The fall armyworm is by far the most important caterpillar pest in pastures and hayfields. These pests can quickly decimate a field of any forage crop, often selecting higher quality material first. If not controlled, the FAW can cause severe economic damage and total forage loss. Reports of fall armyworm have rapidly increased over the last week across the Southeastern US.

Scouting for fall armyworms and their damage 

Careful scouting is the best way to prevent economic losses. The University of Georgia Extension Service recommends using an insect sweep net to identify low populations of fall armyworms before significant feeding damage occurs. Sweep samples should be collected in the early in the morning and late in the afternoon when the fall armyworms are more prevalent. Some birds such as geese or cattle egrets may serve as an indicator of armyworm activity, but this is not always guaranteed especially in the Coastal Plains where the birds are always present. 

Unfortunately, fall armyworms usually go unnoticed until feeding damage occurs. Damage can appear differently depending on the forage type and conditions. Fall armyworms prefer high-quality, well-maintained forage and they will select high quality pastures and hayfields first. Most damage begins with ragged leaves or areas with stripped stems. In hayfields or pastures with tall growth, damage can be devastating with nearly all tender green vegetation being removed. 

Damage may appear as thinned out grass and brown spots in overgrazed pastures which may be misdiagnosed as drought damage. The damaged patch will usually increase in size as the fall armyworm population builds. The brown color results from dehydration of plant tissues following fall armyworm feeding. Often the only vegetation left in the field after a severe fall armyworm infestation will be tough stems. Fall armyworm outbreaks are more prevalent during drought because some of their natural enemies are less active under these adverse conditions. 

 Prevention and Control 

Textbook treatment thresholds are typically recommended at three fall armyworm larvae per square foot, but weather and forage demand may dictate a more conservative threshold. Harvesting a hay crop may be the best option for armyworm control if hay is close to cutting time and the fall armyworms are at the fully grown larvae stage. 

If insecticides are required, be sure to follow all label instructions. Insecticides are labelled for specific life stages of the fall armyworm. In general, small caterpillars are easier to control than larger ones and insecticides are only recommended when the fall armyworms are less than ¾” long. Several insecticides are available for young larva, but we should pay careful attention to grazing or harvesting restrictions related to these products. The products recommended for use in Georgia are listed in the graphic below. You may want to consider a residual control option. Products vary in their length of residual control and application cost, so you will need to determine the potential returns on this investment. For more specific insecticide recommendations for your area, please consult with your local Extension Agent.

By: Dr. Lisa Baxter – University of Georgia State Forage Extension Specialist