Parks Recreation & Culture
Cankerworms & Banding Trees
Cankerworms and forest tent caterpillars both feed on the leaves of shade trees in the spring. During outbreaks they can defoliate trees. A healthy tree will flush out a second set of leaves if it is defoliated, but repeated attacks by cankerworms and/or forest tent caterpillars can cause trees to decline.
Tree banding is an effective method of controlling cankerworms, but not forest tent caterpillars. Adult female cankerworm moths are wingless and need to crawl up the trunks of trees to lay their eggs. Tree bands trap the adult moths as they are crawling up the tree. There are two species of cankerworms; one lays eggs in the fall and the other lays eggs in the spring. Therefore, tree bands are most effective when they are put up in mid-September, re-greased in March, and taken down at the end of May. Trees most susceptible to cankerworms are elm, Manitoba maple and fruit trees. Banding a tree takes just three steps:
- Wrap a 10-15 cm (4-6 inch) wide strip of fibreglass insulation around the trunk approximately 1.5 m (5 ft) off the ground.
- Cover the insulation with cling wrap. Be sure to wrap it tightly so that the insulation is pushed into the crevices of the bark and to leave a bit of plastic above and below the insulation. This may take two or three layers of cling wrap.
- Spread a layer of petroleum jelly, Tanglefoot, or a similar sticky substance on the plastic in a band about 10 cm (4 inches) wide.
Inspect the band regularly and remove any large debris such as leaves. Reapply the sticky material as required. Be sure to remove all tree bands by the end of May to allow the bark to breath. Leaving tree bands on over the summer can cause mold and rot to develop, causing more harm to the tree than the cankerworms. The City hosts an annual tree banding clinic in September, where residents can get a hands-on demonstration of banding trees and take home enough insulation and grease to band four average sized trees.
What do I do if caterpillars are eating my tree?
Both cankerworms and forest tent caterpillars can be controlled using Bt (Bacillus thuringiensis, sometimes listed as btk). Bt is a natural bacterium that acts as a stomach poison for caterpillars but is safe for other insects, birds and mammals. Bt is sprayed onto the leaves of the infested plant, which the caterpillars then eat. It can take several days for the caterpillars to die, but they will stop feeding shortly after ingesting a leaf sprayed with Bt. Forest tent caterpillars can be pruned out of smaller trees and shrubs when they are congregated on their tent in the evening or early morning.
What’s the difference between cankerworms and forest tent caterpillars?
The easiest way to tell the difference between a cankerworm and forest tent caterpillar is the way they move. Cankerworms loop or inch when they crawl, creating a loop in the middle of their bodies as shown in the 1 Cankerworm, James B Hanson, USDA Forest Service, Bugwood.org picture, stretching out, and then pulling their back legs in to create a loop again. Cankerworms are green or brown with a dark stripe down their back. Forest tent caterpillars crawl evenly without creating any loops and have distinct white teardrop or diamond shaped markings along their backs and two blue stripes running along their bodies. We also see low levels of other tent caterpillars, which are true to their name and create a dense white tent near the ends of branches. The Saskatchewan Environmental Society has a printable fact sheet, downloadable here, to help you identify which pest you have and your best control options.