Fall Armyworms - The Surprise of 2021

In late August, we started to receive inquiries about fuzzy, gray patches on house siding, playground equipment, birdhouses, bird feeders, posts, poles, golf carts, vehicles, and trees.  These were the egg masses of the fall armyworm, Spodoptera frugiperda.  Female moths, blown into the state on storm fronts coming from south and east of Iowa, “dumped” their eggs where they landed.  We have documented egg masses in the south-eastern one-third of the state. 

Our unprecedented experience with the fall armyworm follows the devastation to golf courses and lawns experienced this summer in the eastern U.S.  The fall armyworm outbreak of heavy populations caused severe damage to turfgrass from Oklahoma to Georgia and Indiana to Maryland between June and August Fall armyworm egg masses are grayish-tan and appear fuzzy2021.  Multiple generations of fall armyworms in the South and the eastern U.S. created enormous populations of moths that were blown into Iowa by storms in late August.

very tiny fall armyworm caterpillars crawling on white plastic cap
First instar fall armyworm caterpillars crawling on white plastic cap.

By the first week of September, we received reports that the egg masses were hatching.  Hundreds of tiny, first instar caterpillars were crawling on posts, signs, and houses.  See photo.  So far, the tiny caterpillars do not appear to be causing feeding damage to turfgrass, except for limited reports in Lee County.

Turfgrass Damage?

The most common question right now is what will be the level of damage we can expect? No one knows. We know there are high numbers of eggs and potential caterpillars, but we cannot say if there will be widespread damage.  Turfgrass damage from fall armyworm caterpillars will require the successful completion of a chain of events.  The hatching caterpillars must reach suitable food and start to feed before they die of starvation, desiccation, or predation; there must be large numbers of the fall armyworm caterpillars feeding in a concentrated area of turfgrass; conditions must be suitable for the caterpillars to feed and grow for 3 to 4 weeks, to achieve the full-grown size of 1 1/3 inch in length.  The large caterpillars are the most damaging.

Our current temperatures favor the growth of cool-season turfgrasses, which should help limit damage and speed recovery. At the same time, we hope the cool night temperatures will slow the fall armyworm feeding and growth. We do not expect the severe damage that occurred in other states due to these factors. One fact remains, our first frost is coming in the next couple of months, and that will end the fall armyworm problem for 2021.  A frost will kill them, but we hope that frost is still a long way off.


Insecticides provide no control of fall armyworm eggs, and there is no advantage to spraying insecticide on egg masses found on structures and under tree leaves.  Scrape, brush or wash with soapy water to remove the egg masses and hatching caterpillars. 

Fall armyworm caterpillars are orange-brown with lengthwise stripes.
Fully grown fall armyworm caterpillar.  Photo by Marlin Rice.  https://www.ent.iastate.edu/imagegal/

Careful and thorough scouting is going to be critical right now as we wait and watch for damage.  Carefully inspect turfgrass beneath items where the egg masses were laid.  Fall armyworms are surface feeders.  They may conceal in the heat of the day, but they live above ground and feed on the grass blades.  You will not need to dig in the soil to find them.  Young caterpillars will be only 1/8-inch-long and greenish with a black head.  The head becomes more orangish as the caterpillars grow. Mid-sized caterpillars become browner, and the distinctive lengthwise stripes begin to appear.  The largest caterpillars have a reddish-brown head, more distinct stripes, and elevated dark spots on the body. 

There is no threshold for fall armyworm caterpillars.  Evaluate the turfgrass health and vigor, the size of the caterpillars, the amount of feeding damage, and the time till expected frost.

Insecticides need to be ingested by the caterpillar to be effective, which means the insecticide needs to be on the surface where the caterpillars feed.  Label directions specify to “delay irrigation or mowing for 24 hours after application.”  White grub insecticides applied earlier in the season and moved into the soil via rain or irrigation are not likely to control armyworms.

If insecticide treatment is warranted to protect the turfgrass, use a registered insecticide according to label directions.  Pyrethroid insecticides will give control of about 7 days and are a cost-effective treatment.  Note, however, that pyrethroid resistance has been reported in other parts of the country.  If there is not a quick control of fall armyworms after applying a pyrethroid, consider using an alternative such as chlorantraniliprole (Acelepryn) or tetraniliprole (Tetrino).  Both may provide up to 30-days of control.  Combination products containing a neonicotinoid plus a pyrethroid (e.g., Aloft, Alucion) have been successfully used to overcome resistance in other areas.  Insecticides with azadirachtin (Azatin O, Azaguard, and Neemex 4.5) can provide control and are certified organic.  These products may be hard to find.

If you do end up with damage, recovery is possible. The caterpillars will eat the leaf blades, and the crown can become exposed to environmental stress. Supplying regular moisture will help keep the crown alive and allow the plant to recover. You can also add new seed to the area.  Fall is the best time to establish turfgrass.  After seeding, make sure to keep the area supplied with proper soil moisture. Finally, an application of fertilizer will provide nutrients to help the plants recover more quickly. Applying 0.75 to 1 lb. of a slow-release nitrogen source per 1,000 square feet will provide the needed nutrition to speed up recovery.


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