Mineral Ridge Scenic Area and Recreation Trail

Station 8: Witches Broom & Bark Beetles 

Dwarf MistletoeParasitic insects and diseases can infect trees in the forest. A parasite is a living organism that lives on or in another organism called a ”host.” Parasites derive their nourishment from their hosts and return nothing. Insects and disease can kill a tree or weaken it and slow its growth. Weakened trees are less able to compete with healthy trees and can be crowded out. They are also more susceptible to infestation by other parasites. Infestations can thin a timberstand or eliminate the climax vegetation and the plant succession cycle is repeated. In this way, vegetation diversity is maintained and we will continue to see seral vegetation. Wildfire also causes vegetation to diversify.
As you stand at this station, take a look around. Can you find an example of a parasite infecting a tree? The large Douglas-fir below the trail and the other trees in this area have deformities that appear as multi-branched broom-like growths on otherwise normal tree branches. The deformity is called witches broom, and is caused by dwarf mistletoe, a parasitic plant. The branched formation is caused by the tree’s own disease-combating defenses. The tree tries to cure itself by rushing nutrients to the infected area, much as white blood cells are rushed to human wounds. This rush of nutrients causes the sporadic growth pattern. Dwarf mistletoe is a serious parasite in many western conifers and can cause great economic losses. A parasitic insect found in the area is the bark beetle. 

Since the start of the trail, you have probably noticed many dead ponderosa pine trees. They were killed by a tiny beetle that enters the tree and feeds just under the bark. The feeding action of the beetle eventually will girdle the tree, cutting off the flow of nutrients, Bug-killed treesimmediately killing it. Bark beetles are a common and serious pest. They usually attack weakened or dying trees. Many of the dead pine trees you see on Mineral Ridge were killed in the 1980s after several years of warm, dry weather made the trees vulnerable to beetle attack. As you proceed up the trail, see how many ponderosa pine trees you can find that may have been killed by the bark beetle.