Army worms are the destructive larval stage of several moth species, posing a common threat to gardens, crops, and lawns. These pests are infamous for their massive, coordinated feeding habits, which leads to their name, as they appear to “march” across a landscape consuming foliage. Because they can quickly decimate large sections of vegetation, immediate action is necessary once an infestation is confirmed. This guide outlines strategies for controlling army worms.
Identification and Early Warning Signs
Army worm larvae are caterpillars that vary in color, ranging from yellow and green to dark brown or gray, depending on the species and age. A distinguishing characteristic of many army worms, such as the fall army worm, is the presence of an inverted, yellowish-white “Y” shape on the front of the head segment. Their bodies often feature several prominent parallel stripes running along the length, which may be white, yellow, or orange.
The damage they inflict can appear almost overnight due to their nocturnal and voracious feeding habits. Early damage on turfgrass may initially resemble drought or wilting, but closer inspection reveals leaves that are chewed along the margins or skeletonized. In vegetable gardens, they clip seedlings near the soil line or create ragged holes in leaves. A telltale sign of an active infestation is the presence of caterpillar excreta, or “frass,” which are tiny pellets found on the leaves or at the base of the plants.
Immediate Manual and Cultural Removal Techniques
The first line of defense, especially for smaller garden spaces, involves manual removal techniques that require no commercial products. Handpicking army worms is highly effective and involves inspecting plants, particularly at night with a flashlight when the larvae are most active. The collected worms should be dropped into a container of soapy or salty water to ensure they are killed swiftly.
For larger areas, you can create physical barriers or traps to capture the marching worms. Digging a steep-sided trench, at least six inches deep around vulnerable areas, can trap the larvae as they attempt to move into the garden. Another method involves laying down boards or strips of burlap in the evening; army worms will use these for shelter during the day, allowing for easy collection and disposal in the morning.
Adjusting irrigation practices can also make the garden less hospitable to army worms, which prefer dense, lush growth. Excessive watering creates humid conditions that favor army worm development and attract egg-laying moths. Keeping the lawn and garden adequately watered, while avoiding overwatering, helps reduce the site’s attractiveness.
Organic and Biological Control Methods
Once an infestation is confirmed and manual control is insufficient, targeted biological and organic methods offer an effective, low-impact solution. One common biological control is the use of Bacillus thuringiensis (Bt), a naturally occurring soil bacterium. Bt products, particularly the kurstaki or aizawai subspecies, are ingested by the feeding larvae. They release crystal proteins that disrupt the army worm’s digestive system, leading to cessation of feeding and death within days.
It is important to apply Bt when the larvae are small, typically in their first or second developmental stages, as it is less effective against mature worms. Since the bacterium is quickly broken down by sunlight, applications should be timed for the late afternoon or evening to maximize effectiveness while the worms feed at night. For the treatment to work, the larvae must consume the treated foliage, requiring thorough coverage of the plant.
Beneficial nematodes, specifically species like Steinernema carpocapsae or Heterorhabditis bacteriophora, are microscopic roundworms that control the soil-dwelling stages of the army worm. These nematodes actively seek out larvae and pupae in the soil, entering their bodies and releasing a symbiotic bacterium that kills the pest within 48 hours. Nematodes should be applied to moist soil during the cooler parts of the day, such as early morning or pre-dusk, to protect them from heat and UV light.
Organic sprays like spinosad, derived from a soil bacterium, are highly effective against army worms. Spinosad works through both contact and ingestion and is often recommended for heavier infestations or when the larvae are slightly larger. Neem oil, a botanical insecticide, can also be applied to deter feeding and disrupt the insect’s life cycle. Both spinosad and neem oil should be applied in the evening to target active army worms and avoid harming beneficial insects, such as bees, that forage during the day.
Preventing Future Infestations
Long-term management focuses on cultural practices that make the garden less inviting to adult moths and interrupt the army worm life cycle. Crop rotation is a useful strategy, as it limits the availability of preferred host plants in the same area season after season, disrupting the pest’s ability to build up populations. Removing garden debris and controlling weeds is also important, as these areas provide shelter for larvae during the day and serve as sites for moths to lay eggs.
Physical barriers, such as floating row covers or fine-mesh screening, can be deployed over vulnerable plants during peak egg-laying season to prevent adult moths from accessing the foliage. This proactive measure is especially helpful for protecting newly transplanted seedlings. Additionally, encouraging the presence of natural predators, like parasitic wasps, ladybugs, and lacewings, helps suppress army worm numbers.
These beneficial insects feed on army worm eggs and smaller larvae, providing a continuous, natural form of pest control. Reducing the use of broad-spectrum insecticides helps conserve these natural enemies, allowing them to establish a presence in the garden. Tilling the soil in the fall can also expose overwintering pupae to the elements and predators, reducing the number of moths that emerge in the spring.