Ubiquitous Pine Weevil and How to Live with It
By Joe Rankin
The eastern white pine is truly one of the giants of our forest. It can, theoretically, tower 200 feet, lording it over the other softwoods and hardwoods. Except . . . when it doesn’t.
Many white pines never get a chance to fulfill their genetic potential. and in most cases it’s not because they’re cut down, or blown down, or broken down by snow and ice, but because they fall victim to a tiny insect – the white pine weevil, a bug that could be a candidate for the 10 Most Unwanted List.
In my woods I’ve got a few fairly nice specimens. I have a lot more that look like a psycho-pruner has been torturing them for decades. They’re a mass of twisted stems. It would be impossible to imagine them gracing a ship of the line. Maybe brooms for a giant witch. These are “pasture pines” or “cabbage pines.” Unlike many pests in the forests of eastern North America, the white pine weevil isn’t an exotic that hitchhiked from Europe or Asia. It’s a native insect that has preyed on white pine for a long, long time. But it’s considered by many scientists as the most destructive pine pest out there.
“Pines have lots of pests, but when you think pine you think blister rust and weevil,” said Maine State Entomologist Dave Struble. “White pine weevil is pervasive and widespread. It’s everywhere at some level in our part of the pine range. I don’t know anywhere in the state that we wouldn’t find weevil if we looked for it, though some places are worse than others.” And that’s not good news in a state where pine is worth tens of millions of dollars annually.
Perhaps the best you can say about it is the weevil doesn’t kill the pine. But it can effectively ruin them as a timber tree.
The white pine weevil – Pissodes strobi – is a tiny rusty-brown, nondescript beetle. The weevils overwinter in leaf litter, emerging in the warming days of spring, the females searching for a place to lay eggs. They prefer a pine leader that’s thick and warmed by the sun. The mother weevil scissors holes in the bark, then lays eggs. A single weevil can lay hundreds f eggs in one terminal shoot. When the larvae hatch, in about a week, they feed on the sun-warmed bark, girdling the leader and killing it.
The tree, which normally produces hormones that keep the lower branches from growing upwards, copes by turning another branch or branches into a leader. But the damage is done, leaving the tree with a crook in it. Some trees, repeatedly weeviled, take on the appearance of a stubby candelabra.
White pines aren’t the only species vulnerable to this pest. Weevils feed on 20 species of conifers, including spruce; jack and scotch pine; red pine and Austrian pines. In the west, the weevil goes after Engelmann spruce. Colorado blue spruces planted as ornamentals often fall victim. I learned this the hard way.
You can’t wipe out the weevil. And even trying to limit the damage can be challenging. But there are some things you can do.
Key to the female weevil’s choice of an egg-laying site is sunlight, particularly sunlight that warms a pine’s leader in early spring. “It probably has to do with the pheromones the tree is giving off,” said Struble. “The temperature regime is different.”
Planting white pines in an open field is like laying out an all-you-can-eat buffet for these bugs. The pines on my back 20 acres grew up in an abandoned field in the middle of the last century, and it’s not surprising many were weeviled repeatedly. You’ll notice that pines seeded along roads are often uniformly weeviled. If young pines get a start in partial shade, growing among aspens or other hardwoods, they’re more likely to be undamaged – at least for a few years.
Daren Turner of Kennebec Valley Forestry in Norridgewock says there was a lot of weevil damage in the state in the late 1980s and ’90s, when landowners were trying to establish white pine plantations. While tree planting has dropped off as cost-share money dried up, the weevil is “definitely a concern for landowners trying to manage their woodlots,” Turner said. One landowner he knows has done a great job over the years seeding skid trails and log yards. He got a lot of white pine regeneration, but also a lot of weevil damage. The openings were apparently enough, said Turner.
For the woodlot owner trying to grow pines, or the homeowner wanting to protect ornamentals, insecticides are an option. Two caveats: Not all insecticides are appropriate, and they have to be applied in late April or early May when the weevils are searching for a larval host. Timing can differ, depending on the weather in any given year. The good news is you don’t have to spray the entire tree, just the leader. If you do notice a dead leader, cut it off and burn it. Don’t just drop it on the ground, or the pupating weevils inside will provide another crop of bugs next year.
“Knocking down weevils is really about leader protection,” said Struble. “If you can get two to three feet of growth a year, and treat it for eight years, you can get a 20-foot log.” Of course, spraying the leader on a 20-foot tree can be a challenge. And, depending on how many trees you’re spraying, it could be a chore. But as Struble points out, you’re not raising hundreds of pines per acre. Pick your best 80 to 100 per acre to spray and concentrate there. Another thing to remember: “We’re not trying to raise mast trees anymore,” he said. A pine with two 16-foot logs, even with one 20-foot log, is considered a good pine.
If you already have a high percentage of weeviled pines, there aren’t a lot of management options, Turner said. Loggers hate weeviled pine. Felling one with multiple tops is a pain, and most landowners hate the mess. If you have a timber harvest planned anyway, the best option might be to have cabbage pines chipped for biomass, he said. Another option is to girdle the weeviled trees and let them slowly decay. Woodpeckers will thrive.
Science may come up with a weevil solution – but don’t hold your breath. “I can’t imagine us having a way to knock weevil down so it’s not a problem,” Struble said. “And if there is it would be environmentally costly and probably socially unacceptable. So we’re going to have to manage around weevil.”
The key to a decent crop of pines – and retaining your sanity – is managing expectations as much as your woodlot. “We have to live within the realities and limitations” of the natural world, said Struble. “And not let the good be perfection.”