Fall Armyworms are Making Their March

Published 3:04 pm Wednesday, August 11, 2021

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If they have not already eaten your lawn, start scouting now, because this year they seem to be everywhere! The phones have been ringing off the hook, so we know they are already in the area and the numbers this year are extremely high!

The armyworm caterpillars will feed on almost all forage grasses, as well as 100 additional plant species including corn and cotton, which is most detrimental to the cattlemen and farmers.  Most often in the homeowner setting we see them in the lawn and then in the vegetable garden (especially on the tomatoes).

The caterpillar stage of the armyworm develops into a moth that lays eggs and the eggs hatch out into baby caterpillars where the cycle begins all over again.  In Alabama, there may be as many as five to six generations of this pest every summer (Usually beginning in mid-July/ early August).  That being said, continue to scout as we have time for multiple generations of armyworms before cooler temperatures arrive.

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The best method for control is scouting to detect them when they are young.  Fall armyworms can be found on foliage at any time of day, but are more easily detected early in the morning or late in the afternoon. In heavy infestations, you will see caterpillar droppings on the ground underneath your plants.  In the lawn the grass will look like it is moving when they are actively feeding and you will start to see brown patches in the yard where they are eating all of the green tissue off of the grass.

When fully grown, armyworms are 1.5 inches long.  They are always striped, but their coloring is not always the same. Their background color ranges from light green to almost black.

You can identify fall armyworm caterpillars by four black dots on the back end of the abdomen. Larger caterpillars typically have a light-colored, upside-down Y-shape on the head and three white lines on top of the segment just behind the head.

The earlier an infestation is detected the better. Young fall armyworms (under a half inch in length) don’t eat much.  As the caterpillars get bigger, their food demands increase dramatically and the bigger they are, the harder they are to control.  Scouting for the worms and treating when they are young is your best defense.

Scouting for fall armyworms is a relatively simple process, but for a homeowner they are often undetected until you go out to harvest the ripening tomatoes and there you find them boring into the fruit.  In the lawn, they are usually undetected until after the lawn has been mowed and you pull your mower into the garage and walk out an hour later and there are worms all over the ground.

Control of fall armyworms is justified when the population exceeds three 0.5-inch caterpillars per square foot. Fall armyworms need to be treated when they are still small — about 0.5 to 1 inch long. Detecting infestations when the caterpillars are small gives more time for control measures to be implemented.

When armyworms are fully grown, they are less susceptible to insecticides, and therefore are harder to kill. In addition, if most of the caterpillars are nearly grown, most of the damage will already have been done.  In this case, keep the lawn watered well to reduce stress and more than likely the grass will grow back in that area over time, it just leaves an unsightly look to the lawn.

Armyworms do seem to prefer Bermuda lawns over all others, but that does not mean that the other grasses are less susceptible. They will truly eat any kind of grass.  If the worms are large, they have done about all the damage they are going to do and we do not recommend treating.

Treat late in the day when the caterpillars are actively feeding.

Insecticides containing carbaryl or one of the pyrethroids (active ingredients bifenthrin, cyfluthrin, deltamethrin, lambda-cyhalothrin, permethrin) are effective against fall armyworms. Active ingredients are listed on the insecticide label.  Always read and follow label instructions.

The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) is committed to affirmative action, equal opportunity, and the diversity of its workforce.  Educational programs of the Alabama Cooperative Extension System serve all people regardless of race, color, age, sex, religion, disability, sexual orientation, or national origin.

If you have other gardening related questions please call the Lowndes County Extension office at 334.548.2315.   You may also call the Master Gardener Helpline at: 1877-ALA-GROW (252-4769). Visit us at ACES.EDU.