Any insect of the moth family Psychidae, found worldwide, named for the baglike cases the larvae (see larva) carry with them. The bag, which ranges in size from 0.25 to 6 in. (6–150 mm), is constructed from silk and bits of leaves, twigs, and other debris. The strong-bodied male has broad, fringed wings with a wingspread averaging 1 in. (25 mm). The wormlike female lacks wings. Bagworm larvae often damage trees, especially evergreens.
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The Psychidae or Bagworms are a family of the Lepidoptera (butterflies and moths). The larvae of the Psychidae construct cases out of silk and environmental materials such as sand, soil, lichen, or plant materials. These cases are attached to rocks, trees or fences while resting or during their pupa stage, but are otherwise mobile. The larvae of some species eat lichen, while others prefer green leaves. In many species, the adult females lack wings and are therefore difficult to identify accurately.
Bagworm cases range in size from less than 1 cm to 15 cm among some tropical species. Each species makes a case particular to its species, making the case more useful to identify the species than the creature itself. Cases among the more primitive species are flat. More specialized species exhibit a greater variety of case size, shape, and composition, usually narrowing on both ends. Body markings are rare. Adult females of many bagworm species have only vestigial wings, legs, and mouthparts. The adult males of most species are strong fliers with well-developed wings and feathery antennae but survive only long enough to reproduce due to under developed mouthparts that prevent them from feeding. Their wings have few of the scales characteristic of most moths, instead having a thin covering of hairs. Each bagworm generation lives long enough to mate and reproduce the generation for the following year in an annual cycle.
Bagworms species are found globally, with some species, such as the snailcase bagworm, migrating to new continents in modern times where they are not native. The family is fairly small, with about 600 species described.
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 death of their hosts. Trees infested with bagworms exhibit increasingly damaged foliage as the infestation increases until the leaves are stripped bare. Some bagworms are specialized in their host plants while others eat a variety of leaves. Some species also consume small arthropods such as Pseudaonidia duplex.
Since bagworm cases are composed of silk and the materials from their habitat, they are naturally camouflaged from predators. Natural enemies 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 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 dies without laying eggs, and the larval bagworm offspring emerge from the parent's body. Some bagworm species are parthenogenetic, wherein eggs hatch without male fertilization.
Bagworms are considered pests to humans due to the damage done to host trees such as wattle in South Africa and orange in Florida. If detected early, picking the cases from the trees while in their pupa stage is an effective way to check an infestation. Otherwise, insecticides are required.