In the larval stage, bagworms extend their head and
thorax from their mobile case to devour the leaves of host plants, often leading to the defoliation of their
hosts.
Trees infested with bagworms exhibit increasingly damaged foliage as the infestation increases, which may result in gradual mortality accrual, especially on tree plantations in
tropical regions where bagworms are more frequently
multivoltine. Some bagworms are specialized in their host plants (
monophagous), while others can feed on a variety of plant species (polyphagous). A few species also consume small
arthropods (such as the
camphor scale Pseudaonidia duplex, a
scale insect). One bagworm species was found to eat an orb-web of
Plebs sachalinensis (Araneae, Araneidae) entirely. Since bagworm cases are composed of silk and the materials from their habitat, they are naturally camouflaged from predators.
Predators include
birds and other
insects. Birds often eat the egg-laden bodies of female bagworms after they have died. Since the eggs are very hard-shelled, they can pass through the bird's
digestive system unharmed, promoting the spread of the species over wide areas. A bagworm begins to build its case as soon as it hatches. Once the case is built, only adult males ever leave the case, never to return, when they take flight to find a mate. Bagworms add material to the front of the case as they grow, excreting waste materials through the opening in the back of the case. When satiated with leaves, a bagworm
caterpillar secures its case and pupates. The adult female, which is wingless, either emerges from the case long enough for breeding or remains in the case while the male extends his
abdomen into the female's case to breed. Females lay their eggs in their case and die. The female
evergreen bagworm (
Thyridopteryx ephemeraeformis) dies without laying eggs, and the larval bagworm offspring emerge from the parent's body. Some bagworm species are
parthenogenetic, meaning their eggs develop without male fertilization. Each bagworm generation lives just long enough as adults to mate and reproduce in their annual cycle. == Systematics ==