Bark Beetle Information
Bark Beetles in Pines
By Dario Davidson, Registered Professional Forester #2314
Flora & Forest
With many consecutive years of below average rainfall, many Registered Professional Foresters are watching for signs of bark beetle infestations in their trees. Do you know what to look for? Have you properly thinned your forest to protect them from moisture stress and bark beetle attacks? You should keep a watchful eye and consider thinning trees for improved vigor and protection against drought and insects.
Ecological Role –Bark beetles play an important role in maintaining healthy forests. Bark beetles tend to attack trees that are damaged by fire, storms or changes in their environment caused by humans or natural events. They help to thin overstocked forests and they also provide a food source for many predators, from other beetles to woodpeckers. When bark beetle populations are not excessively large they normally go unnoticed by humans. They become more important during large population outbreaks. Trees killed by bark beetles, if not removed, can become a fire hazard or danger to falling on structures, vehicles or people.
When pines are stressed bark beetles can successfully attack trees and build up their populations to very high levels, spreading from one tree to another over successive generations. Adult bark beetles bore through the bark of pines and into the cambium layer where they mate and lay eggs. The eggs hatch into larvae which chew galleries, or mines, through the cambium and inner bark. The larvae pupate in the outer bark, become adult beetles, and chew their way out to fly off to another tree and begin a new generation - inCalifornia, up to four generations per year.
Detection – Normally, bark beetles are detected and identified by the symptoms of their host trees. The beetles themselves are black to red-brown and about the ¼ to 3/8 inch long. The signs that indicate bark beetle attacks on pines include; frass, pitch tubes, changes in tree foliage color and unusually high woodpecker activity.
Frass is fine sawdust and beetle droppings lodged in bark crevices or caught in spider webbing. This is the material that the beetles eject from the tunnel they are building to reach the tree’s cambium layer.
Pitch tubes are droplets or gobs of pitch exuded by the tree in an attempt to expel the beetles. A healthy tree can expel the attackers with gobs of pitch about an inch in diameter. The pitch may be whitish yellow or light brown colored and on close inspection may contain small black beetles and frass.
Foliage color of the host tree will change once a tree has been attacked and severely weakened. The color will change from dark green to a pale green, to yellow brown to reddish brown as the tree dies. When the foliage color changes from the healthy dark green stage to light green, the tree is already nearly dead, yet the beetle pupae may still reside in the tree.
Woodpecker activity indicates an increase in insect presence in the tree. If the surface of the bark changes to a fresh, reddish layer of a few square feet or more, the woodpeckers are likely digging into the bark in search of bark beetles. At this stage the beetle attack has been successful and the tree will likely die.
The most important thing you can do to avoid bark beetle attack is to maintain the health of your pines. Avoid damage and disturbance from trenching, grading, irrigation, compaction, adding fill over the root zone, burning hot fires, changing drainage patterns or toxic wastes spilled onto the soil, especially around older, larger pines.
Groups of younger trees growing in a crowded condition are susceptible to bark beetle attack due to the stresses of competition. Thin by removing and disposing of the weak, damaged or diseased trees. Any material generated from thinning or pruning pines should be burned, chipped or hauled away to avoid attracting bark beetles.
Pines in California have evolved over thousands of years to cope with the summer drought normally experienced here. Overcrowding of understory vegetation (i.e. Manzanita, Ceanothus, etc.) can weaken pines, making them more susceptible to bark beetle activity. Clear most vegetation from beneath pines but maintain a natural pine needle mulch to protect the roots from excessive heat and drying.
Native pines, as well as oaks, can be harmed from abundant irrigation. Excess water during the normal dry season, promotes root decay, weakening the tree. Do not use overhead irrigation beneath the drip line of native pines or oaks.
Red Turpentine Beetles are reddish brown and slightly larger than a grain of rice. They attack pines near the base of the tree, as well as stumps and exposed main roots. Look for light pink to reddish gobs of pitch about 2 inches long near ground level and up to about 6 feet. Also look for piles of granular material on the ground. These beetles are common on pines that have had ground disturbances near them.
Pine Engraver Beetles attack thin-barked areas of trees as well as branches. They do not bore through the thick bark of the middle and lower portion of large pines, as the western pine beetle does, but choose the tops of mature trees or small diameter trees, where the bark is thinner. The pine engraver can also infest tops and branches of trees that are cut down and left without proper disposal. Only fine boring dust will be found in the bark crevices. The beetles can introduce a fungus into the tree, which clogs the water transport system, further weakening the tree. The first sign of the attack is a change in the foliage color at the top of the tree, from dark green to yellow-green, then to yellow. By the time the color of the tree fades the beetles have emerged as adults and have flown to another tree – perhaps miles away.
Pine engraver beetles rarely kill a tree outright. Oftentimes they weaken a tree enough to allow western pine beetles to attack. The tops of trees killed by pine engraver beetles can become a hazard when they break and fall.
Western Pine Beetles are small, black beetles that are the most destructive bark beetles on Ponderosa pines in California. These beetles bore into the middle portion of the tree trunk to mate and lay eggs. The entire life cycle takes about 2 months, and 3-4 generations may be produced in a season. The first sign of attack is pitch tubes – small white to yellow gobs of pitch flowing from the bark. Pitch tubes are the trees defense, and abundant pitch tube production may indicate the tree has repelled the beetle attack.
Additionally, to further weaken the host tree, allowing more beetles to successfully attack, the beetles carry a fungus into the cambium of the tree. As this fungus grows in the moist environment inside the tree, it plugs the circulatory system that allows the tree to take up water and nutrients. The blockage of the water transport within the tree hastens its decline. As the tree weakens the foliage color fades, first to a lighter green, then to a straw color. At this point the successful bark beetle attack is obvious and the tree is already dead.