October 16 , 2003

Life Stages and Development
Western pine beetles pass through the egg, larval, pupal, and adult stages during a life cycle that varies in length from about 2 months in warm weather to 10 months in cool weather. All stages are completed beneath or in the bark of infested trees, except for a period when the adults fly to find new trees to attack.

During an attack period, which may last 3 weeks, each female lays about 60 tiny pearl white eggs individually in niches cut into the sides of the egg gallery. Some of these parent females may emerge and reattack to establish additional galleries elsewhere in the same tree or other host trees. After incubating from 1 to 2 weeks, the eggs hatch.

The larvae are small white grubs that feed first in the phloem, where they construct a short gallery. They then mine into the middle bark where most of their development takes place. After completing four larval stages, they transform into pupae and then into adults. As these brood adults feed on the middle and outer bark, fungal spores collect in their mycangia. Once the adult insects emerge, they are ready to renew the attack-infestation cycle in living trees.

Conditions Affecting Outbreak
Several conditions often work together to influence the number of beetles and the beetle caused tree mortality in a given area. The significant conditions follow.

Food supply -- the availability of suitable host material - phloem and inner bark - is a key condition influencing western pine beetle outbreaks. Most trees are either healthy or too weak to provide material in which beetle numbers can increase. Healthy trees can withstand many attacks before the beetles are successful, the brood is established, and new adult beetles are produced. Weak trees, such as those that have been smog damaged, diseased or suppressed by competition, although easily killed, also produce relatively few beetles.

The thick, nutritious phloem and inner bark of healthy trees become host material for attacking western pine beetles when these trees undergo sudden and severe moisture stress. Healthy trees ordinarily produce abundant amounts of resin, which pitch out or eject attacking beetles. But, when suddenly deprived of moisture, stressed trees cannot produce sufficient resin flow to resist attack, and their nutritious food supply suddenly becomes available to beetles. In these trees, almost all attacking beetles can succeed and reproduce many times their number of offspring, increasing the beetle population to outbreak levels.

Moisture stress results when the water balance between the foliage and the roots changes dramatically. An imbalance may result from increased water loss from the needles, decreased water uptake by the roots or from a combination of the two.

Any condition that results in excessive demand for moisture, such as tree crowding, competing vegetation, or sudden exposure to severe sunlight: or any condition that reduces that ability of the roots to supply water to the tree, such as mechanical root damage, root disease, soil compaction, or drought, can cause moisture stress and increase susceptibility to attack by the western pine beetle.

Landowners have two basic alternatives when choosing the control strategy most appropriate for their needs: beetle population suppression and damage prevention.

Suppression -- Over the years, several suppression methods have been tried to help reduce beetle populations enough to lower tree mortality significantly. These methods have included the removal of infested trees and applying toxic residual sprays to kill emerging beetles. Because adult beetles can fly many miles and produce many offspring, effective suppression methods required the location (spotting) and treatment of all, or nearly all, infested trees over extensive areas in a short period of time.

Timely spotting and treatment are difficult and expensive tasks that require cooperation among many landowners. Consequently, the results have often been unsatisfactory. Also, these projects have failed because the basic underlying cause for the population outbreak - and abundance of stressed trees - has not changed. Typically, if a habitat favorable to high-level western pine beetle populations persists, suppression - by whatever means - will probably fail to reduce tree mortality significantly.

In urban forests, preventing tree killing by the western pine beetle is often more appropriate than attempting to suppress beetle populations. Landowners can prevent unacceptable damage on their land by maintaining thrifty, vigorous trees or stands that do not afford a suitable food supply for the beetle.

Trees with a high risk of damage by beetles characteristically have poor vigor and can be recognized by crown symptoms such as dead tops, branches and twigs, and short, sparse, poorly colored foliage. Also, they may be older slow-growing trees that are heavily infected with dwarf mistletoe, that are root disease, or that have been struck by lightning.

Prevention can also take the form of minimizing injury or disturbance to individual trees or sites. Careful cutting practices and care in developing urban forest land are simple, yet effective, ways to prevent damage by western pine beetles. Information obtained from USDA Forest Service.

Santa Ynez Valley Tree Care
P.O. Box 1147, Santa Ynez, California 93460

(805) 688-5580

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