Bagworms

What are bagworms?  

The bagworm, Thyridopteryx ephemeraeformis, is a caterpillar that forms a cocoon-like bag which hangs from the host plant. The larva constructs the bag from silk and covers it with bits and pieces of leaves and twigs from the host plant. The bag itself will look different depending on what host plant it is feeding on. The larva is dark brown with a yellow head and has black and yellow spots covering its body. Adult female bagworms are almost maggot like; they are wingless and lack functional legs, eyes and antennae. While adult males are densely hairy and black with wings that have a span of an inch.  

What effects do bagworms have on plants? 

Defoliation is the primary injury caused by the caterpillar. Heavy populations of bagworms kill many ornamental arborvitae and juniper. 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.  

Lifecycle of a bagworm  

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. 

How do I get rid of bagworms? 

A very effective control is picking the bags off the host in winter or early spring to help prevent problems in the coming season. The bags, and eggs inside can be destroyed, or the bags can be placed in deep containers that allow any parasites to escape but keep the larvae in. Small larvae are much easier to kill than mature larvae. Therefore, treating infestations in the early summer when they are first noticed is important. Virginia Green’s comprehensive Tree and Shrub Program does a great job at controlling this persistent pest. Call or contact us for a free landscape inspection of this camouflaged pest.