With the holiday season fast approaching, many look forward to the age old tradition of bringing an evergreen tree into the home or decorating a mantle with fragrant sprigs from pines, spruces, or firs. For many years a large eastern white pine in my backyard parted with a few boughs each year for just such a purpose. Last summer my noble pine began to look quite peaked. Over a few short weeks, needles usually deep green in color faded to pale yellow. While I knew this was the beginning of the end for my old friend, I was also intrigued to unveil the villain responsible for my pine’s demise. A closer inspection provided some clues. In addition to washed-out needles, the trunk and branches were riddled with hundreds of tiny hole some of which displayed rings of red sawdust-like excrement called frass and others oozed trails of chalk-colored resin.
Resin is one of the defenses used by pines and their kin to repel attack by nefarious insects called borers whose primary mission is to penetrate the tree and prepare galleries for spawn that consume nutritious tissues beneath the bark. By the size of the holes and their almost perfectly round shape, I knew that a bark beetle attack was well underway. Bark beetles belong to a large family of beetles with common names such as weevils, turpentine beetles, shot-hole borers, and ambrosia beetles. Once an attack is evidenced by pitch leaking from galleries or reddish sawdust accumulating beneath the tree, it is usually too late for therapy. To save my hapless pine the indignity of a slow and lingering demise and to prevent it from crashing into my electrical utility cables, I hired a commercial arborist to remove the victim. After collecting several large branches from the carcass of my woody friend and placing them in a large plastic bag for a few weeks, I was rewarded by capturing several fivespined engraver beetles as they emerged from bird-shot sized holes in the bark. As the name implies, the fivespined engraver bears five spines somewhere on its tiny body. The spines actually occur in two lines of five that ring a small depression on the rear end of the beetle. Other close relatives of this rascal are named for the number of spines found in rows lining their rear ends as well. While the names sixspined engraver and fourspined engraver are not particularly colorful, they do help obsessive entomologists identify and keep track of these miniscule beasts.
In the circle of life, bark beetles are opportunistic members of Mother Nature’s clean-up crew. Under favorable conditions, trees like my pine produce defensive compounds that help ward off attack by pests. Trees stressed by adverse environmental conditions such as prolonged drought or lightening strikes, decline in their ability to defend themselves from borers. Their natural defenses such as a heavy flow of resin fail to keep invaders out. Compromised defenses may allow small numbers of attackers to colonize a tree. In the case of the Ips engraver, the male beetle usually is the first to successfully breach the bark of the tree. If the tree is suitable, the male constructs a nuptial chamber and releases a chemical called a sex pheromone to attract members of the opposite sex, not just one, but several. Once the little polygamist has assembled and serviced his harem, the females construct egg galleries and deposit eggs. Eggs hatch and small legless grubs tunnel through the vital tissues of the tree further sapping its defenses. Beautiful galleries left by beetles as they feed beneath the bark give them the name engravers. At this point in time, the tree is highly vulnerable to attack by other species of bark beetles, several of which complete multiple generations each year. This massive attack causes a rapid spiral of decline leading to death of the tree. An unfortunate set of circumstances may have predisposed my pine to the bark beetle attack. A few years ago, a utility company performed a hasty line clearance that left several large branches of my pine ragged and slowly dying. A few years of drought compounded this stress and opened the door for the vanguard of attacking beetles. Prolonged periods of drought have been linked to outbreaks of bark beetles that killed millions of trees in several southern states. Elevated temperatures associated with climate change have opened vast new forests to bark beetles formerly excluded by cold hibernal temperatures in western North America. Resultant outbreaks of mountain pine beetles have been responsible for the death of millions of pine trees in the United States and Canada in recent years.
Excellent references used in preparing this episode include “Population aggregating pheromone in the bark beetle, Ips grandicollis” by J.P. Vité and J.A.A. Renwick, “Ghost Forests, Global Warming, and the Mountain Pine Beetle (Coleoptera: Scolytidae)” by Jesse A. Logan and James A. Powell, and “Insects that Feed on Trees and Shrubs” by Warren Johnson and Howard Lyon. To learn more about Ips engravers and other bark beetles, please visit the following web sites.