If you live in our area, you have probably seen, or at least heard about, Bagworms. Over the past few years this accidentally introduced pest has caused problems to many trees and shrubs. The bagworm, Thyridopteryx ephemeraeformis (for those that appreciate scientific names), is an unusual but serious pest of many ornamental trees and shrubs throughout the eastern half of the United States. The bagworm gets its name from the bag-like structure created from leaf fragments that are bound together with silk produced by the larva. Most of this insect’s life is spent within the bag, which serves as both a protective structure and an area for the larvae to grow into adult moths. The bags blend in with the host tree’s foliage leaving infestations to often go unnoticed until significant defoliation has occurred. Many people mistake these protective bags for cones or other “normal” parts of a tree.
Bagworms feed on more than 125 species of trees, shrubs and other plants. However, conifers, especially arborvitae, eastern red cedar, ornamental cedars, cypress and white pine are among the preferred hosts. In many parts of the state, deciduous trees are attacked as well, particularly maple, sycamore and locust. Favored species are identified readily in the dormant season by the presence of bags on the plant.
The presence of spindle-shaped bag structures is the most visible evidence of a bagworm infestation. The bags are approximately 1/8 inch long when first noticeable and will reach approximately two inches long and 1/2 inch in diameter when fully developed. The larvae are about one inch long when fully grown (the larvae form of the bagworm is a caterpillar; the adult form is a moth). The body of the larvae is dark brown with a yellow head region and is hairless. The adult female moth is worm-like and lacks wings, legs, antennae, and eyes; she spends most of her life inside the bag. The body is yellowish white and nearly hairless. The male moth resembles a wasp with a black body and dense hairs. The wings are clear and about one inch across.
The bagworm overwinters as yellowish eggs within the bag. Some bags contain only male larvae, and therefore eggs will not be found in every bag on the tree. The eggs hatch from late-May through mid-June; the larvae emerge and begin feeding and constructing new bags. As larvae continue to feed and grow, they enlarge the bag to accommodate their increased body size.
In late-July through early August, fully-grown larvae secure their bags to a twig with silk and pupate – grow into adult moths – within the bag. Only the male moths can fly. The male moths then emerge from the bags after about one month and fly to a bag containing a female, and mating occurs. Yellowish eggs are deposited within the bag by the female, and then she wriggles out and dies. The female never leaves the bag – hence the lack of wings and other structures needed for flight and prolonged movement, as these are unnecessary.
Removing and destroying the bags can control light bagworm infestations on shrubs and small trees. This should be done in the dormant season before eggs hatch. On large trees or plants with heavy infestations, properly timed contact applications with an approved product provide the best results. The applications are most effective when applied to the bags and foliage when bags are small. Large larvae are more resistant to insecticides then the smaller, younger larvae. Systemic pesticides generally provide better control than a contact spray, especially on large larvae, so an application of a soil injected, systemic pesticide is your best option. If you are reading this in the later summer or fall, a soil injection should be applied now or the very early spring. Consult your Certified Arborist or Certified Tree Expert and any state guidelines for specific insecticides, rates and timing for controlling this pest.
As always, I hope you learned something!