By David Smitley
Fall armyworm caterpillars are appearing in large numbers in field crops, pastures and sometimes in lawns, athletic fields or golf courses. In some cases, the caterpillars are eating grass plants down to the base. This type of scalping can damage lawns and fairways, particularly if lawns are sunny and dry. Grass in moist soils may recover from the scalping injury.
If you see some brown or scalped areas on your lawn or fairway, look closely for the armyworm caterpillars. When damage is obvious, you will see hundreds of caterpillars. More damage can be prevented by applying a pyrethroid insecticide labeled for use on turfgrass with one of the following active ingredients: bifenthrin, cypermethrin, sumithrin, deltamethrin, cyfluthrin or another synthetic pyrethroid. Sevin Insect Killer no longer has carbaryl as the active ingredient. It now contains cypermethrin, which should work well. For all of these, use the label rate and wait until the spray is dry before allowing use of the treated turfgrass.
For those who would like a safe insecticide that does not harm beneficial insects including pollinators and predators, Bacillus thuringiensis (B.t.) will work on the small caterpillars but it may not work well when they are more than half an inch long. Also, it lasts only one or two days after it is sprayed, so caterpillars could move back into a treated area from surrounding areas.
Grass pasture damaged from fall armyworms. / Chazz Hesselein, Alabama Cooperative Extension System, Bugwood.org.
An insecticide treatment is not recommended unless some turf damage is observed. However, because of the large scale of the outbreak, which includes most of Michigan, it would be wise to check high value, managed turfgrass sites some time in the next few days and be prepared to spray hotspots.
Fall armyworm, Spodoptera frugiperda, is mostly a pest in the southern United States, rarely reaching Michigan in large enough numbers to become a problem. It does not survive the winter in midwestern states, so infestation depends on moths flying north each spring and summer. This is an unusual year, with heavy infestations appearing in most midwestern states.
David Smitley is an entomology professor at Michigan State University. His areas of expertise include pest management in turfgrass and nursery plants.