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Gypsy caterpillars rebound in Norfolk

Bad news for forests in southern Ontario and beyond: Gypsy moth caterpillars are on the march again after a 10-year hiatus.

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It’s been quiet out there for the past 10 years.

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But don’t let that fool you.

Woodlot owners in Norfolk and southern Ontario were wringing their hands 10 years ago as armies of gypsy moth caterpillars ate through the canopies of local forests.

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Southern Ontario was at the peak of a mounting infestation in the spring of 2008. A well-timed aerial spraying delivered a body blow that cut the caterpillar population down to size.

In Norfolk County at least, the caterpillars were knocked to the bottom of the wheel. But they are never quite defeated. The cycle started anew, and here we are again approaching a new peak and another season of devastation.

Adam Biddle, Norfolk’s superintendent of forestry, says the caterpillars are back and increasingly hungry.

Trees this summer have been defoliated in the Nixon area, along Charlotteville roads 7 and 8 southwest of Simcoe, and in the rural area west of Delhi near Schafer Side Road.

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“It’s a cyclical thing that happens about every 10 years,” Biddle said Thursday. “We are coming up on the 10-year mark. We noticed quite a few adult moths last fall in our surveys.

“People have definitely noticed the damage this year. We expect next year to be much more noticeable. It will be pretty bad, and we’re expecting it to peak in 2020.”

Gypsy moth caterpillars are a concern because they stress trees to death.

In a bad year, caterpillars will strip trees of their foliage in the spring. The timing is bad because trees need to manufacture and store sugars at this time of year. To do this, trees need leaves to capture sunlight for photosynthesis.

Most trees stripped of foliage will re-flush but this takes a lot of energy. Healthy trees defoliated repeatedly eventually die.

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Especially vulnerable are oak trees, cherry trees, blue spruce and white pine. Secondary hosts include hickory, maple, birch and willow.

Biddle noted that pine trees stripped of their needles won’t rebound. Once their greenery is gone, they’re finished.

Gypsy moth caterpillars are cyclical because – after 10 years – they are present in such large numbers that it is easy for a fatal, natural virus in the environment to burn through their numbers.

However – before they crash on their own – a lot of good trees will die.

That’s where aerial spraying comes in. The natural bacterial agent Bt (Baccilus thuringiensis) was sprayed to positive effect in Norfolk in 2008. It could be used again if enough property owners are interested to make it worthwhile.

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Biddle will approach the woodlot community this fall with a proposal for another mass spraying next spring or the spring of 2020.

Community support is essential, he said, as significant acreages are needed to cover the cost and guarantee effectiveness. The 2008 campaign was successful, Biddle added, because many property owners enlisted.

Gypsy moth caterpillars are not to be confused with fall web worm.

Gypsy moth caterpillars are an invasive species from Europe. Web worm is native to North America and is responsible for the highly visible display of web nests in rural areas of Norfolk at this time of year.

Web worm nests are unsightly. However, the pest provides a source of food for birds migrating south for the winter.

As well, they aren’t that damaging because most trees in Norfolk are done producing nutrients by early August. The leaves web worms eat are about ready to fall to the ground anyway.

Locally, web worm is especially visible in Port Rowan, St. Williams and other lakeshore areas. Walnut is the preferred host but it will go after cherry, hickory and orchard crops as well.

MSonnenberg@postmedia.com

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