Southern Ontario (and some parts of southern Quebec) is going through another gypsy moth infestation at the moment. This happens every decade or so (the last few occurred in 1985, 1991 and 2002). The current outbreak started in 2019 and this year is looking like it will break records again, like last year.
Although it is not an event comparable to the periodic cicada infestation going on just south of here, it is still a very noticeable phenomenon: park paths are covered in squishy and squished caterpillars (and their feces), and many trees have been denuded by the voracious caterpillars, mainly oak, birch, poplar, willow and maple. Oak trees in particular have difficulty regenerating so much foliage, and many may die as a result. Last year, a record-breaking 580,000 hectares (about the size of Prince Edward Island, if that helps) of trees were stripped bare, and many of them did not survive. A similar area of devastation is expected this year: this is not a paltry event.
Gypsy moths (or "LDD moths" among the more politically correct, after the Latin name Lymantria dispar dispar) are an invasive species originally from Europe and Asia. They were introduced into North America by a single individual, the amateur entomologist Étienne Trouvelot, in the 1860s. He brought them into the Boston area to see if they would be appropriate for American silk production (they weren't), from where they escaped, and have been spreading in increasing numbers ever since.
The caterpillars are brown-grey and hairy with blue and red dots, up to 6cm long, but usually much less than that. The adult male moths are pale brown and boring, while females are white and boring (and flightless). They can't survive winter temperatures of below -20°C, hence their concentration in balmy southern Ontario, but, with climate change, their range is expected to extend northwards. They particularly like a warm winter followed by a dry spring, like this year (a wet spring encourages the growth of a fungus that kills them).
So, what can be done about them? They can be killed by a bacterial spray, and some municipalities do employ this, but this also kills many beneficial and native caterpillars, so most jurisdictions (and Ontario Parks) are loath to go down that route. Some "high value" trees, particularly the more susceptible oaks, can be treated individually. But the preferred treatment is the more labour-intensive one of manually scraping or vacuuming off the eggs. You can also tie a band of burlap around a tree trunk around chest height; the caterpillars like to hide under it, where they can relatively easily be scraped off and disposed of.
But most scientists are of the opinion that, in the scheme of things, they are not that destructive, and we will probably just have to learn to live with them.
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