Interactions among Bark and Ambrosia Beetles (Coleoptera: Scolytidae) and a Novel Disease of Oaks in California
Brice A. McPherson1, Richard B. Standiford2, David L. Wood3, Andrew J. Storer4, and Pavel Svihra5
Insects have many varied roles in tree diseases. They may disseminate a pathogen by scattering or broadcasting inoculum, or inoculate the host with a pathogen by placing the inoculum on the infection court. They may facilitate ingression of a pathogen into the host tissues, invasion of a pathogen through the host tissues or preservation of a pathogen under unfavorable environmental conditions. Interactions among tree diseases and bark and ambrosia beetles that involve predisposition of the host trees to insect attack have been well documented in a number of pathosystems. For example, conifers infected by the fungus Heterobasidion annosum, a root pathogen, are prone to mass attack by bark beetles, resulting in elevated mortality. Ambrosia beetles, Xylotera domesticum, frequently colonize the bark of beech trees (Fagus spp.) that have been infected by fungal bark pathogens, Nectria spp., which can lead to structural failure (“beech snap”).
Coast live oaks, Quercus agrifolia, infected by Phytophthora ramorum produce a viscous red to black exudate, referred to as bleeding, that is associated with subcortical cankers, usually on the main stem. Bark and ambrosia beetles consistently colonize the bleeding areas of infected trees while these trees maintain functional green foliage. These saprotrophic insects typically attack recently killed trees and branches, but have not been reported to colonize living trees. In disease progression plots that we have monitored every three months since March 2000, up to 53% of living symptomatic (bleeding) trees were colonized by bark beetles. Through June 2002, every symptomatic tree that died (n = 37) had been colonized by beetles while the foliage was still green. A beetle-infested tree may also suffer breakage of the major stem at the point of the beetle tunneling. In an effort to better understand the factors influencing colonization of P. ramorum-infected trees by ambrosia and bark beetles, we monitored the association of these insects with Q. agrifolia in natural forests. During beetle flight seasons for 2 years, sticky traps were placed on trees classified as non-symptomatic, symptomatic (bleeding), and symptomatic/beetle-infested. The mean number of beetles trapped on symptomatic/beetle-infested trees was significantly greater than on both non-symptomatic and symptomatic trees. This result implies pheromone-mediated colonization of diseased portions of trees, which may contribute to tree mortality. There was no significant difference in catch between symptomatic and non-symptomatic trees. Since the course of the disease in mature trees has not been observed in the absence of ambrosia and bark beetles, the ability of trees to recover from P. ramorum infection has not been determined. Studies designed to assess the role of these beetles in sudden oak death syndrome will be described.
1Department of Environmental Science, Policy, and Management, IHRMP/Center for Forestry, 145 Mulford Hall, University of California, Berkeley, CA 94720-3114; (510) 642-5806; aoxomoxo@nature.berkeley.edu
2Department of Environmental Science, Policy, and Management, IHRMP/Center for Forestry, 145 Mulford Hall, University of California, Berkeley, CA 94720-3114
3Department of Environmental Science, Policy, and Management, Division of Insect Biology, University of California, Berkeley, CA 94720
4School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931
5University of California Cooperative Extension, 1682 Novato Blvd, Suite 150B, Novato, CA 94947
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