Galleries under the bark increase in width as bark beetle larvae mature and eat their way toward freedom and flight.Bark beetles kill quickly by introducing a blue-stain fungus that clogs the tree’s plumbing, unlike other insects that generally only weaken trees by chewing on the leaves or sucking sap. As the tree dies, the female beetles lay eggs under the bark in the sugar-rich tissue. When the larvae hatch, they eat their way through a thick layer of nutritious (phloem) cells. Larvae create an expanding network of tunnels as they grow, ending in a hole from which mature beetles fly to breed and attack new trees.
When a tree’s sapwood is completely plugged with blue-stain fungus, it’s functionally dead, although the leaves remain green until the following summer.What could we do to improve the health of huge numbers of trees quickly? Nothing subtle would do. First, we added a half-ton of nitrogen fertilizer per acre to some areas. Elsewhere, we shoveled on sawdust, topped with brown sugar, to caused soil microbes to go into a feeding frenzy that left few nutrients available to the trees. To create the best possible conditions to inspire trees to regain some of their youthful vigor, we felled 80 percent of their neighbors and added nitrogen to the soil. To account for climatic variation from one year to the next, we left some of the forest untreated. Our plots were laid out like a checkerboard across 22 acres. Each treatment was repeated four times, and buffer strips isolated the treatments from one another. We tried to attract beetles to all of our plots by distributing vials filled with a volatile mixture of “essences of pine trees” that mimicked the scent (pheromone) produced by female beetles. We stored the brew in a glass vial with a pin-hole in the screw-top lid to allow the vapor to escape as the temperature rose. In late September, with the first flakes of snow falling, it was time to record tree mortality based on tell-tale sawdust appearing on the bark. The initial results were scary: A higher percentage of trees on the fertilized plots succumbed to beetle attack than on the plots left untreated or sugared. Had we made matters worse? In time, a familiar pattern emerged, but only a few trees responded quickly enough initially to put up much of a fight. By the third year, a different pattern emerged: Many trees with less competition from their neighbors, even on the plots where only the beetles thinned the forest, resisted attack. Only trees on the most nitrogen-impoverished plots, where sugar and sawdust was added, failed to reclaim some of their youthful vigor. The healthier a tree, the more beetles it took to kill it; and the healthiest individuals were rarely attacked by any beetles.