What Are Those Little Bags on Trees?

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Some trees have little bags hanging from their twigs and branches that look like small cones or seedpods. The bags have bits of tree foliage arranged like shingles around them, which makes them appear to be natural growths of the trees. Instead of being part of the trees, the bags are actually cocoons that harbor aptly named bagworms, which are destructive pests of their tree hosts.

Bagworms

  • Bagworms are the caterpillar larvae of the Theridopteryx ephemeraeformis moth. At maturity, the bagworms are 1 inch long and do not resemble the adult male moths, which have black bodies and clear wings. The larvae have black-and-white-spotted heads and thoraxes and brown abdomens. Female moths resemble neither the male moths nor the larvae. They look like white grubs -- wingless, eyeless and legless -- and never leave the cocoons, not even to mate. The adult male moth mates with the females through small openings in the cocoons.

Proliferation

  • After being fertilized, the female moths fulfill their life cycle by laying up to 1,000 eggs before they die. The newly hatched larvae emerge from their cocoon and spin a single strand of silk that sends them airborne when a gust of wind captures it. Called “ballooning,” this allows each caterpillar to hitch a ride to a new location, where it immediately weaves its own bag, or cocoon. The caterpillar begins to feed on its tree host’s foliage and retreats into the protective cocoon when predators threaten.

Cocoons

  • The caterpillars weave bits of foliage into the spindle-shaped silk bags, which camouflages them. Bagworms are the only insect species that interweave plant fragments with the silk they produce to build their cocoons. As the larvae feed on tree leaves, they continue to build their bags larger to accommodate their growth. Cocoons are not only protective cases for the larvae but are also where the caterpillars pupate, a process that takes only two to three weeks. Male moths emerge from cocoons, and the sessile females remain inside to begin the next life cycle.

Damage and Management

  • In severe infestations, bagworms can completely strip a tree of its foliage. If the tree cannot regrow new leaves quickly enough, it loses the ability to perform photosynthesis, which is the process by which it makes its own food. Essentially, it starves to death. Controlling bagworms is easier said than done. The mature height of many trees, coupled with the difficulty in spotting the bags until irreparable damage is done, makes early detection and immediate treatment critical to a tree’s survival. Spraying a tree’s foliage with an insecticide targeted for bagworms may correct the problem, but you must spray all leaf surfaces, and the caterpillars must eat the foliage for this treatment to be effective -- contact pesticides do not work. Products that contain Bacillus thuringiensis kurstaki (Btk) or spinosad are suitable choices. Although adherence to label instructions is important because of variances in different products, 4 tablespoons of Btk or 4 teaspoons of spinosad, each dissolved in 1 gallon of water, are typical mixing instructions.

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