or late in the evening when pollinators are less active.
For more information on the responsible and safe use of pesticides for turfgrass insect pests, go to LSUAgCenter.com and search for “Louisiana Insect Pest Management Guide.” For more information on fall webworms in turfgrass, search for “Louisiana Home Lawn Series: Fall armyworm.”
Healthy and actively growing grass will likely recover fully, even after a heavy infestation and extensive damage. Turfgrasses recover more quickly with their aggressive root growth by rhizomes and stolons that spread across the lawn.
Armyworms feed on turfgrasses, causing large brown patches in lawns. Photo by David Sexton/LSU AgCenter