Last month, researchers started tying white bags around the buds of some trees around the city. If you noticed any of the bags, you’d be forgiven for thinking a strange new landscaping trend is taking off.
“We’re big Halloween fans. We wanted to make it look like Halloween year-round,” says Jon Sweeney, a research scientist with the Canadian Forest Service.
Just kidding. Sweeney is using the bags in an experiment on the beech leaf-mining weevil, a pest native to Europe. As its name suggests, the beech weevil likes to hang out on beech trees, which Nova Scotia has two varieties of: European beech trees and American beech. The weevil feeds on both, but the American trees suffer far more damage than their European counterparts.
The tiny insects have been slowly killing American beech trees since they were first detected in Halifax a decade ago. In a weevil-infested plot of forest Sweeney studied in Sandy Lake, he said 95 percent of the American beeches were dead within three years.
The invasive species lays eggs under beech leaves and the hatched larvae chew through them, which weakens the tree over the years. Weakened beeches are susceptible to rots and diseases, which ultimately kill them. An American beech ravaged by weevils “looks like someone put a flame thrower to the tree,” Sweeney says. But European beech trees, like the ones found in Point Pleasant Park, still appear healthy after the weevils snack on their leaves.
By comparing weevil survival on both tree species, Sweeney’s experiment will help us understand why.
A few weeks ago, Sweeney tied sleeve cages to European beeches in Point Pleasant Park and American beeches at Ashburn Golf Club, in order to keep weevils off the buds. This ensured the buds were exposed to the exact same number of weevils when he and visiting Polish researcher Jakub Goczal added mating pairs to each cage on Tuesday. Sweeney will check back in a few weeks to see how many new weevils hatched and assess the damage on the leaves.
“We're predicting that we're going to see greater weevil survival and more damage on the American beech than on the European beech,” Sweeney says. “It's not going to give us a magic bullet to protect the trees or anything, but it'll tell us more about its biology”—weevil biology—“and why it's behaving so aggressively, because in Europe it never kills European beech.”
Sweeney suspects American beeches are hit harder for two reasons. One, they didn’t co-evolve with the weevil, so they haven’t built up any natural defenses. And two, weevils have a lack of natural enemies in Nova Scotia.
He says a long-term solution to Nova Scotia’s weevil problem could be to introduce a weevil-attacking species from Europe to the province, in a process called classical biological control. The method has been successful before, including with the alfalfa weevil, another invasive past.
Even though Sweeney thinks the beech leaf-mining weevil will kill many of Nova Scotia’s American beech trees, he says we probably won’t lose the tree species entirely. However, a significant population loss of the tree can have cascading effects on the other wildlife in the area. Beech nuts are an important food source for bears, for example. “If [weevils] were to kill all the beeches we lose not just beech, we would lose all those species dependent on beech and it would have quite a significant impact,” he says.
Not to overstate that impact. In a worst-case situation where the American beech was driven to extinction, another species of tree would eventually take its place in the ecosystem. “You still have trees,” Sweeney says, “but it just wouldn’t be the same, you know?”
