Jeff Green | May 26, 2021


Over the past week to ten days, pharmacists and doctors’ offices in parts of Eastern Ontario have been inundated with calls about apparent bug bites or poison ivy causing skin irritation, welts, and swelling on arms and necks. Antihistamines, prednisone and painkillers have been prescribed.

Quickly enough, the true cause was determined: tiny caterpillars falling from the sky. The gypsy moth infestation, which resulted in defoliation of the first set of leaves in deciduous trees and major needle loss in evergreens last summer, is back.

This time, the infestation has already had an impact on people, and there is every indication it will have an even greater impact on forests and backyard trees alike very soon.

Eric Boysen is a forester who has been managing and restoring a 200-acre woodlot on Fagan Lake, near Maberly. He has been studying moth infestations in his own woodlot for a number of years.

He said that the combination of dry weather over the last two weeks, and a windy Monday on May 10th, have been an ideal combination for millions and millions of tiny caterpillars who were hanging on strings high up in trees waiting for a dispersing wind.

The tiny hairs of the caterpillars are toxic and some people claim they have been bitten by them.

The good news, at least for those who have been dealing with itching and swelling skin, is that the caterpillars have moved on to the further stages in their development by now.

“They go through six larval stages,” said Boysen, “they are in the third stage now.”

They have begun to eat the leaves of a large variety of tree species.

According to Boysen, who made a presentation about moth infestations to a meeting of the Eastern Ontario Model Forest in February, which he will present again over Zoom on June 10, the biggest hope to control the population this year would have been a cool, wet May, and that did not happen. With dry weather in store for June, the only real control over the caterpillars this year will be their own success.

“There is a virus that they can handle when the food supply is plentiful, but kills them when they are stressed, which happens when there are too many of them for the food supply. There is also a fungus that kills them,” he said.

If these factors are in play this year, they will be apparent. Caterpillars infected with the virus can be found on tree bark in an upside down V shape, and the fungus causes them to look like they have melted onto the bark. As well, later in the season, the loonie sized brown/beige egg masses that were so ubiquitous last year will be smaller, dime sized perhaps, and less plentiful.

There are things that people can do. Special trees can be protected by using burlap 'traps' which attract the caterpillars as they travel up the bark of trees at night. They can then be gathered up and dumped in soapy water. Many people scraped off egg masses last summer and fall, and there is a product that you can get at hardware stores, BTK by Safer, that can be applied. Aerial spraying is another option, but the company that does the spraying, Zimmer, is fully booked.

“Timing is critical if spraying is going to have an impact. The caterpillars need to be at the right stage, and it needs calm winds and high humidity for the droplets of spray to stick on the trees, and no rain. And it usually needs to be done twice,” he said.

“Human intervention is likely to have little impact on overall population dynamics. All anyone can do is focus on protecting individual trees, or smaller areas, for aesthetic purposes or to protect a favourite tree,” Boysen said in his presentation to the Model Forest group.

Boysen said that he was aware of the gypsy moth increase in his forest in 2019, after 3 years of facing an infestation of the forest tent caterpillar, in our region, that was prevalent in 2016-2018 and was just starting to drop off in 2019.

“The tent caterpillars favoured maple, but also hit oak, poplar, cherrywood, basswood and Ash” said Boysen, who said that he lost about 15-20 mature maples after three years of stress.

When deciduous trees are defoliated early in the season, they will refoliate later in the summer, but that comes at a cost to the tree.

“You can see damage to the crown after a couple of years of this, and trees that are weaker, have other diseases, are the ones that are at risk. It's a lot like the 'co-morbidities' that scientists have been talking about with COVID,” he said.

Ministry of Natural Resources and Forestry (MNR-F) mapping shows that gypsy moth infestations hit pockets of Southwestern Ontario in 2018 and began spreading into Eastern Ontario in 2019. The 2020 map shows a very large infestation in Eastern Ontario.

Gypsy moths prefer oak, but are more versatile feeders than tent caterpillars. They will feed on poplar, cherry, basswood, fruit trees, white birch, elm, willow, beech, and ironwood as well.

They will also eat the needles on conifers, which can be even more devastating to the host trees because the needles do not regenerate the way leaves do on deciduous trees.

White pine, white and blue spruce, balsam fir, hemlock and larch are all susceptible.

Each egg sack can produce 300-500 larva and each larvae can eat 1 square metre of foliage.

“Do the math to get a sense of the scale of this” he said.

2021 may indeed be the tipping point that brings about a population collapse for gypsy moths in our region, but they never do go away.

“They just fade into the background,” he said.

To register for the gypsy moth webinar on June 10, go to Ontariowoodlot.com, and click on events at the top of the page, then click on June10 in the calendar, and click on the registration button.

“These presentations offer information, not really solutions,” said Boysen, “it's kind of a lonely-hearts club.”

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