How Do You Prevent Fall Armyworms?

Fall armyworms are notorious for turning lush green turf into a brown mushy mess, but they can be prevented with careful scouting and timely treatments. These destructive caterpillars are voracious feeders and can strip a field in just a few days. While there are a variety of factors that impact the severity of an outbreak, one of the most important is when treatment occurs. This pest does not overwinter here, so it must be controlled each year from moths that fly in from the south and establish new generations. Infestations are most likely in fields that have been planted late or have a history of armyworm problems.

Getting on hands and knees to carefully scout a field is critical for effective control. Look for larvae in thatch layers, near the base of the plant, and in open ground where birds are congregating. A sweep net can also be helpful in detecting the small larvae. It is not uncommon to have to spray multiple times on a single cutting of hay during outbreak years.

The caterpillars scrape the surface of leaves and sheaths, leaving a white or tan streak and chewing the leaf edges. Eventually the leaves become brown and ragged, resembling drought areas. The caterpillars are so voracious, they can completely defoliate an entire field in a few days, especially on bermudagrass. Grass that has been stripped of its leaves can lose up to 40 percent of its growth, but it is often able to reestablish itself because of the strong root system and vigorous growth.

Scouting for signs of fall armyworms should be a priority every 3 or 4 days until the field is harvested or replanted. It is particularly important to scout a field that has been mowed once or twice, since this can accelerate the development of the caterpillars to damaging numbers.

A severe outbreak of fall armyworms in a hay field will reduce yield and quality, and it can be difficult to recover from this damage. However, if the field is almost ready to cut and the forecast looks good for hay-curing conditions, it may be worth the risk to go ahead and cut the crop. In these circumstances, superintendents should carefully scout the field for caterpillars and be prepared to treat immediately if they reach the treatment threshold of 2-3 caterpillars per square foot.

Fertilizer doesn’t prevent outbreaks of armyworms, but a well-designed fertilizer schedule will help the grass to remain strong and withstand stress. A low phosphorus fertilizer will promote good grass health and allow it to withstand a few feeding episodes by the caterpillars more easily. If the infestation does get out of hand, a low toxicity insecticide such as bifenthrin, carbaryl, or esfenvalerate (or a combination) can be used to control them, although it will take a few treatments to restore a damaged field to its full potential. If an outbreak does occur, prompt spraying will reduce the severity of the damage and give growers time to replant with a less susceptible crop.