Bagworms - June 6, 2018
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Bagworms are unusual insects that are often considered pests in the central, eastern and southern U.S where they defoliate evergreen trees. Defoliation of coniferous evergreens can be a problem because these plants depend on these needles to conduct photosynthesis over several years. They are present in Arizona and sometimes cause cosmetic damage to trees and shrubs. Occasionally, their populations become noticeable on native sycamores along creeks in the Verde Valley, but most often people happen across a bagworm in their landscape. People are not always sure what they are looking at when they come across a bagworm and they rarely see the insect itself, only it's well-camouflaged home.
Bagworm species vary slightly in habits and life cycle, on evergreens the bagworm spends winter months in the egg stage within a cocoon-like, sealed bag produced by the female the previous fall. In the spring, tiny caterpillars hatch, lower themselves on silken strands to new foliage. Very soon after emerging, each tiny larval caterpillar begins spinning a protective bag around itself, leaving an opening at the head end to permit crawling about and feeding. As they feed, they attach small pieces of plant material of the host plant to their bag. As anyone that has tried to tear one open can attest, the silk holding the bag together is very strong and cannot be easily torn open – it can be cut open with a sharp knife or scissors.
Caterpillars remain inside their bags and grow up to 2 inches long before pupating in late summer. Seven to 10 days later, the male bagworm moths wriggle out of the bottom of the bag leaving the empty pupal skin with the bag. Adult males have short (½ inch-long) clear wings, hairy black bodies and feathery antennae. They fly and seek out a female mate. Females do not develop into moths, but remain inside bags and resemble large maggots, with no functional eyes, legs, mouthparts or antennae. After mating, females produce a large clutch (500 to 1,000) of eggs and die. Some bagworm species spend winter months as partially-developed caterpillars that complete feeding and pupate in the spring.
Bagworm caterpillars have chewing mouthparts. Across their range and among differing species, they feed on a wide range of broadleaf and evergreen trees and shrubs. These include arborvitae and other ornamental conifers, box elder, cedar, cypress, elm, fruit and nut trees, juniper, live oak, locust, maple, persimmon, pines, salt cedar, sumac, sycamore, wild cherry, willow and many other ornamental plants.
Young bagworms feed only on the outer layers of cells of the foliage (called skeletonizing), but as they mature, they consume all but major leaf veins. Sometimes, infested plants develop more bagworms each year because adult females do not fly. During leaf-feeding, the caterpillars emerge from the top of the bag and hang onto the host plant with their legs and sometimes with a silken thread. The bottom of the bag remains open to allow fecal material (frass) to pass out of the bag.
As mentioned previously, bagworms are not usually serious plant pests in Arizona. If significant damage occurs, hand-picking the bags and destroying them is a simple way of managing them. In late fall, you can put the bags in coffee cans or jars with coarse screening to allow any parasites in the bags, such as flies or parasitic wasps to emerge and escape. These natural biological control organisms will aid in controlling future bagworm populations. Many birds also feed on bagworms. Extension publications recommend pesticide applications to manage bagworms. These recommendations were developed for areas where bagworms cause significant damage. In our area, they are not usually present in large numbers and are simply an interesting member of the local ecosystem. If significant damage is occurring, start with a least-toxic insecticide with one of the following active ingredients: Bacillus thurengiensis (Bt); Neem; or Spinosad.
As a final note, tent caterpillars and fall webworms are sometimes mistakenly called bagworms. However, these insects create large, translucent webs that contain multiple (often hundreds) of caterpillars. Bagworms are solitary and create opaque, leaf-covered shelters. I have included images and additional information about bagworms below.
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Evergreen bagworm (Thyridopteryx ephemeraeformis, photo credit: Pennsylvania Department of Conservation and Natural Resources - Forestry , Bugwood.org).
Bagworm collected in Yavapai County containing a female larva. The large opening is where the head emerges for feeding (photo credit: Jeff Schalau).
Bagworm collected in Yavapai County showing female larva. The head is on the right side of the image (photo credit: Jeff Schalau).
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Last Updated: May 31, 2018
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