Fall Armyworms

Home Grown  by Keith Reed
This fall is shaping up to be a banner year for fall armyworms. Armyworms are NOT the tree pest on everyone’s mind, those are Webworms. They are a different species. Note for future reference that just because one population is heavy does not mean the other one is, that just happens to be how it is this year.

Fall armyworms feed at or near the ground and prefer grass species. While it is easy to miss seeing these caterpillars, once you’re aware of their presence, you may be alarmed by the sheer numbers and the damage they do. You will also quickly understand where they get their common name as they move by the thousands across the landscape.

If you find your landscape over-run with these pests, there is some good news. The damage they cause is rarely fatal to the plant. Armyworms are primarily leaf feeders, meaning they prefer the softer parts of plants, typically leaving the growth points undamaged.

In other words, if you have a mature stand of healthy turfgrass, control is generally not suggested. The damage, while it might be significant, will be temporary. Oftentimes, armyworm damage looks like a lawn mower with very dull blades has recently been over the area. It is also common to see a turf area showing signs of drought (blue-wilting), even when moisture levels are adequate.

If you have a high value turf area, or one that’s newly seeded, or some other plant armyworms find attractive, controlling these pests can be done with the proper application of an approved insecticide.

There are too many brand names of insecticides to list them all here. However, look for products with one of the following active ingredients; Acephate, Bifenthrin, Carbaryl, Cyfluthrin, Deltamethrin, Permethrin, and Spinosad. Bacillus thruingensis var. kurstaki (Bt) is also a good option if you treat while the insects are still small.

A typical recommendation would suggest these products can either be applied as sprays or granules. And that’s true, but because of the fast-moving and fast growing nature of the armyworm, this is one case where a spray is likely to be more effective. As always, a product is only as effective as the quality of the application. Read and follow all label directions when using any pesticide.

For more information on this or any other horticultural topic, you can contact Keith Reed, the Horticulturist in the Payne County Extension office. Keith can be reached via email at
keith.reed@okstate.edu, phone at 405-747-8320, or in person at the Payne County Extension office, located at 315 W. 6th in Stillwater.

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