Last week, I took a group of Franklin County livestock producers associated with a “Climate Smart” forage program to visit three outstanding beef cattle and horse farms in western South Carolina. One thing we saw was the tremendous damage fall armyworms (FAW)  were having on their pastures and grasslands. Some were saying the early arrival of the armyworm was due to the recent tropical storm Debby blowing up from Florida area. When we returned this week to Franklin County, the armyworm is here damaging our forages. If not controlled, FAW can cause severe economic damage, total forage loss.

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. The caterpillars feed on variety of turf and forage species including: bermudagrass, tall fescue, alfalfa, corn, and sorghum sudan grass. Damage varies in appearance and severity according to the type of forage and management practices.

Identification 

Small fall armyworms tend to stay down in the canopy during the day, and their feeding is less damaging and harder to see. The youngest larvae do not eat much but the mature larvae can eat more than all other ages put together. Large fall armyworms are fairly easy to find because of their size. They are typically most active in the early morning or late afternoon when temperatures are cooler but will feed throughout the day. Most producers will identify fall armyworms at the fully grown larvae stage. They can be up to 1.5 inches long and are light green to nearly black with light and dark stripes that stretch horizontally along the body of the worm. One of their most distinct characteristics is an inverted Y-shape on the head of the worm.  

Fall armyworms. (Photos by University of Georgia Extension.)

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. Extension recommends using an insect sweep net to identify low populations of fall armyworms before significant feeding damage occurs. Sweep samples should be collected early in the morning and late in the afternoon when the fall armyworms are more prevalent. 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. Some birds such as crows, geese or cattle egrets may serve as an indicator of armyworm activity, but this is not always guaranteed. 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.

Many of you have pearl and sorghum sudan grasses with legumes in the fields now, ready to cut for hay. 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 by NCSU for use in North Carolina . See the reference table of Insecticide options for Fall Armyworm from Georgia Extension. 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 our office, Martha Mobley, Ag Agent, 919-496-3344, martha_mobley@ncsu.edu.

Insecticide Options for Fall Armyworms

For specific recommendations, please contact your local County Extension Agent and refer to the Georgia Pest Management Handbook.

Chemical control is only recommended for armyworms less than 3/4″ long.

If armyworms are greater than 3/4″, harvest is recommended.

Low = 0-7 d, Medium = 7-21 d, High = 21-28 d