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Disease Spotlight: Bagworms

Bagworm Nests

What are bagworms?

Bagworms are a non-selective defoliator pest that overwinters in a silk pouch (or bag) on infested tree stems. They may cause moderate to severe damage to trees and shrubs in the landscape and are particularly damaging to evergreens due to their slower ability to regenerate foliage. Severe infestations may even kill trees. Some of their favorite hosts include Arborvitae and Juniper. Bagworms are named for their silk pouches which the female overwinters inside. They are commonly confused for pinecones.

Bagworm
Bagworm emerging from their silk pouch.

Bagworm Infestation

In the springtime, hundreds of juvenile caterpillars emerge from each pouch. These juvenile insects will immediately begin feeding on host plants and produce silken strands that can carry them in the breeze to other potential hosts. This typically happens around early June in the Chicagoland region of Illinois. Between 600 and 900 growing degree days. They consume the foliage or needles resulting in defoliation and weakening of the tree.

Bagworm Nest
Bagworm pouch, often confused for pinecones.

Life Cycle

The life cycle of bagworms begins with the eggs laid by adult female bagworm moths on a host plant. These eggs hatch into larvae that build protective bags around themselves and while concealed feed on foliage to grow. Once the larvae reach maturity, they attach their bags firmly to a surface and grow inside. After this process, the adult bagworm emerges and looks for a mate. Female bagworm moths remain in their bag emitting pheromones to attract males. The males locate the females for mating and the cycle begins again.

Treatment and Prevention

The insect is most susceptible to control as the juvenile caterpillars land on a host and begin feeding. A contact spray with good coverage can be very effective, and multiple sprays are recommended to ensure an infestation is controlled. The caterpillars become less susceptible later in the season as they grow. This can make control challenging since damage can remain unseen for weeks before an infestation is recognized. Late-season sprays are not as effective. In late summer and early fall, the female caterpillar will fully enclose her pouch and become nearly impervious to contact pesticide application.

If an infestation is recognized early enough, a systemic insecticide may be applied to kill off any caterpillars feeding on foliage. The systemic application needs to be done early enough to allow uptake of the active ingredient into the foliage. The chemical is then ingested by young feeding caterpillars. We recommend a systemic treatment in addition to a contact spray as a second layer of protection if the timing is right.

Removing bagworm pouches by hand can be effective in reducing pest populations. Plucking the pouches from an infested tree should be done during fall or winter. Hand-removing eliminates hundreds of potential caterpillars from the following season per pouch and may be effective.

Bagworms are also hosts for a number of parasitic wasps. Planting a flower garden nearby susceptible trees may attract more wasps and contribute to the natural control of bagworm caterpillars. This is not an effective rescue treatment where a plant is heavily infested, but a good addition to have to help mediate potential future pest populations.

Contact Us

2023 was a particularly heavy outbreak year for bagworm with many of our clients discovering new infestations late in the season. Our plant health care team is prepared to assess and manage bagworm infestations in your valuable landscape plants. Contact our arborists today!

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