Gypsy Moth Infestation Prevention: Round Two

Photo Karen Graham.
Egg masses produced by female moths.


This has been a bad year for the Gypsy moth.  While we usually see outbreaks of this pest every seven to ten years, 2021 will be the second year in a row of a severe infestation in many parts of Ontario.

At the larvae stage, the creatures chew holes in leaves or devour them entirely.  They are not fussy eaters.  Gypsy moth caterpillars prefer the leaves of oak, maple, birch, beech, willow and poplar, but if these are not readily available, they will move to conifers including pine, fir and spruce trees as their food source.

Gypsy moth caterpillars can be found from April until June, when they pupate.  The male moths are light brown in colour and fly erratically, focussed on seeking a mate.  The females are larger and white in colour, and while they have wings, they do not fly.  Both have a very short life span, perhaps in part because they do not feed at all.  Their sole mission is to reproduce, and can they ever!

While the obvious threat of infestation is fading from view, it is just lying in wait in the form of hundreds of eggs on the lower parts of trees, understory shrubs, building exteriors, birdhouses and garden fixtures.  Each egg mass is roughly 4 cm in diameter and contains 100 to 1,000 eggs. They lie in wait to start the cycle again in April, unless we take action.

Photo Karen Graham.
Female moths laying eggs in the crevices of a tree in Millbrook, preparing for next year’s invasion.

In the past, infestations fell under the responsibility of the Ministry of Natural Resources, which conducted aerial spray programs during periods of severe infestation in the past, but the control of this pest is now the responsibility of the municipalities.  Spraying programs are expensive and very time-sensitive, and financially-challenged municipalities are reluctant to add this cost to their budgets.  These programs rely on two applications of Bacillus thuringiensis kurstaki (Btk), a natural soil dwelling bacterium that is toxic to young caterpillars when eaten, but the application is only effective early in the season when the caterpillars are small.

During the caterpillar stage, some residents seeking to protect their trees resorted to measures including wrapping trees with bands of oil, burlaps or paper to prevent caterpillars from reaching the leaves they seek to consume.  At this point, prevention activities must focus on the eggs.

White moths should be removed, and if they have already laid their eggs, these should be removed and discarded as well.  According to Jane Zednik’s report in the Millbrook and District Garden Club newsletter last month, simply scraping the eggs off the tree is not enough, as they will overwinter on the ground and remain viable, leading to a new batch of caterpillars in the spring.  The eggs must be totally crushed or dumped in soapy water to be certain they do not survive.

Homeowners may have noticed that some trees that were defoliated early in the season have created new leaves, but the underlying damage remains.  Defoliation makes trees more susceptible to secondary pests, drought, and poor growing conditions.

There is plenty of time to address the egg masses that lie in wait for next spring.  While prevention remedy sound gross, it will be well-worth the “yuck” factor if it protects our green canopies, majestic winter silhouettes and helpful wind blocks in the future.  KG

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