Q: “Why am I seeing these little pine cones hanging from my arborvitae?”
A: Well Cindy, those aren’t pine cones. They are bagworms. The bagworm, Thyridopteryx ephemeraeformis, is an interesting pest of ornamental shrubs and trees. The spindle-shaped bag that the larva carries around as it feeds characterizes this insect. The larva constructs the bag from silk and covers it with bits and pieces of leaves and twigs from the host plant. Therefore, the bag looks quite different when the worm is feeding on juniper than on rose. The larva is dark brown with a yellow head and yellow and black spots on the body. Adult females are wingless and lack functional legs, eyes, and antennae. They are almost maggot like, yellowish and seldom seen. Adult males are typical moths and are sooty black and densely hairy. The wings are nearly clear and have a span of 1 inch.
Bagworms often prefer juniper, arborvitae, and pines, but they are also found on many broadleaf shrubs and trees including rose, sycamore, maple, elm, and black locust. Defoliation is the primary injury caused by the caterpillar. Heavy populations of bagworms kill many ornamental arborvitae and juniper.
Bagworm larvae hatch from overwintering eggs during May. The young larva spins a silken case that it carries about as the larva feeds. As the larva grows, it enlarges the bag and continues to add bits of foliage to the bag. When the larva is resting, it attaches the bag to a twig with silk. Pupation occurs in late summer. In 7 -10 days the adult emerges. Males are free flying and leave the bag. They search out bags containing females, enter the bag, and mate. After mating, the female lays 500 -1000 eggs in the bag and dies. Because the larval stage is the only mobile stage, one bush can have a high population and another one several feet away can be free of bagworms.