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Master Gardener Journal  


Frequently Asked Questions about Bark Beetles



by Jeff Schalau,
Assistant Agent, Agriculture and Natural Resources


HOW DO PINE BARK BEETLES KILL TREES?
Pine bark beetles (Ips sp. and Dendroctonus sp.) feed primarily on the inner bark (phloem tissue). This has the same effect as girdling (peeling off the bark to exposed wood) of the tree. Damage caused by their feeding acts as an internal tourniquet cutting off the flow of nutrients from the leaves to the other parts of the tree. As the damage progresses, sugars and other complex compounds cannot be translocated downward from the leaves to non-photosynthetic areas of the tree. The beetle can also introduce a blue stain fungus, which grows into the wood (xylem). This fungus prevents water from being transported upward to the leaves. Both of these factors contribute to the decline and death of colonized trees.

WHAT ARE THE EARLY SIGNS THAT BARK BEETLES HAVE COLONIZED A TREE?
Needle discoloration is the primary early sign of colonization by bark beetles. Needles fade from dark green to pale green to straw yellow to a rusty red color. The progression from green to red can occur as quickly as a couple of months or as long as several months. Other signs are pitch tubes, boring dust, and galleries (tunnels under the bark). Pitch (resin) is the tree's only natural defense against bark beetles. Beetles colonizing relatively healthy trees will usually create pitch tubes where the beetle entered the tree. If the tube is connected with a tunnel that continues into the bark, then that beetle successfully entered the tree. Fine boring dust is sometimes visible and caused by bark beetles chewing the bark to enter the tree. During initial colonization, the boring dust is bright red. To inspect for galleries, you can remove a portion of the bark with an axe. This should be done only after the tree appears to be dead. Galleries should be visible under the bark and may contain larvae (grubs), pupae (cocoons), and/or adult beetles on recently killed trees. One or all of these signs or symptoms may be present.

HOW DO BARK BEETLES SELECT A SUSCEPTIBLE HOST TREE?
Stressed pine trees emit volatile compounds (turpenes). Bark beetles have evolved to detect these compounds and use them to identify suitable host trees. Many insects communicate with other insects by emitting pheromones (chemical compounds that trigger a specific behavior). Once a bark beetle has located and colonized a susceptible host tree, it emits an aggregation pheromone that attracts other beetles. After enough bark beetles are attracted to that tree, beetles emit an anti-aggregation pheromone signaling them to locate another host tree. In this way, it is thought that bark beetles partition available food among the population.

WHAT ARE THE NATURAL ENEMIES OF BARK BEETLES?
Bark beetles have an array of natural enemies. Woodpeckers and other birds may eat some bark beetles. Some insects are known to help control bark beetle populations under endemic (non-outbreak) population conditions. Predaceous beetles such as the blackbellied clerid (Enoclerus lecontei) and a trogositid beetle (Temnochila chlorodia), a predaceous fly (Medetera aldrichii), and parasitic wasps are natural enemies of bark beetles, but rarely control their populations. These insects are known to have some effect on bark beetle populations, but most experts feel that parasites and predators of bark beetles are a minor factor in controlling bark beetles under the current pandemic outbreak. Northern Arizona University is initiating a study to try to understand the relationship between predators (primarily birds) and bark beetles.

WHAT ARE SOME BARK BEETLE-RESISTANT EVERGREEN TREES THAT CAN BE USED IN NORTHERN ARIZONA LANDSCAPES?
Some suitable evergreen trees are Deodar cedar (Cedrus deodora), Atlas cedar (Cedrus atlantica), and Lebanese cedar (Cedrus libani). These have few, if any, pests and are relatively fast growing. Evergreens have the advantages of providing year-round privacy screening and pine-like foliage. They do require infrequent irrigation to keep them healthy. These species are not as fire retardant as deciduous trees due to their resin content. For this reason, evergreens are not recommended for use next to structures where they may increase risk of property loss in case of forest fire.

WHAT WILL BE THE LONG-TERM IMPACTS OF THE CURRENT BARK BEETLE OUTBREAK?
The disturbance caused by bark beetle mortality will undoubtedly change vegetative characteristics. Many of these changes could be perceived as beneficial to the impacted ecosystems. A significant reduction in woody species will likely be accompanied by an increase in native herbaceous species such as grasses and forbs. These species have reduced water consumption and are desirable forage for wildlife and domestic livestock. In addition, grasses have fibrous root systems that stabilize soil and aid in soil development. The reduced water consumption could also result in increased water yields from impacted watersheds. This increase in water yield could potentially help recharge local and regional aquifers. Other potential ecosystem benefits will certainly be recognized over time.

The risk of catastrophic wildfire is increased in areas where bark beetle-killed trees are not removed. Other long-term impacts are largely unknown. However, potential negative impacts could be: loss of soil due to accelerated erosion; increases of invasive plant species in response to disturbance; decreases in real estate value on properties where tree losses were very high.



Maricopa County Master Gardener Volunteer Information
Last Updated April 29, 2003
Author: Lucy K. Bradley, Extension Agent Urban Horticulture, University of Arizona Cooperative Extension, Maricopa County
© 1997 The University of Arizona, College of Agriculture and Life Sciences, Cooperative Extension in Maricopa County
Comments to Maricopa-hort@ag.arizona.edu 4341 E. Broadway Road, Phoenix, AZ 85040,
Voice: (602) 470-8086 ext. 301, Fax (602) 470-8092