For several years we have been challenged with how best to deal with major infestations of winter moth, Operophtera brumata, in many of the urban gardens where we work. Our native garry oak, Quercus garryana, is a favoured food source of this non-native moth, and large populations can severely damage the foliar canopy, leading to significant stress for the trees.
Not only that, they create problems for the humans who have their homes and gardens underneath the trees. First problem: the feasting caterpillars drop significant quantities of ‘poop’ while feeding in tree canopies, particularly unpleasant if your prime sitting/eating area happens to be in proximity to the oaks.
Second problem is that, once they have finished the feeding phase and are ready to pupate, the caterpillars ‘balloon’ down to the ground, en masse, inadvertently landing on the humans working and relaxing below. A severe infestation results in several weeks of unwelcoming garden conditions during the first few lovely weeks of spring.
So, understandably, many people seek immediate results and resort to spraying trees with Btk, Bacillus thuringienis Kustaki, a naturally occurring bacterium that is a commonly-offered remedy for winter moth. However, I am not comfortable recommending Btk, even though it is ‘organic’ and OMRI-certified, because it is a non-selective, contact pesticide which will kill other Lepidoptera species (that’s all butterflies and moths).
Additionally, spraying is not always effective because a lot of winter moth damage can occur when caterpillars crawl into and feed inside foliar buds before the leaves unfurl. Btk can only contact the caterpillars if it is applied when they are feeding on the outside of the buds or on the unfurled leaves. Treatment often occurs once most of the damage to the tree canopy has already been done.
The Alternatives!
For the past 3 years we have been installing sticky tree bands on tree trunks as a more environmentally-friendly alternative. The winter moth larvae pupate at the base of trees and when they hatch out in November the females, which are wingless, crawl up the trees to lay their eggs in the canopy. Although its not 100% effective, the tree bands capture many of them on their way up the trees. After the first year, we are able to identify the trees that have the highest populations and we add additional or thicker bands to catch as many as possible.
In conjunction, we work with clients to improve the natural ecology on sites so that we encourage the moths’ natural predators, birds and predatory beetles, to live and eat on the property. We do this by providing shelter, nesting materials and year-round food supplies. Leaving leaf litter is an excellent encouragement for birds.
There are many birds that will feed winter moth caterpillars to their nestlings in spring by the 1000s - wrens, chickadees, nuthatches and warblers. Predatory beetles live in leaf litter and debris piles and they will eat the pupae during the phase of their lifecycle when they are in the soil.
A huge part of my work is helping clients to understand the natural processes and ecology in their gardens, and how to balance health and ecology with having their picture-perfect garden.
For more information on winter moth: