Invasive Forest Species' Damage

Gypsy moth egg masses Photo by Jean Levac

"Our forests are suffering from cataclysmic insults these days."

"This year it's gypsy moths. And it was just a few years ago we were hit by the forest tent caterpillars. The trees have recovered somewhat from that, but there's going to be long-term issues."

"Each egg mass can contain 300 to 500 eggs. If you do that math, if each survives, that's a lot of foliage [consumed]."

"If there's two years of gypsy moth defoliation in a row, those trees will probably die. It is more catastrophic for conifers than hardwoods. It doesn't mean hardwoods just shake it off, it just means they have a better chance of recovery."

"On a quiet day you can actually hear the frass [caterpillar poop] that's raining down from the tree tops. There can be so much that cottage owners are sweeping it up into dustbins."

"If you have an outbreak and you're wondering 'How did they get here?' That's the answer. People brought them."

"The virus [that the caterpillars are susceptible to] is a lot like COVID. When people are living crowded and closer together, the virus expresses itself. It's the same for caterpillars."

"[If it's impractical to spray, scrape, trap] Then all you can do is wait it out and hope that nature comes to the rescue."

Eric Boysen, Forester, woodlot owner, Maberly, Lanark county, Ontario

Eastern Ontario is experiencing the worst outbreak of Gypsy moths since the mid-1980s. MNRF IMAGE SunMedia

 

A fact sheet produced and circulated by Eastern Ontario Model Forests in collaboration with the Invasive Species Centre and the Early Detection and Rapid Response Network warns that "Should all of these eggs hatch successfully and develop through all of their larval stages, we should expect 'severe defoliation' [in many parts of eastern Ontario]". As much as we would wish to protect our forests as green sinks, air fresheners, and natural resources for any number of reasons, free agents infiltrate and cause considerable damage, with no way to protect an entire forest from their depredation.

 

In 1869, a failed silkworm breeding experiment in Massachusetts brought gypsy moths to North America. A hundred and fifty years ago, the researchers who hatched this scheme and imported the gypsy moths obviously had no idea what this would mean for the forests of the future in North America. Well beyond their lifetime the imported moths have thrived and now threaten natural woodlands. This didn't happen overnight, it took a century before the moths which had escaped their original laboratory confines, made their way to the forests of Ontario.

 

Adult female Gypsy moth

Once settled in an area, they lay their eggs, the pupae go through the life cycle, starting off by consuming foliage; the greater the number of eggs laid, the more hungry caterpillars will emerge from them to cover a tree from base to crown with dark, wriggling caterpillars focused on eating all the tree's foliage and consigning the tree to a struggle for survival. Repeated infestations will kill trees and if the invasion is sufficiently wide-spread, what will remain is the bleak remnants of a once-green forest.

 

This has led to warnings throughout Eastern Ontario of the peril their forests will face this summer for the second year in a row. Millions of gypsy moth caterpillars are set to hatch and they'll find their way to the tops of trees to begin munching their way from top to bottom, in a feeding frenzy. When July arrives, the sight of stripped=bare trees in a wide swath of forests will evidence itself.

 

Tiny gypsy moth caterpillars emerge from an egg mass on a maple tree in Elmhurst Forest near Lincoln Fields on Wednesday, May 12. Photo by Jean Levac

 

 According to the Ontario Ministry of Natural Resources and Forestry, gypsy moth defoliation increased by 1,200 percent last year, from 47,203 hectares in 2019 to 586,385 hectares in 2020. This year's infestation may turn out to be far more severe, revealed by aerial surveys and ground searches for gypsy moth egg masses. Hardwoods like oak and maple, bass and poplar, and conifers such as white pine, hemlock and blue spruce represent prime feeding grounds for the moths.

Mid-May is when they hatch and the caterpillars; bristly with a double row of blue and red dots down their backs, grow to over six centimetres in length. One single caterpillar is capable of devouring one square metre of foliage before it pupates. Emerging in midsummer to mate, the small grey-brown moths lay eggs for next year's brood. A single tree, in badly infested areas, can be covered in hundreds of egg masses, top to bottom. 

A foliage-stripped deciduous tree will use whatever energy reserve it may be left with in attempting to regrow new leaves. That capability however, is not endowed to evergreens; once their needles are gone, they perish, particularly vulnerable to the devastation the gypsy moth leaves in its wake. They spread through many means, and so readily that they earned their name of 'gypsy'. Tiny young caterpillars cling to silken threads carried by the wind into neighbouring trees.

 

L. d. dispar caterpillars © Jon Hayes

 

Beyond which, the female gypsy moth will lay her eggs just about anywhere; cars, camper vans, firewood; which become 'vehicles' which spread the eggs when they move on, vast distances. In some instances the egg masses can be destroyed by scraping them into warm soapy water or burned in a fire. Aerial spray with a non-toxic natural bacteria can be effective. Burlap can be wrapped around tree trunks where caterpillars find shelter during the day. They can be collected in the burlap, and destroyed.

There is also a virus and bacteria that provide for natural-occurring controls of the caterpillar population, most of which control is actually part of nature's scheme, when by chance there can be an existential encounter between the caterpillar, and the virus or bacteria.