From your very accurate description, it's clear the culprit is the white pine weevil, Pissodes strobi. It's true that the weevil's attack is rarely lethal. However, killing off the tree's terminal shoot not only causes a check in upward growth, it also stimulates lateral branching (in much the same way pruning does). Repeated yearly attacks can cause white pines - which nature intended to grow tall and slender - to become stunted and bushy, a phenomenon foresters call "cabbage form."

The good news, says forest pathologist John McLaughlin of the Ontario Forest Research Institute in Sault Ste. Marie, Ont., is that most trees will eventually grow past the problem, because the weevil doesn't like to go much higher than six to seven metres. Given time, the crook in the trunk you've noticed should virtually disappear as the tree's girth increases. In fact, one study of the growth rings of some very large, old white pines showed that most of them had been infested by white pine weevils when young. In the meantime, you can help young trees that have been attacked return to the straight and narrow, according to Ministry of Natural Resources forest entomologist Taylor Scarr. Adult weevils overwinter in the leaf litter beneath host trees and emerge in spring to feed on the terminal shoots or leaders, depositing their eggs in small punctures there. You may notice these holes - or later, in July, the conspicuous drooping of the leader as the hatched larvae feed on it. When you do see damage, prune out the diseased leader. You'll need to take out the current summer's growth as well as last year's, i.e. down to the whorl of lateral branches that is second from the top. Then, prune out all of these lateral branches except one - the healthiest and most nearly vertical one - which will become your new leader. To help train it to grow straight up, you could then "splint" it to the main trunk with a bamboo plant stake, using soft plant ties that won't do any damage. Burn or bury the prunings, so weevils can't reinfect the tree.

Because white pine weevils like warmth, sunshine, and fat, succulent terminal shoots, foresters usually practise weevil prevention, McLaughlin explains, by planting new seedlings where they'll get some shade - alongside hardwoods, or in thick stands so their terminal shoots stay cool and rather thin. Any trees that have been damaged can then be thinned out, leaving the healthy ones to grow on.