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A Bite Bigger than the Bark

by 28 April 21 2017 , 05:18 pm

Bark beetles are small insects, but don’t let their size fool you. The strong jaws of these beetles chew and create a buckshot pattern of holes on the bark surface of infested branches or trunks of trees, causing widespread mortality in forest and forested communities. About the size of a grain of rice, most species have cylindrical, hard-bodies that dark red, brown, or black.

Native bark beetles cause high levels of tree mortality in California and elsewhere. The extent of mortality is primarily influenced by forest stand and drought conditions. Under a period of inadequate precipitation, a dramatic rise of dead trees is likely. Dense groups of trees are particularly susceptible to bark beetle attacks due to stress caused by limited resources. The main species include: mountain pine beetle, fir engraver beetle, western pine beetle, Jeffrey pine beetle and ips (also called pine engraver) beetles.


Under outbreak conditions, the sheer number of beetles can overwhelm the tree's defenses with resulting impacts on the lumber industry, water quality, fish and wildlife, and property values.

Bark beetle infestation is easily spotted. Bark beetle activity starts on the bark surface and extends to the inner bark of twigs, branches or trunks. A living tree can be killed by the feeding activity of adults and larvae, which damages the phloem in such a way that the tree is girdled. The beetles also introduce several species of fungi which invade and infect the wood. The first sign of a bark beetle infestation in a tree may be the discoloration of the needles that occurs when fungi block the xylem and prevent water transport to the foliage. 

The following are additional signs that a tree may be infested by bark beetles:

In the Bark

  • Trees react by releasing pitch as their natural defense against bark beetle attack. A white pitch tube means the beetle was successfully repelled by the tree. If the pitch tube is reddish brown, most likely the beetle was successful in attacking the tree.
  • The pitch is accompanied by a sawdust-like substance, called frass, created by bark beetles and their larvae as they bore through the bark.
  • Frass has accumulated in tree crevices and may have fallen to the ground, resembling very fine, reddish-brown coffee ground material at the base of the tree.
  • Bark flaking or holes in the bark caused by woodpeckers foraging for bark beetles are also a good indicator that bark beetles are present.

Removing bark sections will reveal holes created by bark beetles, as well as dead or degraded inner bark.

On Leaves or Needles

  • The needles on conifer trees, like pines, begin to turn a reddish-brown color. Often the change begins at the top of the trees and moves down.
  • Some trees may slowly fade in color from green to brown.
  • Some trees may die within a few weeks of infestation, but may not show yellow-green, fading or red foliage for several months. Other types of trees may survive years before dying; by the time a tree appears dead, it cannot be saved.

Bark beetles reproduce in the inner bark of trees. Many species attack and kill live trees. Most, though, live in dead, weakened, or dying specimens. Bark beetles play an important role in forest ecology and can have significant economic impact. In undisturbed forests, bark beetles serve the purpose of hastening the recycling and decomposition of dead and dying wood and renewing the forest. A few species are aggressive and can develop large populations that invade and kill healthy trees and are therefore known as pests.

The staff at Arborist Now is experienced in finding and treating problems created by pests and diseases that can decimate the trees on your property. Our tree care services in San Francisco and surrounding areas are extensive and affordable. Contact us for a consultation. 

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