Fall armyworms are the caterpillar like larvae stage of what eventually becomes a tan colored moth that flies at night. An armyworm infestation can damage large sections of your lawn, very quickly. Unlike white grubs which feed on your lawns roots, armyworms feed on the actual foliage.
These pests, which are about an inch long, vary in color but are generally brown with stripes along the side of its body, and one thin white stripe down its back. If you look closely you should see a white inverted Y shape right at the head.
Fall armyworms are mostly a nuisance in warm season lawns since they aren’t tolerant of cold temperatures. They will not survive winters in northern states, however, large infestations can breakout in Northern states when moths fly, or get blown over from the South.
Generally speaking, armyworms seem to be becoming more of a nuisance in Northern states and are having a negative impact on cool season lawns, especially this year (fall 2021). Weather events such as hurricanes can push these insects further north where they eventually lay eggs, and larvae will feed before succumbing to the Northern temperatures in late fall.
Fall armyworms have 4 distinct life stages: Egg, larva (worm), pupa, and adult (moth). Adult moths fly and mate at night. Females can lay up to 1000 eggs and typically do so in trees, shrubs, and other areas around grass. Once they hatch and become larvae they move into the grass where they spend their days hiding in the soil. In the late afternoon/evenings they surface to feed on grass foliage.
After about 2 weeks the larvae will go into the soil and pupate. In about 10-14 days they will have fully transformed into a moth and the cycle will repeat as long as weather permits.
How to Know If You Have Armyworms
In the early stages of damage, your lawn, or sections of your lawn, will probably just look like normal stress. You might chalk it up to heat stress since it can look pretty similar. As damage progresses you’ll see more significant patchy brown spots.
This is why it’s so important to play detective. If you have an area in your lawn that is stressed, do some digging! Take a square shovel and dig up a small section. Even if it’s just a one foot square section. Dig it up and investigate the soil. If you have a serious armyworm infestation than you will see them.
You’ll never know what’s really going on in your lawn unless you dig. You can put the square section right back in place and it will be fine so it’s not like you’ll be harming your lawn.
Another great test you can do to see if you have armyworms is the Soap Flush Test. Simply fill a 5 gallon bucket with water and add a squirt of dish soap and mix it. Pour the 5 gallons in a spot where you are seeing damage or just pick any spot where you want to test for armyworms. Flood one specific area, about 2 square feet with the bucket. All you’re doing is just slowly dumping out a bucket of water onto one specific spot of your lawn. Waita couple of minutes and you’ll see armyworms at the surface. If you don’t see any than you most likely don’t have an infestation of armyworms. Test several areas throughout the lawn to see if you have a widespread infestation.
Armyworm Treatment Options
Once you’ve identified that armyworms are in fact the problem in your lawn, there are some simple ways to treat them.
The first product I recommend is the same product I recommend as a grub curative treatment: BioAdvanced 24 Hour Grub Killer Plus. The active ingredient is Trichlorfon. I really like that this one product can treat for both grubs and armyworms especially since these two pests are present around the same time. Apply this granular product immediately after spotting armyworms. You can reapply 2-4 weeks later depending on how severe your infestation is but don’t apply more than 3 times a year. As always follow the label for application instructions.
Another great product to treat armyworms is Talstar P which contains the active ingredient Bifenthrin. Talstar P is a liquid concentrate that can be mixed with a back pack sprayer or used in a hose end sprayer. This concentrate gets mixed at a ration of 1 oz. to 1 gallon which should cover 1000 s/f. Apply to your lawn right before sunset since that’s when the armyworms will be active at the surface. Reapply Talstar P a month later if necessary.
Feel free to use one, or both of these products to treat for armyworms. My preferred way to treat for armyworms is to use a combination approach. In this case, I recommend applying the BioAdvanced 24 Hour Grub Killer and then a week later apply the Talstar P. Two weeks after that do another application of BioAdvanced 24 Hour Grub Killer. A week after that you can apply the Talstar P again. This is an aggressive approach but since armyworms go from egg to adult in about 28 days there may be several cycles that continue until the problem is solved.
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