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Bagworms

WHY WE DON’T LIKE THEM

Bagworms prefer juniper, arborvitae, spruce, pine, and cedar but also attack deciduous trees. They feed on the leaves or needles and large populations can cause severe defoliation. Defoliation for multiple years can reduce tree growth or cause sparse foliage and poor appearance. In addition, the brown bags can become very noticable and unsightly in large numbers.

WHAT ARE THEY ?

Bagworms are common landscape pests because they feed on many of the most common ornamental plant species. They can be readily identified by the cone-shaped bag they spin from silk and embed with bits of host plant and other debris. The bags range in size from 1⁄4 inch to over 2 inches to accommodate the growing caterpillar inside. Sometimes the bags are mistaken for pine cones or other plant structures.

WHAT CAN WE DO?

Bagworms can cause severe damage to landscape plants by defoliating branches that alter plant shape and foliage density. The decision of when and how to control bagworms will depend on the size of the populations and on plant location, value, and purpose.

Cultural controls for bagworms include planting less suseptible plant species if bagworms are common. Bags can be mechanically removed by pruning infested branches or hand picking bags. Remember each bag could contain 1,000 eggs so picking them off in fall and winter could make a big difference in spring.

Bagworms are parasitized by several kinds of parasitic wasps. In many cases natural parasitism and predation may keep populations below noticeable damage levels. Because females do not fly, infestations can be concentrated and localized in a landscape. For example only four or five trees in a hedge row of 50 plants may be infested. Scouting can verify this.

Biological Controls that specifically target feeding Bagworms are the ideal method for controlling Bagworms. Because this treatment only targets caterpillars actively feeding, beneficials are unlikely to be harmed.

Chemical controls are a last-resort for Bagworm control and may fail because bagworms live inside a water-proof bag.

If the bagworms are first noticed in late summer, it is too late to do anything but physically remove the bags. By then they have already mated and females have already laid eggs inside their bags for next year’s generation. It’s best to use a very sharp knife or utility razor to cut the silk band that bagworms wrap around and around the twig just before they pupate inside. If you merely pull the bag to remove it, the silk band may slide along the twig and shear off the needles. If not removed, the silk bands sometimes girdle the twig, which causes it to eventually die and break off at that point.

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