The bagworm is not a worm, but the larval stage of a moth belonging to the family Psychidae. This caterpillar lives entirely within a portable, protective case constructed from silk and plant material. The spindle-shaped structure, which can grow up to two inches long, is camouflaged with bits of leaves, twigs, and bark from its host plant. This mobile case allows the caterpillar to feed and move while exposing only its head and the front portion of its body. Once the caterpillar reaches full size, the life cycle reveals a remarkable biological divergence between the sexes.
The Transformation Process
Once the caterpillar reaches maturity, typically in late summer or early fall, it prepares for pupation. The larva firmly attaches the top of its silk bag to a sturdy branch using a strong band of silk. After anchoring the case, the caterpillar closes the open end and transforms into a pupa inside the bag. This transitional pupal stage lasts approximately four weeks, during which the insect is inactive and does not feed. Inside the pupal case, the insect’s body is reorganized into its adult configuration.
The Adult Male
The male insect undergoes a complete metamorphosis, emerging as a fully developed moth in late August or September. The male pupa wriggles partway out of the case before the adult emerges, leaving the empty pupal skin visible. The adult male is a small, hairy moth, often black or dark brown, with a wingspan of about one inch. It possesses large, feathery antennae used to detect pheromones and has functional, nearly clear wings. The adult male has non-functional mouthparts and does not feed, resulting in a lifespan of only a few days. Its sole purpose is reproduction, and it is the only stage that leaves the host plant to locate a female.
The Adult Female
In stark contrast to the male, the adult female undergoes an incomplete metamorphosis and never develops into a winged moth. She remains inside the silk bag, essentially a larviform, grub-like organism. The female is soft-bodied, yellowish-white, and lacks functional eyes, antennae, legs, and mouthparts, making her completely immobile. Her entire adult existence is dedicated to reproduction within the bag. She releases a pheromone to attract a flying male, who must enter the bag to mate. After mating, the female lays a large clutch of 500 to 1,000 eggs inside her pupal casing. She then dies, and her body often remains inside the bag, protecting the overwintering eggs until they hatch the following spring.