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Act Now to Avoid Unsightly Bag Worms

Bagworms hang off of trees like little diabolical ornaments, eating the needles and leaves. If you didn’t have them on your trees last year, you likely saw them elsewhere.

When the Japanese tree lilacs are in bloom, it is time to scout and control bagworms. This species flowers later than other lilacs, with large fluffy white blossoms on a 20- to 30-foot tree. Known for fragrant flowers in early-to-mid June, Japanese tree lilacs are common in the urban landscape.

But don’t get swept away by the aroma. On nearby evergreens, there are tiny caterpillars climbing to the top, spinning a silk to catch the wind, and setting sail to find a suitable host.

Bagworms overwinter in the bags from the previous season, then hatch from their eggs in mid-to-late June, so these paratroopers’ flights tend to coincide with the blooming of Japanese tree lilac.

Arborvitae and Juniper are particularly popular hosts. However, hundreds of other species can be attacked, including pine and spruce. And, as we discovered at The Refuge Food Forest last year, black currant.

Small caterpillars feed on the outer layer of the leaves or needles of their host, causing browning, usually starting at the top of the tree. As they grow, their appetite increases, and they begin to eat all the foliage. They feed the entire summer but are much easier to kill when they are small. These caterpillars remain susceptible to chemical treatment into early July. Heavy infestations cannot only be unsightly with all the eaten foliage but can kill branches or whole plants.

As the caterpillar feeds, it uses silk and foliage from the plant to create a bag. In late summer, these caterpillars pupate for seven to 10 days. The adult female remains in the bag and is without legs, wings, eyes or mouthparts. The male emerges from the bag as a black moth and finds a female within a bag to mate. Neither adult feeds. After mating, she produces 500 to 1,000 eggs, keeping them protected inside her body. Dissecting a bag in the early spring will reveal a dead female with lots of little eggs ready to hatch. Protected by the bag, control is futile until the eggs hatch and the juveniles emerge.

By the time August comes, when bagworms are most likely to be noticed, these caterpillars have already formed their bags, it is too late for chemical control. Hand picking is an option.

The following chemical treatments are effective for bagworm: Bacillius thuringiensis kurstaki (also known as BTK, and found in Dipel and Thuricide), Spinosad (organically derived and found in products like Conserve) and cyfluthrin (Tempo). As always with pesticides, read and follow the labels to ensure safe and effective application. Follow-up applications may be needed.