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Master Gardener: Armyworm invasion can be destructive
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Master Gardener: Armyworm invasion can be destructive

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It looks like my yard has been invaded with these inch-long caterpillars that are devouring my turf and some of my plants. How do I get rid of them? — T.G.

The insects you are describing are armyworms. We have been getting quite a few questions about armyworms, so let’s talk about what they are and how to get rid of them.

First of all, we call them armyworms, but in reality, they are caterpillars on their way to becoming a moth. These moths will migrate south later this year in search of the warmer climates of the Gulf Coast, Texas or Mexico, only to return to Oklahoma in June next year. Until they leave, they can be quite destructive.

When the armyworm moths return in June, the female moth can lay up to 1,000 eggs in smaller groups of fifty to several hundred. These eggs hatch in July as tiny caterpillars and begin to eat, and they can eat. They can do quite a bit of damage to your lawn or plants, not so much individually, but you can imagine what it would be like if a hundred or so females each laid 1,000 eggs in a concentrated area. A hundred thousand armyworms could do some damage.

The good news is that they don’t have a long feeding season — only about 2-3 weeks. After this feeding frenzy, they go down into the soil to pupate. Two weeks later, the next generation of adult armyworm moths emerge. There can be several generations all the way into October or even November. With this relatively quick life cycle, it’s easy to see how their numbers can build up quickly. Their preferred food is turfgrass, but they will really eat anything. We’ve seen some photos of them even devouring tomatoes this year.

If you suspect you have armyworms eating your turf, there is a quick and easy test you can perform to confirm their presence. Just mix up about an ounce of dish detergent with a gallon of water and pour that solution in about a square yard of turf. The soap will serve as an irritant and the armyworms will come to the surface to get away. If you find 5 to 10 armyworms in the test area, you should consider treatment.

One of the safest ways to treat for armyworms is to use an organic pesticide called bacillus thuringiensis, or BT for short. Spray BT on the turf or plant experiencing the issue. The armyworms will eat the pesticide and die. One good thing about BT is that it is only effective on caterpillars, so this minimizes potential damage to any beneficial insects as long as you are careful where you spray.

With large infestations, you might want to consider getting a product containing bifenthrin such as Ortho Bug-B-Gone among others. These are available in containers you attach to the end of your hose to spray. The challenge with this strategy is that this type of pesticide does not discriminate and will kill any insect it comes in contact with, even the good ones.

Since their feeding frenzy only lasts 2-3 weeks, if your lawn or garden can take the hit, you might choose not to do anything about it. Yes, there will likely be some damage to your turf, but Bermuda and Zoysia will grow back. Fescue will likely be damaged, but it’s almost time to re-seed fescue anyway. Good luck.

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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 St., or by emailing us at mg@tulsamastergardeners.org.

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