- Red and white oak
- Maple (including boxelder)
Cankerworms are inchworm-like caterpillars that feed on the foliage of many deciduous trees. Outbreaks occur every few years, and are usually concentrated in urban areas or near deciduous woodlands. Outbreaks can cause complete defoliation of trees in early spring. Typically, the cankerworm population will increase to an upper limit where it will remain for 2-3 years, and then decline for the same duration. Both the fall and spring cankerworm species are native to North America, and have many natural enemies that regulate outbreak populations. In Utah, the fall cankerworm is most common.
- Adult moths have slender bodies and large, broad forewings. Males are about 1 1/8˝ long, and are dull gray-brown in color. Female moths are wingless, dull gray-brown, and about 5/16˝ long.
- Fall cankerworm eggs are laid in flat, neat rows that encircle twigs or small stems. Individual eggs are barrel-shaped, and shiny gray in color with a light-brown ring around a dark brown spot. Spring cankerworm eggs occur in smaller clusters and are not as neatly arranged. The individual eggs are spindle-shaped and silvery-beige in color.
- Fall cankerworm larvae range from light green to dark green or light brown to black. Usually, they have three small stripes on either side of the abdomen. Spring cankerworm larvae range from yellow-green to yellow-brown to black. They have one larger stripe down either side of their bodies.
- Cankerworm pupae are dark brown, 1/4˝ long, and wrapped in a cocoon of silk and soil particles.
Fall cankerworms spend the winter as eggs on tree bark. Spring cankerworms overwinter as pupae and adults lay eggs in the spring. Eggs of both species hatch in early spring soon after bud break. Larvae feed for approximately 6 weeks. When mature, they drop to the soil to pupate. Fall cankerworm moths emerge in fall after the first hard freeze, and spring cankerworm moths emerge in early spring. Females crawl from the ground and up tree trunks, and emit a sex pheromone to attract males for mating. Females lay approximately 50 eggs in loose clusters in bark cracks, under bark scales, and other protected sites. Cankerworms have one generation per year.
- Tattered leaves
- Random holes and chewed areas of foliage
- Complete defoliation, with re-growth occurring several weeks later
- If defoliation occurs more than 3 years in a row, trees lose vigor
Monitoring for cankerworm eggs and larvae will help to determine egg hatch and the extent of larval feeding. Native Utah trees can tolerate cankerworm feeding and defoliation. To help trees recover, provide optimal watering, fertilize, mulch within the drip zone (instead of turf), and prune when necessary.
Ways to monitor include:
- Examine twigs of susceptible host trees in late winter for egg clusters. Flag a cluster, and at bud break, watch for larval hatch.
- Shake multiple branches over white cardboard or cloth in early spring to look for dislodged larvae. Repeat every other day until larvae are found or leaves have fully expanded.
- Examine terminals of several branches for early feeding in the spring, which appear as small, irregular holes between veins.
- Use a sticky barrier around tree trunks to trap females and prevent egg laying on twigs.
Although insecticidal control for cankerworms is generally not recommended, persistent defoliation can be stressful to trees and may warrant application. Horticultural oil applied during dormancy can smother eggs. To treat larvae, an effective organic option is a product containing Bt (Bacillus thuringiensis). It must be applied while larvae are still less than 1/2 inch long. Conventional materials can be used if you miss the reduced risk/early treatment period, and must suppress larvae greater than 1/2˝ in length. These treatments include carbaryl, cyfluthrin, esfenvalerate, and permethrin.