Pesticide injection developed to attack destructive beech leaf weevil

This year, for the first time, a pesticide is available for use against an invasive bug that has been slowly killing beech trees across Nova Scotia.

Jon Sweeney, a research scientist with the Canadian Forest Service in Fredericton, has been testing the pesticide TreeAzin against the beech leaf-mining weevil since 2013. He said it can be injected into the trees in fall or early spring.

"In an ideal world, everybody could treat their trees. We'd really suppress that population."

In the first two weeks of May, when beech buds burst open, adult weevils start to feed on them. They lay their eggs on the leaves and when the larvae hatch, they eat tunnels, or "mines" through the middle of the leaves.

Beech trees under attack

The weevils were first discovered in Halifax in 2012, although Sweeney said they've likely been infesting American beech trees in the area since as early as 2007.

As weevils feed year after year, causing beech trees to lose large amounts of foliage, the trees get progressively weaker, Sweeney said. This repeated stress can eventually lead to tree death.

TreeAzin, an insecticide derived from the neem tree, kills weevil larvae when they bite into the beech leaves. Although it doesn't kill adult weevils, Sweeney said, this still helps to decrease the population.

Ron Neville, a plant health survey biologist with the Canadian Food Inspection Agency, said it's important to try to get rid of the weevils because beech trees are also at risk from beech bark disease and a pest called the scale insect.

"Unfortunately, the beech leaf-mining weevil is a compounding problem for beech trees, further stressing them, putting them at risk," he said.

Biological solution sought

Sweeney said although TreeAzin is good for use on high-value trees in residential areas, it's not practical for use in larger beech forests. Trees must be injected individually, which takes labour and money.

He's on the lookout for a more permanent solution.

His team is just beginning to research biological controls, having recently received the funding. It will be at least four or five years before they have enough information to apply to release one into the environment, Sweeney said.

In the meantime, both Neville and Sweeney urge people to avoid spreading the weevil, especially as the summer camping season approaches. They warn against moving firewood from place to place, as weevils, along with other pests, can hibernate under the bark.

"We really encourage people to always source their firewood, you know, buy it where they burn it," Neville said.