Bagworm FAQs for Connecticut

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Do your evergreen shrubs, such as arborvitae and juniper, have brown leaves and dead branch tips? It might be bagworm, aka Thyridopteryx ephemeraeformis. This insect also goes by the names eastern bagworm, evergreen bagworm, and North American bagworm.

Learn how to deal with bagworms infesting your evergreens, including identification, targeted plants, timing, spread, signs of damage, and control options.

What does a bagworm look like?

Whatever you call it, you can probably imagine what this insect pest looks like: a worm in a bag.

Although it’s classified with butterflies and moths, bagworms are not showy. Some female bagworms don’t develop wings or even eyes – in their adult stage, they can look like caterpillars or grubs inside their bags. Adult male bagworms are about an inch long, can fly, and are identified by their dark, sooty color and transparent wings.

Male bagworm with transparent wings

Male bagworms can be recognized by their dark color and transparent wings

What do bagworms eat?bagworm larval case on evergreen

You’ll most commonly find bagworms eating your arborvitae (Thuja species) and eastern redcedar (Thuja plicata). However, they’ll eat dozens of coniferous and broadleaf species including:

  • Pine (Pinus)
  • Willow (Salix)
  • Apple (Malus)
  • Elm (Ulmus)
  • Maple (Acer)
  • Black locust (Robinia)
  • Linden (Tilia)

Bagworms start by eating the leaves at branch tips, causing dieback. They’ll then continue down, eating leaves along stems and branches.

How can I tell if my tree has been damaged by bagworms?

Severe bagworm damage causes partial or complete defoliation, and can kill the host plant. It’s especially important to stop bagworm early on conifers, as the plants may already be dying by the time the bagworm is identified. Unlike broadleaf trees, conifer leaves or needles can remain green and persistent on branches, so their decline isn’t as readily noticed.

Why did my trees get bagworm?

While bagworm is a common insect pest in the eastern U.S., it is more often found on trees that were under stress (from excessive heat, lack of water, severe pruning, or disease) before the arrival of the insect. This is a common scenario for many insect pests that have the uncanny ability to locate stressed trees with a weakened ability to fight off infestations.

Another reason for bagworm infestations is their exponential population growth: a bag can hold 500 or more larvae. As the larvae hatch, they may not need to travel far to find a host plant, so the whole “family” can rapidly infest a small area. You can go from having zero bagworms to having hundreds of them in a very short time.

When are bagworms active?

  • Bagworm larvae hatch in late spring.
  • In summer, they are actively eating leaves and mating, and the female bagworm constructs her bag.
  • In late summer, the male bagworm locates the female in her bag (from pheromones the female releases) and they mate. The adult-stage male bagworm lives only a few days and dies after mating. The female stays in her bag during mating, and dies not long after laying her eggs in it.
  • The larvae stay protected in their bag over the winter until they hatch the following spring, are carried off on the breeze by strands of silk, and begin the cycle again.

To control bagworms, it’s important to find the female worms and the larvae, and kill them.

How do I find female bagworms?Bagworm with green leaves against a white wall

Female bagworms are masters of camouflage so you’ll have to look carefully to find them. She weaves a 2-inch, cocoon-like “bag” from her silk and then attaches small leaves and pieces of twigs to the outside of the bag. When the bagworm uses bits of leaves from the same plant she’s feeding on, it can be harder to distinguish between dead foliage and the bag. Think of it as a ghillie suit for insects.

The bag that the female creates is also a mobile home that she lives in as she moves around to eat leaves, so it doesn’t stay stationary.

You’ll find the female bagworm hanging from plant leaves, twigs, or branches. You can also find larvae-filled bags hanging from fences, gates, and other surfaces.

How can you tell whether the bag has a female inside or is filled with larvae?

If you see the female’s grub-like form outside the end of the bag, eating leaves, or if the bag is moving, then there probably aren’t any larvae. If the bag is sealed, it means the female has already died and there are larvae inside.

What’s the best way to control bagworm?

Depending on the number of these pests on your property and the time of year, there are three ways to help reduce their numbers.

1. Spraying

Bagworms can be harder to control with sprays than other insects because both the adult female and her eggs are inside their namesake bag, where they’re protected from insecticide sprays.

Spraying with BT (Bacillus thuringiensis) is safer than using traditional neurotoxin pesticides. Traditional insecticides will kill the beneficial insects you want to keep around and should be used only as a last resort.

BT is most effective in late spring and early summer when the bagworms have recently hatched and are in their younger stages, or instars. This is when the female’s protective bag has not yet been completed and before males have mated and died.

2. Encourage natural predators

Natural predators will also keep bagworm populations under control, which is another reason to ensure that your garden welcomes them. In Connecticut, there are several predators that can help reduce the number of bagworms in your yard.

  • Having flowering plants in your garden is the easiest way to attract both beneficial pollinators and beneficial predators, and you get beautiful flowers in the deal! Not sure what native flowers to use? Check out the Connecticut Botanical Society’s list of native plants for ideas.
  • Several species of parasitic wasps will kill bagworm.
  • Helpful birds like sparrows and finches will eat the insects, the eggs overwintering in their bags, and hatching larvae.
  • Little white-footed mice will also eat the larvae as they search for food among plants and trees in fall and winter.

3. Pick them off by handBagworm larval case in fall

For small infestations and smaller-sized species, the simplest and fastest way to reduce or eliminate your bagworm population is to remove them by hand.

Once the females create their bags in summer, you can find them and pluck them off the plant. To find bags filled with larvae, especially on deciduous broadleaf species, look for them in fall through early spring. You just want to make sure you get to the larvae before they hatch with the warming spring weather.

Once you’ve located the bags, pick them off the branches or structures they’re attached to and destroy them. If you’re squeamish, wear gloves or put those barbeque tongs to work! Drop the bags into a heavy paper or plastic bag that you can seal and then discard in your trash (not in a green waste can or compost pile where they may survive). You can also fill a bucket with soapy water and drop the bags in that. Just make sure you completely destroy the case and the female bagworm or larvae inside.

WANT SOME HELP?

We offer a range of insect and disease treatment options and can help you with the most efficient treatment methods that are targeted to specific pests and applied at the right time. Because bagworms (and many other pests) attack stressed or unhealthy trees, we can also help prevent attacks through proper pruning, soil enhancement, and regular maintenance.

We want you to have healthy and vigorous trees and a garden that supports beneficial insects and pollinators. Give us a call to find out what we can do for you!

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