ISSUE 3 May 27, 2010
TREE INSECT PESTS - CANKERWORMS AND TENT CATERPILLARS EMERGING
Cankerworms and tent caterpillars are being observed in eastern North Dakota (Cass and Traill Counties). Homeowners, arborists, and park managers should be on the lookout for cankerworm and tent caterpillar activity.
Cankerworms: Spring cankerworms (Paleacrita vernata) belong to the inch-worm family Geometridae. They attack a variety of hardwoods including basswood, bur oak, elm, green ash, maples and white birch.
There is one generation per year and they overwinter as larvae (caterpillars) in earthen chambers in soil. Larvae pupate and the spiny, wingless female emerges in early spring. Females lay masses of about 100 eggs in crevices of bark on the lower trunks of hosts. Young larvae hatch from the eggs and feed on the buds and unfolded leaves. Larvae range in color from yellow-green to almost black and have a yellow stripe along the side of the body and about ¾ to 1½ inches long when mature (Fig. 1). Another differentiating characteristic of spring cankerworm larvae is the two pairs of abdominal prolegs and a pair of tubercules on the dorsal surface of the abdomen. Larvae crawl via a looping behavior and are often blown by the wind as they hang from their own strands of silk.
Figure 1. Spring cankerworm larva (J.B. Hanson,
USDA Forest Service, Bugwood.org)
Defoliation is caused by larval feeding. Repeated defoliation for three or more consecutive years will stress trees, decrease aesthetic value, and may kill tree branches.
Banding of tree trunks with a sticky material such as Tanglefoot® is a common technique used for cultural control. A 3- to 4-inch band of sticky material applied in April is used to prevent the wingless female moth from crawling up the tree (Fig. 2). It's recommended to apply the sticky material to a waterproof material wrapped around the tree. This prevents absorption and allows for removal of the sticky substance at the end of the pest season. Insecticidal treatments should be directed at the larvae in late spring or early summer. A residual insecticide sprayed on the trunk, branches and newly expanding leaves will kill the larvae as they emerge from the eggs. Spray ten days after egg hatch for the best control. Some active ingredients of insecticides labeled for homeowners include: carbaryl, cyfluthrin, imidacloprid, esfenvalerate and permethrin. Biorational pesticides include Bacillus thuringiensis var. kurstaki (Bt), insecticidal soap, spinosad and pyrethrin.
Figure 2. Adult spring cankerworm moths on
sticky band trap (W.A. Carothers, USDA Forest
Tent Caterpillars: Three species of tent caterpillar occur in our area: eastern tent caterpillar (Malacosoma americanum), forest tent caterpillar (M. disstria), and prairie tent caterpillar (M. californicum ssp. lutescens). Host plant damage by these moth species is caused by the larvae, or "caterpillars." Several hardwood hosts may be used, depending on the species.
Eastern tent caterpillar prefers chokecherry, though it will feed on other hardwood species occasionally. Larvae of eastern tent caterpillar are gregarious and construct tent-like nests of silk in the forks of branches and smaller trees (Fig. 3).
Figure 3. Tent of eastern tent caterpillar
(Photo by P. Beauzay, NDSU)
Tents are used for shelter or as resting places. Larvae forage during the day in new foliage on nearby branches. Larvae feed for 6 to 8 weeks and are about 2 inches long when mature. Larvae (Fig. 4) are black and rather hairy, with a whitish-yellow stripe down the middle of the back, narrow broken orange stripes just to either side of the whitish mid-stripe, and lateral white and blue markings.
Figure 4. Eastern tent caterpillar larva
(Photo by P. Beauzay, NDSU)
Larvae disperse when mature and spin cocoons in sheltered places. Adult moths appear in late June and early July. Females lay eggs in a band-like cluster of 150 to 350 eggs around a small twig and cover the eggs with a frothy excretion called spumaline. Eggs overwinter and larvae emerge in the spring; thus, there is one generation per year. Larval feeding disfigures ornamental plants but usually does not result in permanent damage unless the feeding is severe. Tents and masses of larvae are unsightly. Eastern tent caterpillar populations usually peak every 10 years.
Forest tent caterpillar utilizes a wide variety of hosts, including ash, aspen, basswood, birch, cottonwood, elm, maple, and oak. Larvae emerge in the spring from overwintered eggs. Emergence coincides with the flush of host plant foliage. Larvae feed for 5 to 6 weeks and are about 2 inches long when mature. Larvae are identified by keyhole-shaped spots along the midline of the back and by broad bluish lateral bands (Fig. 5). Unlike other tent caterpillars, forest tent caterpillar does not form a tent. Instead, larvae gather and spin silken mats on branches. Larvae tend to feed in wandering masses. Mature larvae form silken cocoons, and adult moths emerge about 10 days later. Females deposit 150 to 200 eggs around small twigs and cover the eggs with spumaline. Light defoliation has little effect on tree growth, but severe feeding can affect growth and cause twig mortality. In North Dakota, outbreaks of forest tent caterpillar typically last for 2 to 4 years.
Figure 5. Forest tent caterpillar larva
(Photo by P. Beauzay, NDSU)
Prairie tent caterpillar can utilize a variety of hardwood host, though chokecherry is its preferred host. Prairie tent caterpillar is the most common tent caterpillar species in North Dakota. Praire tent caterpillar overwinters in the egg stage and larvae emerge in the spring with the flush of their host plant foliage. Larvae feed for 6 to 8 weeks and are about 2 inches long when mature. Larvae are black with a white mid-line stripe broken into dashes, and light blue lateral stripes also broken into dashes. Like eastern tent caterpillar, larvae of prairie tent caterpillar form silken tents in the forks of branches and small trees and feed on nearby foliage. Mature larvae spin cocoons in curled leaves or in leaf litter. Adult moths emerge in mid-summer. Females lay eggs near the base of the host plant in the ground. Larval damage is similar to that of eastern tent caterpillar.
Control of all tent caterpillar species should target larvae. Actively feeding larvae are easily controlled with conventional foliar insecticides including acephate, carbaryl, imidacloprid, or any of several pyrethroids. Biorational pesticides include Bacillus thuringiensis var. kurstaki (Bt), insecticidal soap, spinosad and pyrethrin. Boiling water can also be poured directly on tents that contain larvae. Tents may also be physically removed and destroyed.