July 9, 2022

Fall Armyworms: All You Need to Know

Fall armyworms are grass eating caterpillars that, as their name indicates, move in large groups and can cause significant damage to lawns, especially if new sod was recently installed. Find out more about fall armyworms, how to identify them, and how you can get rid of them if they are dining on your grass.

What are Fall Armyworms?

The fall armyworm is the most common cause of damage in turfgrass on golf courses, athletic fields, and home lawns. The fall armyworm arrives in the southern United States almost every year and can rapidly damage previously healthy grasses.

Armyworms are the caterpillar (or larvae) of a small brownish-gray moth that moves north from Florida each spring and summer.

Since the armyworm can not survive even mild winters, it reinfests the southern states of the US all over each year. It takes time for generations of these moths to spread from south to north, which is why armyworms typically appear in late summer and early fall, hence the name fall armyworm.

*Be sure to pay close attention to your lawn for armyworm activity every year from July until the first frost.

Armyworm Life Cycle

Armyworms have four stages of life: egg > larvae > pupae > adult

Stage 1: Egg

Adult armyworm moths fly and mate at night. The female will lay eggs in batches of a few dozen to several hundreds. The eggs are small, white and laid in clusters.

The female does not discriminate when choosing a place to lay her eggs. Eggs are known to be to be found on just about anything; the underside of tree leaves, fences, bleachers, water coolers, light poles, even golf flags.

Stage 2: Larvae

After only a few days the eggs hatch and the caterpillars (larvae) feed and grow for 2-3 weeks. Armyworms feed throughout the day but are typically most active early in the morning and late in the evenings .

The larvae phase is when the caterpillar cause damage to grasses and plants.

Stage 3: Pupae

Once the larvae are full grown they will burrow into the soil 1-2 inches and form the pupal stage. The pupal stages lasts between 11 to 13 days. Pupae are smooth and reddish brown in color.

Stage 4: Adult Moth

Adults then emerge to mate and lay eggs. Adults typically live about 2 weeks. There will be several generations each year.

How to Identify Armyworms

Fall armyworm caterpillars range in color from yellow-green, green, or shades of brown to grey.
Newly hatched armyworms are lighter in color, but darken as they mature.
Their most identifiable trait is a light colored inverted Y between the on the head.

Armyworms have a broad, pale band along the top of its body, contrasted with dark bands along its sides.

Still not sure if you have armyworms?

If there is any doubt whether armyworms are present in your lawn, pouring soapy water on the grass (1/2 oz. dishwashing soap/gallon water) will bring armyworms to the surface quickly. If present, the flush will irritate the caterpillars, causing them to leave their hiding places in the thatch to escape the irritant, and will become visible in about 30 seconds.

Armyworm Damage

Armyworms can cause serious damage to turfgrasses. The damage caused by fall armyworms is usually very distinctive, but not always. Armyworms cross the grass as a group and therefore will create a noticeable line between damaged and undamaged grass.

Typically damaged areas will begin around where the egg mass has been laid and radiate outwards.

Damage caused by fall armyworms often appears browned or burned patches resembling drought stress. If you have distressed patched of grass in your lawn, be sure to go take a closer look.

While armyworm damage may seem to appear overnight, it is likely the larvae have been feeding for a week or more before noticeable damage appears. Newly installed sod is highly attractive to fall armyworms because it is well fertilized and well watered, and therefore is more susceptible to damage.

Damage to established grass is mostly aesthetic, but newly sodded areas can be severely damaged or even killed by armyworm feeding. If armyworms are identified and controlled early, grass recovery is expected without the need to replace the sod.

Fall Armyworm Control

It is often said that knowledge is power. Knowing what when and what to look for is the first step to controlling armyworms. Look for signs of lawn and plant damage often, especially areas of your lawn that have recently turned brown.

If you spot the brown adult moth form of the armyworm, it is highly likely the armyworm caterpillar is nearby feeding on plants. Spotting armyworms early is critical in controlling the amount of damage they cause.

Natural Enemies

Preventative insecticide treatments are not practical since outbreaks of fall armyworms tend to be random. Natural enemies, such as birds, paper wasps, and ground beetles are quite good at controlling armyworms. Using unnecessary insecticide can eliminate those natural enemies and potentially cause a worse armyworm problem. By spot treating instead of blanket treating, armyworm’s natural enemies can be conserved.

Insecticide Treatment

Proper timing of insecticide applications results in the best control. Normally, insecticide control is warranted when an average of three or more armyworms are present per square foot.

Sevin, Dursban, Orthene or Conserve are effective insecticide treatments. Please be sure to read the label carefully and follow the directions.

Credit: UGA Corporative Extension

Insecticide Application Tips:

Time of Day: Treat in the early morning or late afternoon, when the caterpillars are most active.
Lawn Care: Mow before you treat if possible and then do not mow for three days after treatment.
Follow the Label: Be sure to read and follow all insecticide instructions carefully for targeted pest.
Repeat: Armyworm reoccurrence is common, so even after you have treated it is important to diligently check for armyworm damage. You may need to treat multiple times to break the armworm life cycle.

Summary:

Fall armyworms typically appear in Northern Georgia from July until the first frost
Inspect your lawn daily for armyworm activity from July thru the first frost
Understanding the armyworm lifecycle informs best management practices
Control is easy and inexpensive from multiple products
If identified and controlled early, damaged grass caused by armyworms is expected to recover