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By Extension: Beating back the armyworms

Last week while I was gone to California for the National Canon Envirothon competition with the Rockdale 4-H Envirothon team, we had our first confirmed sighting of armyworms in lawns in Rockdale County. In late summer, almost every year, these caterpillars invade turfgrass throughout the state. Some years it’s bad in Rockdale County and some years they miss us. But they are here this year.

The damage to established turf is mostly aesthetic, but newly sodded or sprigged areas can be more severely damaged or even killed. Their favorite turf to feed on is Bermuda grass, but they will eat any succulent grass so our well maintained lawns are perfect fodder. If you are irrigating and fertilizing regularly, your lawn is more at risk. The adult armyworm moths are active at night and lay several hundred eggs at a time.

Caterpillars feed for 2 to 3 weeks before pupating. The entire life cycle from egg to adult moth takes about 28 days in the warm weather of August and September, so there can be several generations. If there is any doubt about whether caterpillars are present, pouring soapy water on the grass (1/2 oz. dishwashing soap/gallon water) will bring them up very quickly.

Control can’t really wait until the weekend. Your lawn could be decimated by them and the caterpillars will have moved next door. The old active ingredient standby carbaryl (Sevin, etc.) still works well, as do all the pyrethroids (Pyrethroids are those active ingredients that end in "-thrin") and triclorfon. If the worms are detected while they are still small, Bacillus thuringiensis (Bt) based "biocontrols" like Dipel can provide good control. Just read the label on the insecticide to make sure it can be used on the lawn and is labeled for armyworms, or turf caterpillars. Follow the label directions carefully.

Applications should be made as late as practical for best results. It is not necessary to water after application but an application rate of 20 - 25 gallons of solution per acre as a minimum will ensure good coverage. Do not cut grass for 1 -3 days after application.

I once had armyworms in my fescue lawn. There were hundreds of tiny caterpillars. Being the entomologist I am, I wanted good pictures, so I decided to wait until they got a little bigger for good pictures. The next morning my fescue was eaten down to the ground and they had moved into my neighbor’s lawn. They are named armyworms because they are found in great numbers and move fast. This is one insect that you do need to control immediately — if you like your lawn, pasture or hayfield.

Julie-Lynne Macie is the Rockdale County Extension Coordinator. She can be reached at (770)278-7373.