Resin canals measure resistance to beetle attacks

Doctoral student's work could help forest managers remove susceptible trees, enhancing beetle-resistance of the stand

By Jeff Mitton

A female mountain pine beetle flew through the forest and landed on a lodgepole pine. She walked around on the bark to assess the tree, surely sniffing, perhaps nibbling bark.

But she flew off.

She landed on another tree, and after assessing, bored into the bark to reach the phloem, severing vertical resin canals. Resin poured into the borehole and spilled out of the tree, forming a pitch tube that signaled the beginning of a life-or-death struggle.

If the female could attract a sufficient number of females to join the attack to reduce resin pressure to zero and inoculate blue stain fungi, the tree would die. But if the beetles could not stem the flow of resin, then beetles would die and the tree would survive.

Mountain pine beetles are native to western North America and have been utilizing pines such as ponderosa, lodgepole and limber pine as host trees for many thousands of years. The widespread western conifers have very high levels of genetic variation, and given that western species have been attacked for ages, it is reasonable to expect that they maintain genetic variation for resistance to bark beetle attacks.

Resin is a pine's sole defense against mountain pine beetles, and three decades ago a study clearly showed that the greater the volume and pressure of resin, the better the defense. The perimeter of a beetle epidemic was advancing through an Arizona forest, and the year before it reached a stand of ponderosa pines, resin pressure was measured in the trees.

After the epidemic had swept through, the resin pressure was compared between unattacked and killed trees. In two successive years, resin pressure was about 10 times higher in the trees that escaped attack in comparison to those that died.

The difficulty with estimating resistance with resin pressure is that it fluctuates dramatically between wet and dry periods.

In 2010, forest biologists Jeff Kane and Tom Kolb published a paper that compared the size and number of vertical resin canals in ponderosa pines that died during a mountain pine beetle epidemic with trees that were not attacked. In the most recent five years, trees that escaped attack produced 55 percent more resin canals than those that died. Using measures of growth rate and resin canals, they were able to correctly classify 70 to 90 percent of the trees into their correct group: not attacked, or killed.

The advantage of using resin canals to identify resistant and susceptible trees is that they are retained as a permanent record in the tree. While a tree may proliferate resin canals during a beetle mass attack, the trauma-induced resin canals only appear in one portion of the current growth ring and cannot influence the counts from previous years.

University of Colorado Ph.D. student Scott Ferrenberg conducted a study that identified a general role for resin canals in resistance. His study included both lodgepole and limber pines that had been mass attacked by mountain pine beetles at CU's Mountain Research Station. Many months after the attack, he recorded which of the trees had survived (resistant) and which had died (susceptible). In both species, the number of resin canals was significantly higher in resistant trees. Using only resin counts from the last five years, he was able to correctly identify 72 percent of lodgepole and 81 percent of limber pines as susceptible or resistant.

By comparing killed trees with unattacked trees, Kane and Kolb showed that beetles prefer to attack trees with fewer resin canals. By starting with mass-attacked trees and comparing surviving trees with trees that died, Ferrenberg was able to show that trees with more resin canals are more likely to survive a mass attack.

Ferrenberg has used his results to design a simple, quick and economical method for thinning a stand by removing susceptible trees, enhancing the resistance of the stand.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

December 2013