Western Pine Beetle (Dendroctonus brevicomis)
The western pine beetle (WPB) is a bark beetle which attacks and kills 6″ diameter and larger ponderosa pine in Nevada. In California, Coulter pine is also attacked by this aggressive beetle. Typically the beetle attacks trees weakened by drought, old age, overstocking, wind damage, fire or disease. Construction related damage around roads and home sites also predisposes ponderosa pines to attacks by the WPB.
In this role, the beetle performs the important ecological function of recycling the nutrients contained in trees, and creating conditions which favor the germination and growth of new trees. However, when there is a widespread decline in tree vigor, WPBs available food source, weak trees, can cause large increases in the beetle population (epidemics) which can mass attack, overwhelming even healthy trees.
Adult beetles emerge from trees attacked the previous year as the spring temperatures warm, normally in late spring. Females initiate attacks on new host trees about half way up the stem by boring through the outer bark, into the phloem. If the attack is successful, the female emits a powerful chemical scent which attracts males and other females to the tree (mass attack). These beetles attack the tree above and below the initial attack.
Once in the phloem, the female mates with a male, and bores a tunnel with niches along the sides in which she lays her eggs (egg gallery). The eggs hatch in 1 to 2 weeks into small white grubs or larvae. The larvae then feed on the phloem until moving into the middle bark where they turn into pupae, and finally adult beetles. The new adults emerge from the bark ready to begin the cycle again. The development from egg to adult takes from 2 months in warm weather to 10 months in cool weather.
The beetles transport a fungus on their bodies into the host tree as they bore into its bark. The fungus grows in the trees water conducting wood, clogging the system. The combination of this fungus and the beetle feeding on the inner bark kill the tree.
Evidence of Attack
From a distance the first sign of a successfully attacked tree is a fading of the trees foliage. Normally green foliage turns yellow then red-brown. The tree is dead by this time. Closer inspection will often reveal globules of pitch (pitch tubes) along the trunk where the beetles attacked. The pitch tubes are formed as the tree tries to expel the attacking beetle. An unsuccessful attack will be evident by the white creamy color of the pitch. Successful attacks are marked by smaller tubes which are reddish in color from sawdust mixed with the pitch. Very weak trees may not produce any pitch tubes. Boring dust is often evident in bark crevices and at the tree base. Peeling bark of infested trees will reveal the winding maze of egg galleries characteristic of WPB. Woodpeckers feeding on the larvae will often peck through the bark, leaving the inner bark exposed and piles of bark chips accumulated at the base of the tree.
The best method of controlling WPB problems is prevention. Keep trees growing vigorously by maintaining low stocking through thinning, and selectively removing trees which are unhealthy. When thinning, tree spacing should follow the diameter plus ½ guideline. That is, the distance, in feet, between adjacent trees should be equal to 1½ times the average tree trunk diameter in inches. Retain upright, undamaged, full crowned trees. Look for signs of poor vigor such as dead tops, branches and twigs. Short, sparse and poorly colored foliage are also indications of poor health. Diseased and lightning struck trees, and those heavily infected with dwarf mistletoe (see Forest Health Notes No. 5) are also good candidates for removal. Avoid thinning and pruning during active flight periods as fresh wounds may attract beetles.
In urban settings, watering trees during drought periods can help keep them healthy and vigorous. Avoid injuring trees during construction and day to day activities.
High value trees in the home landscape or elsewhere can be sprayed with an insecticide which the adult beetles ingest while boring into the tree. The insecticide must be applied before adult beetles attack the tree, by early April. Trees killed by beetles the previous year may still have green foliage the following spring when spraying is done. Look for pitch tubes, boring dust and other signs infestation before spraying a green tree. Spraying dead trees is expensive and of no value. Contact the Nevada Division of Agriculture for a list of pesticides registered for use against bark beetles on pine. read, understand, and follow instructions on pesticide labels.
Infested trees should be cut and removed by early April, before the adult beetles emerge. If infested trees are kept on site after cutting, they should be burned, chipped or sprayed with insecticide as directed above. If kept for firewood, the tree(s) should be cut up, piled in direct sunlight and covered with thick (= 3 mil), clear plastic, making sure all edges are buried. The heat buildup under the plastic will bake the beetles. Keep covered for three months if weather is sunny, longer during cloudy periods. Peeling the bark off firewood is another effective, although difficult, control method.
Authored by: John Christopherson- Resource Management Officer- Nevada Division of Forestry
Forest Health Specialist
885 Eastlake Blvd.
Carson City, Nevada 89704
Phone: 775-849-2500 ext 241