Arborvitae Leafminer: Appearance, Territory, Damage and Life Cycle

Latin Name: Argyresthia Thuiella Appearance: There are four species of arborvitae leafminers in the Northeast, with Argyresthia Thuiella being the most prevalent in New York State. Arborvitae is the primary food plant. The adults are small white to light gray moths with brown patterns on the forewings (3/8-inch wingspan). The larvae or caterpillars are about an eighth of an inch long, with a light green (occasionally with a reddish or yellow tinge) body and a glossy black head. Hosts Plants: Arborvitae (Thuja) Territory: This moth may be found in eastern Canada and the United States, from Maine to Missouri. It is also a problem throughout Europe. Damage Insect Cause: The older, slower-growing trees that are exposed to sunlight attract these miners. Trees that have been badly damaged can produce new leaves later in the growing season. On the other side, heavy defoliation over a long period of time may result in tree death. Because the parasites of this leaf miner are so abundant and effective as a natural control, artificial control is rarely needed, especially on attractive trees. Pesticides can then be applied at 3-week intervals between early June and mid-July to control females before they deposit eggs. Life History and Habits: Each year, one generation is born. Adults are visible from June through mid-July. The female lays 1 to 25 eggs behind the margins of the current year’s leaves shortly after emergence. Larvae emerge towards the end of June and immediately mine into the leaves to feed and grow for the remainder of the season. The larva puts all of its waste to one end of its tunnel and overwinters in the fall. In the spring, the frass is evacuated from the tunnel, and the tunnel is stretched into the interior of the crown. Adults appear shortly after larvae pupate in the mine.