The large aspen tortrix, Choristoneura conflictana, is in the category of tree pests known as defoliators or leaf rollers. Its principal target host is the quaking aspen, Populus tremuloides, and the insect’s range coincides with that of the tree for the most part. The range extends from Nova Scotia south and west into northeastern and northcentral U.S., further west into the Canadian Prairie Provinces and the Rocky Mountains, ranging all the way to interior Alaska and as far south as northern Arizona and New Mexico.
Populations of the tortrix build-up to massive outbreaks over 2 to 3 years, then usually collapse. However, between 1969 and 1975 an outbreak was reported in northern Ontario that spanned over 25 million acres. And between 1988 and 2000, upwards of 700,000 acres of aspen forests in Minnesota and Michigan were defoliated. Insects will only reach outbreak levels in areas of dense aspen growth.
After eggs hatch in July, the first instar larvae begin voraciously feeding. They produce webs which join flat leaf surfaces together and skeletonize leaves within this shelter.
The primary host species of the large aspen tortix is quaking aspen, however, bigtooth aspen, Populus grandidentata, is also affected by this pest. During large outbreaks feeding can also occur on other nearby species of trees and shrubs, including; black cottonwood, balsam poplar, some birch, willow and alder species, as well as choke cherry trees.
Adult large aspen tortrix moths are brownish-gray with indistinct and variable brown markings on their forewings. They emerge the spring following egg hatching.
Patches of skeletonized foliage will be the first indicator that the tortrix is present. Eggs hatch in July and the first instar larvae web together flat leaf surfaces creating a shelter in which they will begin skeletonizing leaves. These first larvae will move into hibernation mode starting in August, nestling in bark crevices or under moss at the base of trees where they will molt into second instars and overwinter. Come May of the following year, these larvae make their way up to the crown and mine buds. Pupation occurs in late June and adults emerge approximately 10 days later.
Continuous defoliation season after season stresses aspen populations severely, causing even vigorous stands to experience growth reduction. Older stands of trees are more susceptible to mortality following top kill and defoliation. Defoliated or dead trees are unsightly in recreation and urban settings and can also pose danger to users and inhabitants. The large aspen tortrix does have a number of natural predators including; predaceous ants, wasps, and large ground beetles, as well as chickadees, vireos and woodpeckers. Additionally, larvae can be susceptible to fungal and viral diseases. However, these may not be enough when outbreaks reach levels mentioned above and sites that incur human usage may require intervention for safety purposes. There are manual, cultural and chemical treatments that can be used in combination and in concert with natural predation. If you have concerns or questions about aspen trees on your property or in your community contact your arborist and visit: https://www.savatree.com/insect-management.html.
