How to Get Rid of Fall Armyworms

If you see patchy brown spots in your lawn or chewed foliage on food plants like sweet potatoes and beans, you may have armyworms.

While there are several species of these invasive pests, two common types are true armyworms (Mythimna unipuncta) and fall armyworms (Spodoptera frugiperda). We'll focus on fall armyworms here.

What Are Fall Armyworms?

Both true armyworms and fall armyworms are in the Noctuidae moth family and cause most of their damage in the larvae, or caterpillar, stage. They reproduce fast and travel in large numbers — think of a hungry army marching along, feeding on plants and grasses and moving on when they've devoured almost everything in the way. A lawn that's severely infested can be wiped out in days.

Freezing weather kills armyworms, which are native to tropical areas, but they can overwinter in mild climates. In the adult moth stage, they're able to fly long distances and migrate from state to state.

While true armyworms show up in the spring, fall armyworms appear towards the end of summer and persist until winter frosts kill them off, says Marc Mayer, director of technical operations for TruGreen. They can be hard to spot, he adds, because they're most active at night, when the temperatures are cooler.

While these surface feeders don't harm roots or crowns, Marc says, you'll see signs of their destruction — chewed grass blades — and piles of frass (droppings) indicating their presence.

The Life Cycle of Fall Armyworms

Marc says fall armyworms complete all life cycle stages year-round within 100 miles of the Gulf Coast and can produce multiple generations in southern US states, but only one generation migrates north during the growing season.

An armyworm can complete its life cycle between 25-30 days and in four stages:

1. eggs
2. larvae (feeding caterpillars)
3. pupae
4. adults

Adult female moths lay masses of eggs on foliage, tree limbs and other surfaces and cover them with gray scales from their bodies. The eggs hatch in about four days into larvae that go through six phases of development, maturing in approximately 14 to 30 days.

Fall armyworm larvae typically pupate in the soil — or on top of hard soil — in loose, oval-shaped cocoons. If a long period of cold weather doesn't kill them, adult moths emerge.

Full-grown caterpillars — which cause the damage to your lawn — are 1 1/2 to 2 inches long and have nearly hairless, striped bodies and an upside-down Y shape on their heads. They live roughly seven to 21 days, depending on the weather and geographic location.

What Does Fall Armyworm Damage Look Like?

Fall armyworms feed on some 80 host plants but prefer grasses like Bermudagrass and tall fescue; they'll also eat crabgrass.

Marc says signs of their damage include:

• Nibbled, ragged grass blades
• Skeletonized leaves
• Discolored grass at lawn edges
• Increased bird activity (birds prey on them)
• Green fecal pellets in damaged areas

Marc says that preventing armyworms can be challenging due to their rapid movement but suggests that homeowners take these steps to protect their lawns and gardens.

• Look for armyworms underneath chewed leaves. Egg masses may also be hidden under leaves."Scout for damage and the actual caterpillars while outdoors in the evening or early morning," Marc suggests.
  
  
• Get rid of grubs, insects and other pests as soon as you see them, before they weaken or harm your grasses and other plants.
  
  
• Maintain your lawn by practicing correct watering and mowing techniques. A healthy lawn can better withstand pest damage.
  
  
• Remove fallen leaves and other debris that can be used as hiding places, so birds and other predators can find and devour the armyworms. Dethatch your lawn if needed.
  
  
• Use professional pest control. "Opt for professionally applied treatments designed to keep lawn pest populations in check, preventing potential damage," Marc suggests.

Non-Chemical Methods for Fall Armyworm Control

Start your attack on armyworms with environmentally friendly, non-chemical practices. Step up to stronger methods if these don't solve your problem.

• Hand-pick the armyworms and drop them into a bucket of soapy water. Armyworms don't harm people directly unless they've been treated with insecticides, but you may want to wear gloves.
  
  
• Encourage beneficial insect predators like ladybugs and lacewings. Buy them online or attract them by planting herbs and flowers that offer nectar and pollen.
  
  
• Use beneficial nematodes, available from garden centers or online. They're most effective on larval armyworms.
  
  
• Apply Bacillus thuringiensis (Bt), a naturally occurring bacteria that produces a toxin that will kill the caterpillars when they eat it.

Chemical Solutions for Fall Armyworm Control

If you use chemicals, make sure the product is labeled to control fall armyworms and follow all product directions carefully. A study has shown that neem oil, a natural insecticide, can prevent fall armyworms by disrupting their life cycle during the larval stage.

Many homeowners prefer to leave the application of chemical treatment to the professionals.

Preventing Future Fall Armyworm Infestations

How do you keep fall armyworms from coming back, and how do you handle the damage they've already done?

"Homeowners will be relieved to know that damage from the pest isn't permanent," Marc says.

Water affected areas regularly to promote regrowth. "For cool-season turf like tall fescue, consider aeration and overseeding, especially if the crowns are damaged or dead," says Marc. "Schedule a professional armyworm treatment if they're still present. If you have a lawn care service, let them know about the armyworms so they can customize the next feeding for your lawn."