Bagworms: A Destructive Threat to Emerald Arborvitae and How to Stop Them

Emerald arborvitae (Thuja occidentalis 'Smaragd') is a favorite landscape plant for homeowners and commercial landscapers alike. Valued for its dense evergreen foliage, upright growth habit, and low maintenance, it's a go-to choice for privacy screens, foundation plantings, and ornamental hedges. Unfortunately, its popularity has also made it a frequent target of a particularly destructive pest: the bagworm.

If left unchecked, bagworms can cause serious, even fatal, damage to emerald arborvitae. Understanding their life cycle, the signs of an infestation, and how to control them with insecticides is critical to preserving the health and beauty of your landscape.

What Are Bagworms?

Bagworms (Thyridopteryx ephemeraeformis) are moth larvae that feed on a variety of ornamental plants and trees. Despite their name, they are not worms at all. These caterpillars are the larvae of a moth species that build protective cases—“bags”—from silk and bits of plant material, such as needles and twigs. They live and feed within these bags, making them difficult to detect until serious damage has already occurred.

Each bagworm case is distinctive, blending in with the host plant. This natural camouflage not only protects the larvae from predators but also from many forms of pest control.

Why Emerald Arborvitae Are Especially Vulnerable

Emerald arborvitae are a favorite target of bagworms. Their soft, evergreen needles provide an ideal food source, and their dense foliage offers perfect protection for larvae. Once a bagworm infestation begins, damage can quickly escalate:

• Defoliation: Bagworms chew through the soft foliage, causing browning and thinning. If left untreated, branches may become completely bare.
• Stress and Decline: Loss of foliage reduces the plant’s ability to photosynthesize, weakening it over time.
• Permanent Damage: Severe infestations may cause entire sections of the tree to die back. In extreme cases, especially on young or stressed arborvitae, the entire plant can be killed.

Unlike deciduous plants, evergreens like arborvitae don’t readily regrow lost foliage. This makes bagworm damage more visually apparent and longer-lasting.

Bagworm Life Cycle: Timing Is Everything

Understanding the bagworm life cycle is crucial for effective treatment. Bagworms go through one generation per year:

1. Egg Stage (Fall to Spring): Females lay hundreds of eggs inside their bags in late summer. These eggs overwinter and hatch in late May to early June.
2. Larval Stage (Early Summer): Tiny caterpillars emerge and begin feeding almost immediately. They start constructing their protective bags, growing rapidly over several weeks.
3. Pupal Stage (Late Summer): In August, the larvae pupate inside their bags. Males will eventually emerge as moths and seek out wingless females for mating.
4. Adult Stage (Late Summer): After mating, females lay eggs inside their bags and die shortly after. The cycle then begins again.

Early summer is the most effective time for treatment—when the larvae are small and more vulnerable to insecticides.

Signs of a Bagworm Infestation

Bagworms can be tricky to spot until damage is already underway. Look for:

• Small, spindle-shaped bags hanging from branches—often mistaken for cones or debris.
• Brown or thinning foliage, especially at the tips of branches.
• Movement: On close inspection, you may see the larvae moving their bags slowly along the branches as they feed.

By mid to late summer, the bags become larger (up to 2 inches) and easier to spot. However, by this point, much of the damage is already done and treatment options are more limited.

Insecticide Treatments: What Works Best?

For homeowners and landscape professionals dealing with a bagworm infestation, insecticide application is one of the most effective control methods—if timed correctly.

Best Time to Spray

Late May to mid-June is ideal.

This is when the larvae are newly hatched and most susceptible to chemical control.

Once the bagworms are larger or have entered the pupal stage, insecticides become far less effective.

Recommended Insecticides

Here are some of the best insecticide options for treating bagworm infestations on emerald arborvitae:

1. Bacillus thuringiensis var. kurstaki (Btk) – Organic Option

• Type: Biological insecticide (naturally occurring bacterium)
• Mode of Action: Targets the digestive system of caterpillars, killing them without harming beneficial insects.
• Effectiveness: Best when used early in the larval stage (June).
• Brand Names: Thuricide, Dipel, Monterey B.t.
• Pros: Organic, safe for pollinators, pets, and people.
• Cons: Requires good timing and thorough coverage. Less effective on older larvae.

2. Spinosad

• Type: Organic, broad-spectrum insecticide derived from soil bacteria.
• Mode of Action: Affects the insect nervous system.
• Effectiveness: Very effective on young larvae.
• Brand Names: Captain Jack’s Deadbug Brew, Monterey Garden Insect Spray.
• Pros: Kills a wide range of chewing insects; less toxic to beneficial insects than conventional options.
• Cons: Slightly more expensive; still needs to be applied early.

3. Pyrethroids (Synthetic Insecticides)

• Examples: Bifenthrin, Permethrin, Cyfluthrin, Deltamethrin.
• Mode of Action: Attacks the nervous system of insects on contact.
• Effectiveness: Fast-acting and broad-spectrum.
• Brand Names: Talstar (bifenthrin), Tempo (cyfluthrin), Ortho Max, Bayer Advanced.
• Pros: Readily available and effective on a wide range of pests.
• Cons: Can kill beneficial insects; requires careful application to avoid environmental impact.

Application Tips

• Thorough coverage is key. Spray the entire canopy, especially the inner branches where larvae hide.
• Use a sprayer that can reach the tops of tall arborvitae, or hire a professional if necessary.
• Repeat applications may be needed depending on the product and infestation severity.
• Always follow label directions for mixing, application rate, and safety.

Other Control Methods

While insecticides are effective, they work best when combined with cultural and mechanical methods:

Hand-Picking

In late summer and fall, remove visible bags by hand and destroy them.

Especially useful for small plants or light infestations.

Pruning

Heavily infested or dead branches may be pruned to improve appearance and reduce egg loads for next season.

Monitor Annually

Check arborvitae every spring for early signs of infestation.

Early detection makes control easier and more effective.

Conclusion

Bagworms are a serious pest for emerald arborvitae, capable of causing significant damage or even death if ignored. However, with proper timing and the right treatment strategy—particularly in early summer—you can effectively manage and prevent infestations.

Whether you choose a biological product like Btk, a natural solution like Spinosad, or a conventional pyrethroid, the key is early intervention. Regular monitoring, hand-removal of bags, and proactive spraying will help ensure your emerald arborvitae stays healthy, green, and beautiful year-round.

Got a bagworm problem you need help with? Reach out to your local extension office or licensed arborist for professional advice tailored to your area.