Ask a Master Gardener

fall army worm

Fall Armyworms

9/3/22

There are a bunch of caterpillars eating my turfgrass. How do I get rid of them? GS

Sounds like you are talking about armyworms. Last year it seems like they were everywhere. While it doesn’t seem as bad this year, armyworms can always be a problem.

Like many insects, their common name is a bit confusing because they are not worms and not in an army, they are caterpillars on their way to becoming a moth. Interestingly, these moths have flown in June or so from the Gulf Coast, Texas, or even Mexico to make their home here in Oklahoma.

Upon their arrival, each female moth can lay up to 1000 eggs, typically in groups of 50 to 100. The first round of eggs hatch in July and these small caterpillars begin to eat. The good news is that they don’t feed very long as caterpillars - only about 2-3 weeks. After their feeding frenzy, they enter the soil to pupate. A couple of weeks later, the adult moths emerge, and the cycle begins again. This process can continue all the way into October or even November. With this rapid life cycle, what began as a mild grouping of armyworms can quickly become a ravenous tribe, able to devour your lawn and then move on to the next yard. They seem to prefer fescue, but really aren’t that particular.

If you think you have armyworms eating up your turf, you can perform a quick test to confirm your suspicions. All you need to do is mix about an ounce of dish detergent in a gallon of water and then pour that solution into about a square yard of your lawn.

This mixture of detergent and water serve as an irritant to the armyworms and will cause them to get active, trying to get away. If you only see a few armyworms after performing this test, I wouldn’t worry too much about them. However, if you see a bunch, you might want to consider treatment.

One of the safest pesticides to use is an organic one called bacillus thuringiensis also known as bT for short. This pesticide can be found at most garden centers. Just spray it on the turf where they are eating, they will eat the pesticide, and the threat is removed.

One of the great things about bacillus thuringiensis is that it is only effective on caterpillars, so there won’t be any collateral damage on other insects.

With large infestations, you might want to consider calling a pest removal company to treat the entire yard. Or you can just let them do what they do because your turf should grow back. Good luck.

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org. Photo: Frank Peairs, Colorado State University, Bugwood.org