Armyworms are voracious caterpillars that can quickly decimate lawns, pastures, and crops if left uncontrolled. Getting rid of armyworms in your yard takes proactive monitoring and control methods. This comprehensive guide covers everything you need to know about armyworm identification, life cycle, damage, and control for yards and gardens.
What Are Armyworms?
Armyworms are the caterpillar stage of several moth species in the genera Spodoptera and Mythimna. There are both native and invasive armyworm species in North America. Some of the most common armyworm pests include:
- Fall armyworm (Spodoptera frugiperda) – Native. Feeds on over 80 plant species. Most destructive armyworm in warm regions.
- True armyworm (Mythimna unipuncta) – Native. Damages grasses and cereal crops. More of an issue in cooler areas.
- Common armyworm (Mythimna convecta) – Invasive. Arrived from Eurasia. Feeds on many garden plants and grasses.
Armyworms get their name from their habit of “marching” across fields or lawns in large groups as they feed. High infestations can completely destroy leaves, stems, and heads of plants overnight.
What Do Armyworms Look Like?
Armyworm caterpillars are smooth-bodied and hairless, with brown, green, black, and yellow stripes running lengthwise along their backs and sides. They have light undersides. Mature larvae reach 1 to 1 1/2 inches long.
Armyworms can be difficult to spot due to their coloration providing camouflage. Look closely to detect them crawling low on leaves and plants. Heavy infestations may result in visible leaf damage before caterpillars are noticed.
Armyworm Life Cycle
Armyworms undergo complete metamorphosis with four life stages:
Eggs
- Laid in clusters on leaves and stems
- Tiny, spherical, and pale yellow to greenish-white
- Hatch in 2-10 days
Larvae
- Feed for 2-4 weeks after hatching
- Molt 6-7 times, reaching up to 1 1/2 inches long
- Cause the most plant damage during larval stage
Pupae
- Form cocoons in soil or plant debris after final larval molt
- Overwinter as pupae in cold climates
- Emerge as adult moths in warm seasons
Adults
- Active at dawn/dusk
- Live 7-10 days
- Mate and lay eggs shortly after emerging
- Females lay eggs in clusters of 50-200
Armyworms develop from egg to adult in as little as 30 days in peak summer. Multiple generations occur annually in warm climates. Peak damage is usually in mid to late summer.
Armyworm Host Plants
Armyworms are generalist feeders, meaning they attack a wide variety of grasses, grain crops, and broadleaf plants.
Some of their most common garden and turfgrass hosts include:
- All lawn grasses – Bermuda, zoysia, St. Augustine, centipede, ryegrass, fescue
- Corn, wheat, oats, barley, rice
- Vegetable crops – Tomato, pepper, cole crops, lettuce, beans
- Ornamentals – Marigold, aster, zinnia, rose, dahlia
Fall armyworms are especially problematic, feeding on over 80 different crop and garden plants. No plant with green vegetation is immune to armyworm feeding.
Recognizing Armyworm Damage
Being able to identify armyworm damage is key for effective monitoring and control. Watch for these common signs:
Skeletonized leaves
Armyworms chew irregular holes and notches in foliage, leaving just veins and stems behind. This skeletonizing damage is most notable overnight or in the early morning as armyworms feed at night.
Dead spots or thinning turf
In lawns, armyworms graze grass blades down to the crown. This causes dying circular patches that expand rapidly across turf. Damage may first appear along sidewalks or lawn edges.
Plant cutoffs
Armyworms sometimes attack at the base of plants, completely severing them from roots and killing the plant. Look for missing seedlings or cut-off flowers and vegetables.
Frass
Armyworm droppings (frass) may accumulate under plants or armyworm hiding spots. Frass looks like greenish to black sawdust-textured pellets.
Chewed leaf margins
Less severe armyworm feeding causes chewed, ragged leaf edges. This is often found on outer leaves or hidden interior leaves.
Finding Armyworms in Lawns and Gardens
Finding armyworm caterpillars takes diligent scouting. Here are some of the best places to look:
- Undersides of low-hanging leaves – armyworms avoid light and often feed upside down on leaves.
- In lawn thatch – armyworms hide at the soil surface during the day. Part turfgrass thatch to find them.
- Inside curled leaves or folded corn ears – armyworms like protected sites.
- Near damaged areas – focus efforts where you see feeding injury or dead patches.
- At base of plants – armyworms sometimes cut off plants as they feed on stems.
Handpicking and destruction is recommended if only a few armyworms are found. For heavier infestations, continue reading for control options.
Natural Armyworm Control
For mild armyworm invasions, try these eco-friendly control solutions first:
Encourage natural predators
Many beneficial insects and birds eat armyworms. Avoid broad-spectrum insecticide use which can kill these predators. Provide habitat like bird houses, diverse plants, and insectaries. Common predators include birds, parasitic wasps, tachinid flies, ground beetles, and insect pathogens.
Use pheromone traps
Pheromone traps draw in male moths with sex attractants and prevent mating and egg-laying. Use them as a monitoring system to determine armyworm population levels.
Apply Bt (Bacillus thuringiensis)
This microbial insecticide contains a bacterium deadly to caterpillars when ingested but harmless to people and pets. It must be reapplied frequently because it breaks down quickly.
Introduce nematodes
Beneficial entomopathogenic nematodes can be applied to soil or turf to control armyworm larvae. Look for species like Steinernema carpocapsae or Heterorhabditis bacteriophora.
Use horticultural oils and neem oil
These botanical insecticides smother eggs and interfere with armyworm feeding when sprayed directly on larvae. They must contact insects to be effective.
Deploy row covers
Floating spun polyester row covers can physically exclude moths from gardens and prevent them laying eggs on crops. Use them only for small plantings.
Trap with boards
Place boards on the lawn or soil surface at night. Armyworms will gather underneath them. Destroy collected caterpillars in the morning.
Manage lawn health
Healthy, vigorously growing turfgrass is less vulnerable to extensive armyworm damage. Raise mowing height, irrigate, and fertilize appropriately to improve lawn vigor.
Chemical Armyworm Control
For moderate or severe armyworm invasions, chemical insecticides are the most effective solution. Treat as soon as feeding damage appears:
Insecticidal dusts – Diatomaceous earth, pyrethrin dust
These mineral and plant-derived dusts are low toxicity. Spread over grass and leaves. Avoid inhaling dusts. Reapply after rain or watering.
Insecticidal soaps and oils – Potassium salts of fatty acids, neem oil
Spray on plants and turf to kill larvae through contact. Also smothers eggs. Treat in evenings to avoid leaf burn.
Microbial insecticides – Bt-kurstaki, spinosad
These bacteria and their byproducts are toxic when eaten by armyworms but have very low toxicity to mammals.
Synthetic insecticides – Carbaryl, bifenthrin, cyfluthrin
Broad-spectrum chemicals like carbaryl (Sevin), synthetic pyrethroids, and organophosphates provide 1-2 weeks of residual caterpillar control. Use as a last resort and follow label precautions.
For any chemical control:
- Read and follow all label directions
- Spray both sides of leaves and down into turfgrass thatch
- Scout again in 3-5 days, reapply if needed
- Avoid treating flowers in bloom to protect pollinators
- Irrigate treated lawn lightly after application
Preventing Armyworms
While armyworm outbreaks can’t always be prevented, the following practices help reduce risk:
Avoid early season over-fertilization
Heavy nitrogen fertilization of lawns or crops in spring creates the succulent tissue armyworms prefer. Moderate fertility prevents susceptibility.
Eliminate weed grasses
Armyworm moths prefer laying eggs on grasses like crabgrass, foxtail, and ryegrass. Keeping turfgrass weed-free limits egg-laying sites.
Clean up crop residue
Destroy or bury leftovers after harvest. Armyworm moths are attracted to lay eggs in plant debris.
Monitor population levels
Use pheromone traps and regular field scouting. Take action at first signs of infestation before it intensifies.
Time plantings carefully
Staggering successive crops helps avoid periods of peak armyworm activity.
Promote diverse habitat
Polycultures with a mix of plant families support more predators. Mono-cropping increases armyworm risk.
FAQs About Controlling Armyworms
What kills armyworms naturally?
Many biological controls can kill armyworms including parasitic wasps, ground beetles, tachinid flies, birds, insect pathogens like Bt (Bacillus thuringiensis), beneficial nematodes, and horticultural oils/soaps. Promote these predators don’t use broad-spectrum pesticide that might kill them.
What is the best insecticide for armyworms?
For home lawns and gardens, microbial insecticides like Bt-kurstaki, spinosad, or insecticidal soaps are the best choice and safest for people and wildlife. In commercial agriculture, synthetic pyrethroids and carbaryl work best for armyworms but have higher environmental impact.
How do you stop armyworms from returning?
There are no foolproof ways to prevent armyworms year after year but cleaning up debris, maintaining plant diversity, avoiding early over-fertilization, monitoring for moths, and timing plantings carefully all help reduce recurrence.
What kills armyworms in the lawn?
Effective lawn treatments for armyworms include parasitic nematodes, insecticidal soaps/oils, microbial insecticides like Bt-kurstaki and spinosad, and synthetic chemicals like bifenthrin and carbaryl as a last resort. Treat armyworms at first signs of damage before populations explode.
Do armyworms come back every year?
Armyworm moths emerge every year in spring and summer to lay eggs. But major outbreaks don’t happen annually in the same location. Good sanitation, plant diversity, and monitoring moth flights can help prevent annual armyworm damage.
How do I know if I have armyworms?
Look for signs like skeletonized leaf damage, dying circular lawn patches, cut off seedlings, chewed leaf margins, and armyworm droppings around plants. Search under low leaves, thatch, and debris for striped smooth caterpillars. Act promptly once feeding evidence appears.
Conclusion
Armyworms are stealthy but destructive lawn and garden pests. Stay vigilant for signs of feeding damage, especially in mid to late summer. Implement preventive cultivation methods to reduce armyworm susceptibility. Monitor carefully and treat promptly at the first evidence of armyworms before populations explode. Use natural control methods like beneficial insects, pheromones, and microbials when possible, or synthetic chemicals for severe infestations. Swift action is key to get rid of armyworms before irreparable lawn and crop damage occurs. With a proactive integrated pest management approach, armyworms can be effectively controlled and your yard and garden protected.