Fall armyworms can be a menace to lawns, pastures, and grass crops such as corn and sorghum. They arrive every year in the late summer or early fall. They can be very damaging to your plants so let’s look at what they are, how to spot them quickly, and what you can do for management of them.
Fall armyworms have four life stages. They over winter in Florida, as they can only survive a mild winter. Each successive generation moves northward, causing a lot of damage as they advance. The females will lay egg masses of up to 700 eggs at a time. After coming out of their eggs the larvae, the caterpillar stage, will be about one quarter to three quarters of an inch long. The larvae go through six stages of development. Each stage eats more than the one before it. However, the final stage of larval development eats more than all the other stages combined. Armyworms take two to three weeks to reach the pupal stage, where they go into cocoons. After about a week in the cocoon the armyworm moth will emerge. These moths will travel further northward and lay eggs repeating the cycle.
Larval stage is when they will do damage to lawns, pastures, and grass crops. At this stage they can get up to 1.5 inches long. Their coloring can vary from light green to nearly black with dark stripes that stretch horizontally down the length of their body. Their most distinct characteristic and the best way to identify them is by the inverted Y shape on the head. The point of the Y is at the back of the head.
Signs to look for are flocks of birds in the morning and evening. That is when armyworms will do most of their feeding. During the heat of the day they will retreat to the thatch below. The armyworms will preferentially feed on tender green vegetation. Damage from armyworms can also look like thin brown patches. One method of checking for armyworms is to mix two tablespoons of lemon-scented dish soap with a gallon of water, and then pour that into a one square foot area. That will irritate the worms and make them crawl up so that they’re more visible. The soapy water method works on all caterpillars, not just armyworms. Usually treatment is recommended if there are more than three armyworms per square foot.
If you have a well-established lawn armyworms may feed on the grass but most likely will not kill the grass, however the damage is unsightly. The only really effective way to control armyworms is through insecticides, and even then control will be very difficult on full grown caterpillars. Therefore, scouting early is essential. Sevin is an effective product that has been around for a while. Products containing spinosad or Bacillus thuringiensis have organic options and good control of armyworms. Applying as late in the evening as possible is best because that’s when armyworms will be the most active and pollinators the least active. Whenever using pesticides always read and follow the label before the application.
The Extension is available is you need help identifying a pest. You can email a picture of the insect to Jacob.Williams@uga.edu or bring one into the Extension office in a sealed container.