But what are armyworms? These moth caterpillars are so called because they gather into groups or “armies” while feeding. At least, the fall armyworm does. There are 3 common species of armyworm in the Central Great Plains and 2 that are important pests in the lawn and landscape.
Armyworms are members of the Noctuid Moths, which has over 2,500 members in North America. This family of moths includes armyworms, cutworms, dagger moths, owlets, darts, and miller moths. Moth identification can be difficult, and entomologists are constantly changing family groups, but armyworms are definitely in this group.
The 2 species of armyworms that we need to recognize are the yellow-striped armyworm and the fall armyworm. They are both important pests in our gardens and lawns.
Here is a pest that seems to show up everywhere. Almost every day from March to November, I wander through my client’s gardens as well as my own, searching for insects, both pests and beneficials. Some are both, like butterflies and their caterpillars. The yellow-striped armyworm is easy to find.
Feeding on a wide range of plants, yellow-striped armyworms are usually dark brown to nearly black with 2 yellow lines running lengthwise down the back of the caterpillar. Here is a partial list of plants you may see the caterpillars feeding on:
- Flower heads of many annual and perennial flowers
- Bush Beans
While they are general feeders, most persistent damage is on the fruits of tomatoes, peppers, and other vegetables in the garden. If you are like me or other gardeners with chickens, you can just handpick the caterpillars into a bucket and feed them to the hungry birds. Otherwise, just drop off the plant and squish them with you shoe.
I have not seem a case yet where yellow-striped armyworms required chemical treatment or prevention. It is good to note that there are several insects which either parasitize or eat yellow-striped armyworms, in either the caterpillar or egg stage. There are 6 species of wasps known to parasitize the caterpillars and 9 species of tachinid fly.
Predatory insects include minute pirate bugs, big-eyed bugs, and damsel bugs. Spiders, birds, and assassin bugs all eat the adult moths.
Another pest in the garden, which is more prevalent on lawns, is the fall armyworm. Just like yellow-striped armyworms, the fall armyworm feeds on a wide variety of plants, but mainly on turf grasses in our region. It is likely that this insect is where the term armyworm originated, as they feed in huge “armies” on turf, devouring blades of grass.
The eggs are laid in large masses by the adult moth and then feed gregariously over the hatching site. Lawns can become brown quickly if predators are not available. And in our highly treated lawns and landscapes, predators may not be available. Here is a partial list of some of the host plants for fall armyworms.
- Timothy Grass
- Tall Fescue
Fall armyworms can be difficult to predict from one year to the next. Sometimes you may go 3 to 5 years without getting an explosion of caterpillars in your lawn or garden. In Kansas, we can have up to 2 broods per year, if the weather is right. The best treatment for stopping an invasion of armyworms across your lawn is a chemical application.
There are however, many animals and birds which happily feed on large numbers of armyworms in the garden or lawn. A few parasitoid wasps and flies will attack the caterpillars, while ground beetles, predatory bugs, and earwigs all feed on the caterpillars.
Plants to Attract Predatory and Parasitic Insects
When discussing the above treatment options for armyworms, we need to think more about how to attract predatory and parasitic insects to the garden. The biggest method is of course to limit the amount and kinds of chemicals you apply to both the garden and the lawn. The second is by adding certain plants into the garden.
Both predatory and parasitic insects feed on pollen and some plants are better at attracting them than others. The following list has 12 species of annual or perennial plants that attract these beneficial insects.
- Sedum ‘Autumn Fire’
- Helenium ‘Dakota Gold’
- Common Boneset
- Butterfly Milkweed
- Slender Mountain Mint
- Agastache ‘Blue Fortune’
- Agastache ‘Blue Boa’
- Ornamental Onion ‘Millenium’
- Ornamental Onion ‘Summer Beauty’
- Rattlesnake Master
- Swamp Milkweed
Armyworms are a common pest that can make either a big or small impact in the garden and lawn. Keep your eyes open for these pests, but also for their predators. Combatting insect pests in the garden should be a mix of providing habitat and food for beneficials, handpicking and monitoring, and using pesticides (as a last resort).