This is the first of a four part series on fall tree insects.
In fall, while preparing for winter dormancy, many plants shed their leaves in a burst of color. During this period, certain tree insects can be easily identified. The following examines some of the most frequently observed fall tree insects, and how they impact their hosts.
Bagworm (Thyridopteryx ephemeraeformis)
Bagworm, also referred to as common bagworm, common basketworm, eastern bagworm, evergreen bagworm, and North American bagworm, is an insect that infests numerous deciduous and evergreen trees. The insect is notable for the bag that it weaves during its larval stage. The larvae use the bags to conceal themselves as they feed. Bagworm larvae are voracious feeders. When populations are dense, the larvae can rapidly defoliate host trees.
Bagworm infests a multitude of hosts. It is most partial to arbovitae and red cedar, but will also infest apple, birch, black locust, cypress, elm, honeylocust, Indian hawthorn, juniper, ligustrum, maple, oak, pine, poplar, spruce, sycamore, viburnum, and willow.
Symptoms of Infestation
Bagworm larvae feed on the upper foliage of host plants. They gnaw small holes in the leaves or needles. On evergreens, the persistent feeding causes the branch tips to turn brown. Severely infested trees often experience significant defoliation. Repeated defoliations can reduce tree vigor. Eventually, infested trees may become unstable, and collapse. Upon being defoliated, most deciduous trees will produce a second flush of foliage by the end of the growing season. The larvae weave silken threads, that they use to fasten their bags to nearby branches. If the silken threads remain attached to a branch for several years, the branch may become girdled, and die back. By winter, the bags turn brown.
- Bags from the previous year may be removed by hand in early spring, before the eggs hatch.
- Bags may be pruned from infested plants during late fall, winter, or early spring. The bags become more conspicuous in fall once they have turned brown. Severe the silken threads that holds the bags aloft, and safely dispose of them.
- Several registered insecticidal sprays can be utilized to control bagworm infestations. This method is ideal when bagworms become too numerous to control naturally. Applications should be performed in June, when the larvae are small. The larvae become more resistant to insecticidal sprays as they mature.
- Releasing entomopathogenic nematodes into the environment can help to reduce larvae populations. For this method to be effective, conditions must be favorable to the nematodes’ development.
- Bagworm has a plethora of natural predators that help to limit its populations. These predators comprise at least eleven species of parasitic wasps, including Pimpla disparis, Itoplectis conquisitor, and Gabrus ultimus. White-footed mice, and sparrows feed on the eggs and larvae.
Fall Cankerworm (Alsophila pometria)
Fall cankerworm is a member of the family of moths called Geometridae. The insect is native to North America. The larvae strip the leaves from trees, and consume them, causing extensive defoliation. Trees that are infested by fall cankerworm often experience a significant reduction in plant vigor.
Fall cankerworm consumes the leaves of apple, ash, basswood, beech, birch, black cherry, boxelder, dogwood, elm, hickory, red maple, sugar maple, red oak, and white oak. Oak is the insect’s preferred host. Extensive outbreaks have occurred in forests comprised primarily of oaks.
Symptoms of Infestation
Severe infestations can result in the widespread defoliation of host trees. If defoliated for three or more consecutive years, infested branches may die back. Eventually, trees may weaken, and become prone to failure.
- Begin monitoring for fall cankerworm in early May, when the larvae first emerge.
- Numerous insecticides can be applied to reduce larvae populations. Bacillus thuringiensis, carbaryl, cyfluthrin, esfenvalerate, malathion, and permethrin are the most effective insecticides available for commercial use. These insecticides will not harm beneficial insects, wildlife, or humans. For the best results, applications should be made when larvae are less than ½ an inch in diameter. Larvae become more resistant to insecticides as they develop.
- Cool, wet spring weather has an adverse effect on populations.
- Late spring frosts can kill newly emerging foliage on host trees, eradicating food sources for early stage larvae, and causing a collapse in larvae populations.
- Several insects assist in limiting cankerworm populations. A tiny wasp called Telonomus alsophilae parasitizes the eggs. Calasoma frigidum, a ground beetle, preys on the larvae.
- On isolated shade trees, bands composed of a sticky substance can be placed around the trunk in late fall. This will deter females from ascending the tree, and depositing their eggs. This method is not as effective in forested settings where there is an abundance of trees, and infestations are more difficult to manage.
Photo courtesy of Alicia Lafever