The Western bark beetle and oak ambrosia beetle have become a serious pest of oak, particularly Coast live oak and California black oak, and tanoak in the western United States. Over the past five years or more, the population has increased tremendously. These beetles are usually associated with trees infected with Phytophthora ramorum, the causal agent for sudden oak death. These pests are found in stressed, dying or recently dead trees. Western bark beetles tunnel under the bark surface, whereas ambrosia beetles tunnel deep into the sapwood, creating egg galleries and larval tunnels, which weaken the branch. Seemingly healthy branches, complete with green leaves, are known to suddenly break off in minimum wind. The tunnels also enable the Phytophthora fungus easy access into the interior of the trunk. The adults of both beetle species complete their development in the trees, then emerge to infest new targets.

In an effort to slow the spread of beetles, researchers at the University of California examined the effects of treating the trunks with permethrin to reduce the emergence of adult beetles. Three samples from each of eight heavily infested trees were collected, then halved. One half of each sample was surface-treated with permethrin; the other half was unsprayed. Emerging beetles were collected daily from July 8 through Dec. 9. There were 13,808 total beetles collected from the untreated samples, whereas, there were only 131 total beetles collected from the permethrin treated samples. In the permethrin treated samples, neither beetle species was able to complete normal development. Researchers were not clear on how emergence was prevented exactly, but reduction in the shear number of potential pests was dramatic and worth the effort to reduce the population overall.

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