Fall cankerworm (Alsophila pometaria), a native insect, can be one of the most damaging defoliators of Toronto’s urban forest.
Members of this family are often called inchworms, loopers or spanworms. Caterpillars have a very peculiar means of movement; they loop when they walk. There are two forms; one green, the other brown in colour.
Fall cankerworms overwinter in the egg stage, on the outer crowns of host trees. Larvae hatch and begin feeding on unfurling leaves in the spring. Infestations can spread great distances as the larvae can suspend themselves on silk threads to be carried by wind to new areas. Feeding lasts for about four weeks. The larvae then descend to the ground and enter the soil. Here each spins a sac, about 3 mm below the soil surface, where they pupate. Adult moths emerge in the fall at about the time of the first frost; wingless females initially, followed by the winged males. After mating near the ground the females must crawl to the outer branches where eggs are laid. Adult moths then die, but the eggs remain until spring to complete the cycle.
Hosts and Damage
The preferred hosts are Manitoba maple, red and black oak and crab apple. Other hosts include ash, basswood, beech, birch, elm, hawthorn, hickory, honey locust, linden, as well as other maple and oak trees.
Young larvae, only about 1 mm long when they hatch, first chew out holes in the leaves (called “shot holes”). As feeding continues the holes are widened until most of the leaf between the veins is consumed. The entire tree crown may become defoliated. Healthy, vigorous trees can usually put on new leaves later in the summer. Older, less healthy trees may become stressed, experience crown die back, or even die if this defoliation recurs over 2 or 3 consecutive years.
Natural or beneficial organisms, such as parasitic wasps, birds, pathogens, mice, ground beetles and insect-attacking nematodes usually keep fall cankerworm population levels in check. However, serious infestations do break out from time to time, usually every 8 or 12 years in forested areas. These infestations are historically less frequent in urban forests.
Specific Management Practices for Control of Fall Cankerworm
Trap wingless female moths by installing a band around the main stem of host trees in October. This physical barrier prevents egg laying in the upper crown. The instructions for banding can be found below.
Attract to your yard birds and other beneficial organisms, like ground beetles by planting appropriate plants (herbs, flowers, ground covers and shrubs).
Increase the number of beneficial organisms by releasing insect attacking nematodes and/or parasitic wasps. Check your local Garden Centre for availability.
Discourage the use of synthetic pesticides, as spray drifts are harmful not only to humans and pets, but also to many beneficial organisms that naturally help control infestations.
Apply Bacillus thurigiensis var. kurstaki (Btk) in severe infestation to early larval instar when the host leaves first start unfurling. This bacterial insecticide affects only actively feeding moth and butterfly caterpillars.
Fall Cankerworm Banding
Fall cankerworm damage can be controlled in your backyard in an environmentally friendly way. A sticky band installed around the main stem of host trees, traps the female moths and prevents them from climbing up the tree to lay their eggs. The female moths are wingless and cannot fly. All host trees should be banded.
Materials and Tools
Quilt batt, cotton batt, or foam roll cut into 10-20 cm wide stripes, available from most department stores.
- Plastic wrap cut into long 20-30 cm wide strips, available at grocery stores.
- Sticky material such as Tree Tanglefoot, also known as Stickem, available at most garden centres.
- Application tool - a wooden paint stirrer is ideal.
- Select trees for banding. All tree species identified as preferred host species above should be banded. Do not band coniferous trees or any other objects.
- Wrap the quilt batt at a convenient height (over 1.5 m above ground level) around the tree main stem.
- Overlap the plastic wrap and pull it tightly over the quilt batt. This will help seal bark crevasses to prevent female moths from crawling under the band.
- Apply a thin layer of the Tree Tanglefoot or Stickem to the plastic wrap using a paint stirrer.
- Use baby or vegetable oil on your hands to make wash up easier from the sticky material.
- Tighten bands if they become loosened to prevent female moths from crawling up the tree to access the crown.
- Apply more sticky material if the bands become covered with debris, leaves or moths.
- Replace the tree bands if they become worn out or fall off before mid December.