All across the state we are hearing reports of this pest damaging lawns and forages.
Written by: David Buntin and Will Hudson, Entomology Department, UGA; Lisa Baxter, Forage Agronomist, UGA
(Photograph by James Castner.)
By late summer, fall army worms (FAW) are present in every county of GA every year. The difference year to year is when they show up and in what numbers. The FAW does not overwinter in GA. Our worms are the result of moths flying north from FL or east from TX and LA each year and spreading north throughout the summer. We usually see increasing problems beginning in July in southern parts of the state and in August or even September in middle and north GA. Serious problems develop when, like this year, populations start to build early. We began to see damaging levels in June in parts of south GA this year and early July in middle GA. This gives more time for the moths to spread and produce an extra generation compared to “normal” years. Pastures, hayfields, and turfgrass throughout the state are experiencing heavy pressure and damage now. To make things worse, many growers are finding control difficult to achieve even with repeat applications of insecticides. The most commonly used (and cheapest) insecticides growers turn to first are the pyrethroids including familiar names like Mustang Maxx and Baythroid along with dozens of generic formulations. We have had evidence of reduced efficacy for pyrethroids for some time now, and levels of reduced sensitivity (AKA resistance) have been documented in FAW populations in FL for 30+ years.
The reduced efficacy our producers have observed is all too common this season. If the pyrethroids don’t provide the control you need, there are other more expensive options to choose. Spinosad products have so far held up pretty well but at a price point well above the pyrethroids. Dimilin (diflubenzuron) is an insect growth regulator (IGR) that only affects the worms when they molt the next time, so don’t expect to see them drop dead on the spot. Intrepid (methoxyfenozide) is another IGR that is specific to caterpillars (pretty much) and has also been effective if given time. Many growers choose to tank mix the IGR with one of the other products, and that approach can be effective. The top end price-wise includes the diamide products like Vantacor, Prevathon and Besiege (a combination with a pyrethroid kicker to give quicker control). These last longer but take time to give full effect and cost considerably more. Whichever product a grower chooses, best results will be achieved with smaller worms. Once they get close to an inch long control is a real challenge.
Large worms are tough, and large worms in tall grass are unlikely to be effectively controlled in time to make much difference in hay yield for the immediate crop cycle. We recommend that growers cut what they have if close to harvest and save the insecticide treatment for the regrowth if the worms persist. Mowing the grass deprives the worms of their food, and they will either pupate (if large enough) or move out looking for something to eat. Either way, they’re gone, and the grass has
some time to recover before the next wave arrives. From the standpoint of coverage, short grass is easier to treat than tall grass so the results will be better if you need to spray the regrowth. Remember that the pyrethroids and spinosad kill by contact or ingestion of treated grass. In the hot, bright weather typical of late summer in GA you can expect control for a few days to a week. The IGR’s are taken up by the foliage but do not move in the plant. They are more persistent but do not protect new growth. The diamides are taken up and moved around in the plants and provide protection for new growth for up to 2-3 weeks depending on conditions. In any case, slowing down and applying the highest volume of spray solution possible will improve your chances of success. There are other options , like carbaryl and methomyl, but carbaryl carries a 14 day grazing and harvest restriction and methomyl is highly toxic (skull and crossbones toxic), restricted use, and labelled only on bermudagrass.
