Bagworms: What to Know

Medically Reviewed by Jabeen Begum, MD on January 13, 2023
5 min read

You’re enjoying a sunny morning when you notice what looks like a tiny pinecone on your favorite juniper bush. When you inspect it, you notice that it’s not part of the foliage — it looks more like a cocoon. Upon closer inspection, you find a tiny caterpillar inside.

What do bagworms do to trees and bushes, and are they harmful? Should you pull its “bag” off the tree and throw it away, or will it one day yield a beautiful butterfly? Learn more about bagworms and understand how to handle them in residential areas in the following guide.

If you assumed that the bag was a type of cocoon, you’re partially correct. Bagworms are simply the larval form of the bagworm moth. Inside the bag, you may find eggs, grublike worms that are still in their larval form, or wingless, adult females that are preparing for the next generation.

Juvenile and adult bagworm characteristics. What do bagworms look like inside their odd-looking shelters? It’s difficult to identify a bagworm caterpillar on sight because, unlike other caterpillars, they don't leave their bags until they’re fully grown (if they’re male). Young bagworms are shaped like caterpillars with brown, black, and gray markings. 

Adult males are black, fuzzy moths with translucent wings. Adult females do not develop wings, and they resemble like large caterpillars throughout their lives. Their color varies from pearly white to black.

Bagworm life cycle. Bagworms begin their life as eggs laid by their mother inside the bag. The bag serves as a shelter that protects the eggs from cold temperatures throughout the late fall and winter. These eggs usually hatch in the spring to early summer, depending on the location.

After hatching, the small caterpillars feed on the needles and leaves of their host tree. It might surprise you to learn that bagworms aren’t as stationary as they seem: Young worms can easily transport their bags with them as they move around to look for food.

In the fall, bagworms enter the pupal stage and become adults. The female adults can't fly, so they stay in their bags. The males leave their bags to find females and mate with them. After the mating season, the female lays over 300 eggs in her bag, leaves these eggs to last through the winter and hatch the next spring, and dies.

“Bagworm” is just a nickname for the larval stage — and sometimes the adult, wingless females — of nearly 1,000 different types of moths that lay eggs in bags. The North American bagworm, which has spread throughout the continental U.S. and southern Canada, has counterparts in Central and South America as far south as Argentina.

You can easily discover where bagworms live by asking yourself, "What do bagworms eat?" Bagworms can devastate entire residential areas, but they aren’t considered a pest in forest areas or in the home. These moths and their larvae reside in willow, spruce, maple, oak, and pine trees — among many others that are popular in suburban yards. Their favorite trees are typically arborvitae and juniper, but they’re quite adaptable. When trees aren’t available, they have no problem settling in decorative shrubs that can offer a food source.

You can identify a bagworm's bag easily. It looks like a small pine cone or a collection of pine needles stuck together, and it probably hangs from a tree branch or leaf. When they’re freshly made, the bags range from green to light brown. As they age throughout the winter, they’ll fade in color to a grayish-brown.

Fortunately, it’s easy to identify whether you will develop a bagworm infestation in your yard or garden. You may first notice a few bags on trees they enjoy consuming — such as the arborvitae species. At this stage, you can easily pick the bags and throw them away. 

You won’t notice any smells, sounds, or anything strange about your yard aside from the placement of the bags. If you notice that a tree or bush looks like it’s losing foliage, investigate it thoroughly to determine whether bagworms are the true cause.

Bagworms simply look for nice places to live that can also serve as food sources for themselves and their larvae. While they can survive on several types of trees, they prefer conifers. Because these plants do not produce a new crop of foliage every year, they can be easily damaged by bagworms. It’s best to eradicate a bagworm population before these creatures become a problem for your entire yard.

No, bagworms do not injure humans or animals. If you pick up a bagworm or its bag, it won’t hurt you or make you sick in any way. However, pesticides commonly used on bagworm-infested trees can be toxic to humans, animals, and beneficial insects. 

Bagworms can kill trees by defoliation (foliage loss) if there are a lot of them on the tree. Evergreen trees, or plants that don't lose their leaves or needles in the winter, are at high risk for death by defoliation because they don't grow a new set of leaves every spring. Deciduous trees can survive bagworm infestation, but it’s best if you don’t let it get to this point in the first place.

It’s difficult to get rid of bagworms if there are a lot of them. You can do your best to decrease the threat to your trees by picking the bags off the trees and drowning the worms in a bucket full of water — or discarding them far from your home — at any point during the year. If there are too many bags, you have two options for eradication:

  • Use a bacterial Bacillus thurigiensis spray, which is nontoxic to humans and other animals, in the spring to eradicate the hatching larvae. This is best sprayed in the late spring and early summer so that it works on younger bagworms. Once they’re older, they’re more difficult to eradicate.
  • Use a chemical insecticide to eradicate more mature worms. It’s best not to use chemical control as a first resort. Pesticides can cause health problems if you don't use them correctly.

Bagworms present many problems for homeowners, but they are typically easy to eradicate if you catch the problem early. If you’re having trouble removing the pests by picking the bags, talk to a pest control expert about how to get rid of bagworms before the spring begins and a new generation of bagworms hatches.