Bruce Spanworm Outbreak

The west side of Rocky View County has had a large number of properties affected by small green caterpillars. These caterpillars have been confirmed to be the Bruce spanworm, an insect from the Order Lepidoptera which includes moths and butterflies. The main host plants include poplars such as aspen and balsam but they also affect other species such as willow, birch, choke cherry, saskatoon, and wild roses.

Image of spanworm

The caterpillars overwinter in egg stage at the base of the tree in bark crevices as well as in moss and lichen. Eggs hatch in spring and the larvae “inch” their way up the trunk and begin feeding on the leaves. Often the caterpillar will roll the leaf around them and secure it with silk as a protection mechanism. Once they destroy the entire leaf crop, larvae descend from the tree on silk strands where they could continue to feed on lower shrubs. There may be large silk webs on the understory and surrounding trees when infestations are severe. Historically, outbreaks haven’t lasted more than two years, with limited tree mortality.

Image of spanworm webs in trees

Control Options

For smaller stands and ornamental trees a biological, non-toxic Btk (Bacillus thuringiensis kurstaki) spray can be used and found at local hardware / gardening stores (Home Depot, Canadian Tire, Lowes). However, in most cases the trees and shrubs are resilient and can handle defoliation as they will have a second flush of leaves after the pest has run its course. Defoliation from rare infestations usually does not kill trees and is more of an aesthetic concern.

Posted in: Agriculture Weeds & Pests

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