Like swaddled infants wrapped in lined camo fabric, bagworm caterpillars are encased in an outer covering with an opening just large enough for their heads. Unlike infants, the caterpillars are left to fend for themselves as they transform into moths. During this process, you may mistake their cocoon coverings for tiny cones in your pine trees (Pinus spp.).
Bagworm caterpillars are the larval stage of the Theridopteryx ephemeraeformis moth. The caterpillars have heads and thoraxes with black and white spots and brown abdomens, and they are typically 1 inch long. The 1-inch-long adult male moths take flight with clear wings and black bodies, but the female moths are the most unrecognizable. They cannot fly because they don’t have wings, and they also lack eyes and legs.
The female bagworm's largely immobile existence is spent entirely inside a cocoon, where she lives for only a couple of weeks to lay eggs that ensure the next generation. The life span of a male moth is even shorter -- he lives only a couple of days to fulfill his role of fertilizing the females, which he accomplishes through an opening in the females' cocoons. Females may lay up to 1,000 eggs, which hatch into tiny caterpillars that leave the cocoon immediately to spin their own, individual cocoons. The first silk strands they weave upon exiting their birth cocoons catch wind currents and transport them to new spots where they weave their own cocoons -- a process called “ballooning.”
When the hatchling caterpillars spin their new cocoons, they weave bits and pieces of pine needles around the silk-lined case like little shingles, camouflaging it. Bagworm caterpillars have the distinction of being the only insect that builds this type of home. They leave a hole at the top of the cocoon, from which they dart in and out to feed on the pine needles. As they grow, they enlarge their cocoon home -- essentially, it grows with them.
Pine trees grow in U.S. Department of Agriculture plant hardiness zones 2 through 9, depending on species. Bagworms attack pines in USDA zones 5 through 9, because their northernmost range is in southern Michigan’s USDA zone 5. As the caterpillars feed on pine needles, they extract plant sap that contains chlorophyll, which impairs a pine's ability to photosynthesize. The needles initially turn brown, but in severe infestations, a tree may lose all its needles before dying.
Managing bagworms on your pine trees can be challenging. Handpicking and destroying the cocoons is an effective control if trees are within a ladder’s reach. The height of some mature pines puts the cocoons beyond the reach of sprayers, which prohibits effective chemical control. If you’re able to coat all leaf surfaces thoroughly with a sprayer, using a biorational insecticide such as spinosad or Bacillus thuringiensis kurstaki are effective controls. Although product labels vary, typically you dissolve 4 tablespoons of spinosad or 4 teaspoons of Btk in 1 gallon of water and spray on trees. Store these chemicals out of the reach of inquisitive kids, and keep kids and pets out of the way while you're spraying.