Cankerworms can be a huge nuisance because they defoliate many species of trees in springtime, causing the tree to turn its energies into creating a new set of leaves, reducing the tree’s defences. Several seasons of defoliation can also cause branch die-back.
Cankerworms (also known as inchworms because, when they reach their full size as larvae they are about one inch long and have a hump-backed way of “inching along”) are unpleasant visitors to gardens because when they have finished feeding on the leaves of your trees, they drop from the branches by means of a long silken thread. They also exude a sticky “honeydew” in the same manner as heavy infestations of aphids.
There are two species of cankerworm, one where the wingless female adult moth is on the march in fall and the other in spring. The female moths crawl up host trees and, after mating with their winged male counterparts who visit them there, lay their eggs around small twigs in the tree. If you examine fruit trees in your yard, you may see their egg bands tightly wrapped around these twigs. In spring, both species hatch as larvae at the same time and often on the same tree.
Here they gorge themselves until the caterpillar drops to the ground where it will burrow a few inches into the soil and pupate. The fall cankerworm emerges as an adult after the first hard frost in fall; the spring cankerworm emerges as an adult in late March or early April, depending on the weather, after overwintering as naked brown pupae about four inches under the ground.
The wingless females of both species look less like moths and more like fat brown or grey, oval-shaped bugs with spidery legs. The males are typically beige- or grey-coloured moths.
Spraying trees with dormant oil in late winter can get rid of the eggs.
An environmentally safe prevention and control for these nasty invasions of ornamental fruit trees, maples, oaks, and lindens, is to wrap a sticky band around your trees in fall on or just after the Labour Day weekend. Keep the bands up, renewing the sticky substance in March, until just after the May long weekend. Make a band using a strip of insulation, wrapped with plastic and then smeared with a sticky substance. An effective brand of this sticky substance is Tanglefoot. By banding trees in September, you’ll catch both populations as they climb heavenward to mate. Be sure to remove the bands after Victoria Day to avoid damaging the bark of the tree.
You do catch other bugs at the same time, including the native elm bark beetle, which travels southward to the bark at the base of the elm to overwinter.
An infested tree can be sprayed with BT (Bacillus thuringiensis), a naturally occurring bacteria that attacks and paralyses the digestive system of the larvae, causing death in anywhere from 12 hours to five days. To be effective this must be done when the trees are fully leafed out and the larvae are only about half grown. You may have to spray more than once as the larvae hatch at different times starting in April or May and lasting until late June. If you’re too late for this, then forbearance is about the only remedy. The worms, and their nasty brown droppings that are foul everything beneath the trees they infest, will be gone in two to three weeks.
Healthy, mature trees will refoliate, but young trees or trees under a lot of stress may be at risk. Preventative tree banding is important to protect a considerable investment, and to avoid all the nasty experience of these fellows dropping first their honeydew on you and then their fat bodies when they are finished dining.