Armyworms in Turfgrass

Lawn Armyworm Diagnosis and Control

Allan Robinson: Ask a Master Gardener

Tuesday, August 15, 2017

Q: Something is eating my grass, and my neighbor says he thinks these are armyworms. How do I tell if they are armyworms and what should I do? A.M., Tulsa

A: Armyworms are not actually worms. They are the larvae, or caterpillar, of a moth, and they love to eat grassy plants. They prefer grain crops (peanuts, cotton, soybean, wheat), but they can spill over into nearby lawns and move from lawn to lawn from there. Large numbers can consume all above-ground plant parts, and they are capable of killing or severely retarding the growth of grasses. The overall numbers are more pronounced in dry years.

Fall armyworms don’t necessarily wait until the fall to do their damage. Larvae are present by late July, so they are here now. They produce several generations per summer, but the September generation is the one that damages lawns.

Caterpillars or mature larvae are green, brown or almost black and about 1½ to 2 inches long, with black and reddish brown stripes on each side of the body and four small, black spots on the dorsal side of each abdominal segment, with a marked pale, inverted “Y” on the front of the black head capsule.

If you find a patch of lawn suddenly wilting, move the grass aside and look at the soil. If you see several beige-gray caterpillars eating the grass at the soil-line, you probably have armyworms. But, to effectively search for them, mix one tablespoon of liquid dish washing soap to a gallon of water and pour it onto a square-foot area of grass. This should bring worms to the surface within a few minutes, and you can see them by separating the blades of grass. You should do this on two or more areas of the lawn and compare results.

If you find two to three worms per area, you should treat the lawn

Garden tips

• It is not advisable to use post-emergent broadleaf weed killer during the peak heat of summer. Wait a bit. Weeds will start to grow when it cools.
• The birds need a handy source of water, as well as food. Put out a big saucer of water and watch them not only drink but also take baths to cool off and remove parasites. Another saucer of water filled with stones and sand will be a watering hole for butterflies and other beneficial insects.
• If you are seeing scattered tips of limbs on trees turning brown with some falling to the ground appearing to be broken off, this is likely cicada damage. The adults dig into the bark of a stem about 6 inches from the tip and lay eggs. The limb tip usually dies and may fall to the ground. It commonly involves oaks, but other trees can be affected as well. The amount of damage is nominal and will be of little consequence.