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Master Gardener Journal  


S O U T H W E S T   R E P R I N T S



Pine Bark Beetle Outbreak in Arizona

by Tom DeGomez,
Forest Health Specialist, with the University of Arizona Cooperative Extension Forest Health Working Group and the Arizona Bark Beetle Task Force which includes professionals from University of Arizona, Northern Arizona University, the United States Forest Service, and Arizona Public Service


Arizona's ponderosa pine and pi–on forests have sustained significant impact from the bark beetle outbreak of 2002. Conservative estimates, based upon U. S. Forest Service aerial surveys of federal lands, place the number of dead ponderosa pine statewide at 2 million on 503,000 acres. This estimate is admittedly low, because the surveys were done between late July and October and many additional trees were detected during the fall. One area was re-flown in October, and levels of mortality increased more than 300 percent over the earlier estimates.

The most heavily impacted forests of the state are the Tonto, Apache-Sitgreaves, and Prescott National Forests, and the San Carlos Apache Reservation and adjacent state and private lands. Some stands in these forests have 80 to 90 percent tree mortality; others have less than one percent mortality. Mortality in pi–on pine woodlands is equally high. A late-season survey of 28 square miles of pi–on woodland southeast of Flagstaff revealed 700,000 dead trees, or more than 90 percent of the mature pi–on trees in the area.

Bark beetles have also attacked trees in the juniper and spruce families. Native junipers, native Arizona cypress and Leyland cypress are among those being killed by the cypress bark beetle. Spruce bark beetle activity has occurred on over 35,000 acres of spruce as well.

The two main reasons why bark beetles are killing so many trees is that the forest has too many trees and the trees are very dry. Overcrowded forest conditions, coupled with drought, lead to the high probability of beetle attack. Unfortunately the winter of 2002-03 has not been as wet as hoped for. Moisture levels for the winter of 2002-03 (October to Jan) are running 3 inches below normal. Current dry conditions, coupled with very high levels of over-wintering bark beetles, could very well lead to greater bark beetle outbreaks in 2003. Recent rains in mid-February could greatly improve forest health; however it may take several years for stressed trees to rebuild carbohydrate stores. Stored carbohydrates are used by the tree to produce beetle-fighting resin (pitch).

The forests of Arizona have been able to survive in relatively dry conditions because in past centuries, low-intensity fires helped to maintain a low density of trees in the forest. Whereas, in the past century we have controlled fire, which allowed many forested areas to become overcrowded.

The best way to avoid having trees attacked by bark beetles is to take preventive measures. First and foremost is to lower tree density through thinning. Many people are unsure as to which trees should be removed. In these cases it may be best to consult with a certified forester or arborist. For a listing of certified professionals consult the yellow pages, call your local University of Arizona County Extension office, or log on to www.safnet.org/certified/directory.htm to find a certified forester, or www.isa-arbor.com to find a certified arborist.

When removing trees it is important to treat the logs and slash properly, or you may promote increased beetle populations in the down material. If you are not interested in saving the logs, they can be hauled to the landfill or chipped. If they are chipped, don't pile the chips deeper than 3 inches next to live trees, as the chips may attract bark beetles. Try to keep chip piles in the open sun and as far from live trees as possible.

If bark beetles are found in the logs and you wish to keep the logs for firewood, you have several options: Utilize the firewood prior to April 1. Peel the bark from the logs to expose the brood to natural enemies. The bark should then be raked into a pile and burned.

Covering sun-exposed stacked logs with clear plastic in an attempt to cook beetles that are over-wintering in the bark may not work as well in practice as in theory. If you use this method, keep the stacks small (2 to 3 layers high), and check the plastic often for tears and any other openings that may allow the adult beetles to escape.

The small slash (limbs and tops less than 3 inches in diameter) is less likely to be used by beetles. This material can be chipped, or piled for burning this winter. When piling, put the smallest diameter material in the middle with the largest on the outside.

Often property owners will have several trees that have significant value in their landscape, either because of their size or their location. These high-value trees can be irrigated or sprayed with insecticides to prevent infestation.

Bark Beetle

If these trees are irrigated, they should be given enough water to wet the soil at least 2 feet deep. The water should be applied in a donut-shaped pattern at the drip line or outer edge of the trees branches. It generally takes about 2 inches of rain to soak soil to a depth of 2 feet. Check the soil 6 to 8 inches deep just outside the drip line of the trees monthly. If the soil is dry, then water. Generally, the months that most often warrant watering are May, June, and October. However, depending on weather patterns watering may be needed any month of the year. If current dry conditions continue this winter you may need to irrigate in March or April. Keep in mind watering restrictions that may be in effect in your community and follow those guidelines as well.

Applications of fertilizers will not help protect trees from the effects of drought, and will not protect against bark beetle attacks.

Trees that are not yet infested can be protected from beetle attacks by spraying with insecticides. When spraying, the entire trunk and the bases of large branches of the tree 4 inches in diameter and greater must be soaked. Spraying large trees is generally not a practice that homeowners can do themselves. To locate a certified pesticide applicator, call the Arizona Structural Pesticide Control Commission at 800-223-0618. The only registered chemicals for this purpose are carbaryl and permethrin. You must use a product that is especially formulated for bark beetles, such as Sevin SL, Dragnet, or Astro. This is a protective measure only; it will not kill beetles once they enter the tree. Typical home and garden products containing carbaryl or permethrin will be ineffective. If the correct material is applied properly, it should be effective for an entire season. Spraying should be completed prior to April 1 to ensure a full season of protection.

The only known direct control method is the removal of infested trees. A good rule to remember is "IF A TREE IS BROWN CUT IT DOWN, IF IN DOUBT CUT IT OUT." If we leave dead trees standing, we run the risk of the new generation of beetles leaving the tree and attacking more trees. Finding reddish-brown boring dust in the bark crevices of a tree indicates that the tree has been successfully attacked, and the tree should be cut down even if the tree is still green at that point. If dead trees are next to houses or other structures, they can become a hazard tree.

Insecticide injections or systemics have not proven effective against bark beetles. Many trees have been injected with seeming success, when what has actually happened is that the treated tree successfully pitched out the attacking beetle with resin prior to the treatment. The tree was then injected with insecticide when in fact no beetles were actually in the tree. The tree saved itself! Studies have proven that injecting chemicals will not kill bark beetles attacking conifers.

There are several miracle cures being promoted to save trees from bark beetles. These materials may not have gone through extensive research to test their effectiveness. Buyer beware! If what is being marketed sounds "too good to be true" it generally doesn't live up to its billing. Remember, it is against the law to use unregistered pesticides, and using pesticides for insects not listed on the label is unwise.

The University of Arizona, Northern Arizona University, and U.S. Forest Service will be engaged in research to test materials to prevent and control bark beetles in Arizona. When these studies are completed and reviewed, the results will be released to the public as soon as possible.

Many trees may only have the top half of the tree dead. Most often what happens is that the lower half of the tree will be killed shortly thereafter. Do not cut the top out of the tree hoping that the rest of the tree will recover. It is best to remove such trees to prevent the spread of beetles to other trees and to prevent them from becoming a hazard tree. You need not wait until the entire tree turns brown; many adult beetles may have flown from the tree before turning brown.

Remember, the most effective method for preventing bark beetle infestations is to thin overly dense stands of trees. If you need more information, please contact your local University of Arizona Cooperative Extension office, State Land Department, or your local fire department. Additional information can be found at http://ag.arizona.edu/extension/fh/, or http://ag.arizona.edu/yavapai/.



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