Last year, three of my balsam fir trees looked awful—brown needles, tunnels in the bark, piles of sawdust near the roots. A local arborist identified a sawyer longhorn beetle infestation. Unless we cut down the trees—which were dead—the beetles could kill neighbouring ones too.
We’d seen these creepy insects before—many, but not all, longhorn beetles are native to Canada. Only some longhorns—including some native species—damage trees, and usually only trees already under stress, says David Dutkiewicz, the entomology technician for the Invasive Species Centre in Sault Ste. Marie, Ont. “Healthy trees have defences against beetle infestations, but there’s no way to prevent the beetles from attacking stressed trees,” he says. In fact, trees affected by lightning, heavy snow load, or drought release a stress scent the beetles can detect. Sawyer longhorn beetles are just one adversary your trees face. Here’s how to look for and guard against some other common foes
1) Needles turning brown? Could be spruce budworm
“Check conifers for dead branches or areas that aren’t very green,” says Dutkiewicz. Brown needles can result from overcrowding or root rot, but the most likely culprit is spruce budworm. The insect can also cause needles to die back over several years.
In Quebec and Eastern Ontario, budworm outbreaks are widespread, with moth larvae chewing on the needles, often killing the trees, says Dutkiewicz. Last year, Ontario had almost 2 million hectares of defoliation; Quebec had more than 10.4 million hectares.
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Small caterpillars typically appear on the trees in May and June, pupating in late June or July and emerging as small brown speckled moths in August. If caught early, trees can be treated with biological pesticide. An efficient approach, suggests Dutkiewicz, is for cottage associations to hire a pro to treat a large area.
2) Random branches in odd positions? Might be emerald ash borer
“With deciduous trees, epicormic shoots—rapid growth of a branch—show there’s something wrong with the tree’s internal vascular system,” says Dutkiewicz. This random branch growth and bark cracks can be signs of the emerald ash borer, which kills ash trees from Manitoba eastward.
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“Spring is a great time to inspect for emerald ash borer; woodpeckers feed on their larvae throughout the winter so you can see the holes they make,” he adds. Pros can inject trees with insecticides to treat the infestation in its early stages.
3) Tiny holes and sawdust? Look for engraver beetles
Like sawyer longhorn beetles, engraver beetles attack stressed trees, creating tiny holes in the bark. But trees can rebound from this infestation, says Dutkiewicz, describing an infested pine he looked at that had piles of sawdust at the base. Once the owner removed the firewood he was stacking against the tree, it eventually recovered (see No. 5 below).
4) Yellow slime? Could be oak wilt
Watch for evidence of tree pathogens or fungal infections. “Telltale signs on a tree’s surface include strange growths or slimy yellow secretions, or discoloured foliage,” says Mike McCutcheon, the sales and operations manager at Pevach, an arborist company in Bonnyville, Alta.
Early detection is important, because oak wilt can kill a red oak within six weeks, says Dutkiewicz. “I find the fungus smells like Juicy Fruit gum, malt wine, or stale beer. It attracts tiny sap beetles that feed on the fungal fruit.” Avoid pruning oaks from April to October, when the beetles are active—and don’t transport firewood.
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Maintaining tree health protects against pests and disease. McCutcheon suggests consulting a tree health technician who can set up a deep root fertilizing and irrigation program, especially for high-value or heritage trees. Dutkiewicz recommends having an arborist inspect your trees about once a decade. “People plan their gardens, but not their forests,” he says. “Think of your treescape area as a larger garden—one that will last a lot longer.”
5) Are your trees stressed?
Stacking firewood or equipment against trees is a stressor, says McCutcheon. And securing floating docks to trees with rope or leaning heavy items against them can compromise the tree’s circulation, leading to poor health. (I wished I’d known this before—my husband and I had been tying up our Laser sailboat against our fir trees for years, which likely stressed them out.)
This article was originally published in the May 2024 issue of Cottage Life
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