Bagworms on Arborvitae Shrubs

Pest Identification

• Arborvitae is among the preferred plant hosts of bagworms. Other favored evergreens include juniper, spruce, pine and cedar. The pests also attack a number of deciduous trees including black locust, honeylocust, sycamore and sweetgum.

Description

• Bagworms are the larval stage of a small, black moth with a hairy body and a wingspan of about an inch. The maggotlike adult females are yellowish white and do not have wings, functional legs, antennae or eyes. The females lay their eggs in the 1/4-inch-long silken bags that they weave with their silk and pieces of host plant foliage. The eggs hatch in early spring and the size of the bags increases as the larvae grow within it. Adult larva is about 1 inch long with a dark brown abdomen and a white head and thorax. Young larvae start to weave their bags as soon as they are born.

Damage

• Bagworms start feeding on the foliage on infested trees as soon as they hatch, coming out of their bags at feeding time and then retreating. Mature larvae eat entire needles of evergreen trees. Repeated bagworm infestations over the years on already stressed and weak trees is likely to cause tree death.

Management

• In case of light infestation on arborvitae shrubs, handpick the bags and drop in a pail of soapy water to destroy eggs and adult females. The best time to destroy bags is during early spring, fall or winter prior to egg hatching. The application of insecticides is most effective at the younger larval stage when pests are less than 1/2 inch long. This is usually in early June. Larger, mature larvae are very hard to control with chemicals. Recommended chemicals for arborvitae shrubs include acephate, carbaryl or azadiractin.