Bagworms are common and destructive pests of many trees and shrubs, especially evergreens. They are easily recognized by the spindle-shaped bags they construct from needles or leaves of their host plant.
When populations are high, bagworms can completely defoliate and kill junipers, arborvitae, and spruce if left untreated.
Quick Bagworm Control Tips
- Bagworms hatch in late May
- Treat after eggs hatch, while bagworms are small
- Small caterpillars are easier to control
- Large bagworms require stronger products
- Hand removal works in fall and winter
What Are Bagworms?
Bagworms are caterpillars that protect themselves inside bags made from the foliage of their host plant. These bags start very small—about 1/8 inch long—and grow to over 1½ inches as the caterpillar matures.
Male bagworms pupate into clear-winged moths and leave the bag to mate. Females never leave their bags. Each fertilized female can lay up to 1,000 eggs inside the bag before dying.
The eggs overwinter inside the bags, leading to new infestations the following year if bags are not removed or treated.
Plants Most at Risk
While bagworms can feed on many plants, including crabapples and Japanese maples, conifers are most vulnerable to long-term damage.
- Junipers
- Arborvitae
- Spruce
Heavily infested trees may develop a brown or scorched appearance during summer as foliage is consumed.
When to Treat Bagworms
Timing is critical for effective bagworm control. Treatment success depends on the size and life stage of the caterpillars.
Why Preventative Treatments Don’t Work
There is no effective preventative insecticide for bagworms. The eggs overwinter safely inside the protective bags, where insecticides cannot reach them.
Bagworms must be treated after eggs hatch and larvae begin feeding in late spring or early summer. For best results, treatment should target young caterpillars before bags grow larger.
Regular inspection of susceptible trees in late May and June is the most reliable way to catch infestations early.
Treatment Recommendations
- May – June: Treat young bagworms with Bonide Captain Jack’s Deadbug Brew (Spinosad) or Natural Guard Caterpillar Killer Spray (Bt). These products are most effective when caterpillars are small.
- Mid to Late Summer: As bagworms grow larger, use products containing permethrin (Bonide Eight) or lambda-cyhalothrin (Hi-Yield Bug Blaster II) for better control.
- Late August: Bagworms stop feeding. Insecticide applications are no longer effective.
- Fall – Winter: Hand-picking bags from small trees and shrubs is an effective way to reduce next year’s population.
Preventing Future Infestations
Because eggs overwinter inside the bags, removing and destroying bags during fall and winter helps reduce the number of bagworms that hatch the following spring.
Regular inspection of susceptible plants in late spring allows for early detection and easier control.