Forest tent caterpillars, cankerworms, and leafrollers are an annual issue for the city’s urban forest. The trees may experience heavy defoliation, but unless there are underlying issues, the trees will recover and produce new foliage.
Forest tent caterpillars feed on a large variety of trees, including ash, poplar, and chokecherry. In some cases these insects can completely defoliate a tree, but trees will typically recover. After several years of heavy defoliation trees can decline.
Outbreaks typically last 3 to 7 years. Saskatoon is in the third year of an outbreak. During an outbreak of forest tent caterpillars, the large number of insects can be a nuisance, but can be removed from the tree with a blast of water.
Cankerworms, also known as loopers and inchworms, are found primarily on the leaves of American elm and Manitoba maple trees. The fall and spring cankerworm species often feed together during June and July. They can be found hanging from silken threads underneath infested trees and can be a nuisance.
Leafrollers feed while concealed within leaves that are rolled or tied together with silk webbing. Ash trees are their preference, but they also attack other tree species. The larvae will feed in mid- to late-June, but the leaves will remain rolled throughout the summer.
Tree banding can be used to control cankerworms, however, it is not effective against leafrollers and tent caterpillars. The best time to place bands on the trees is from September to May to help prevent the wingless adult female cankerworm moths from climbing host trees to lay their eggs. It is important to remove the bands at the beginning of June because the band can collect moisture and cause the trunk to rot. Unlike the adult female cankerworm, the female leafrollers and tent caterpillars have wings and will fly from tree to tree to lay their eggs.
The City of Saskatoon does not control cankerworms, tent caterpillars, or leafrollers as they typically do not impact the health of the trees they attack. Homeowners interested in controlling them on their own should consult their local garden centre for advice. For information on urban pests, such as mosquitoes, Dutch elm disease, wildlife, and tree defoliators, visit www.saskatoon.ca/pests.