Re: Management of blue gum eucalyptus in California by Kristina Wolf and Joseph DiTomaso (vol. 70, no. 1, January–March 2016)

January–March 2016

The article discussed, at length, the various aspects of the management and environmental impact of the introduced blue gum tree in California, and also included an extensive list of references. However, the article completely ignored the introduction and impact of bark boring insects (Phoracantha sp.) on the survival of blue gum plantings in California. It also ignored an article on the subject in California Agriculture (Beetle from Australia threatens eucalyptus) by Scriven, Reeves and Luck in the July-August 1986 issue (volume 40, number 7).

The Phoracantha bark beetle species continue to have an impact on eucalyptus species including blue gum, especially in Southern California. The extended drought has also enhanced the successful attack of the beetles on stressed trees.

The ignoring of the impact of insects on the planting of Eucalyptus in California seems to be a significant omission in the article.

Glenn Scriven, UC Riverside (retired)
Homeland, California

Authors Kristina Wolf and Joseph DiTomaso respond:

Thank you for noting the impact of the eucalyptus longhorn borer (Phoracantha semipunctata) on eucalyptus species in Southern California. Our review on E. globulus (blue gum) in California focuses specifically on the traits of this tree species that might make it invasive in certain regional or climatic contexts. Therefore, we did not assess the impacts of this particular pest on eucalyptus populations in California. As there is little information documenting invasive populations of blue gum in Southern California, the possibility of this beetle species having any potentially negative impact on already noninvasive populations was not reviewed for the purposes of our article. Hanks et al. (1991) found that this beetle cannot colonize the bark of live, vigorous eucalyptus trees (although drought-stressed trees of this species may be more susceptible; see Hanks et al. 1995), and it is thus unlikely to have major impacts in terms of biological control of blue gum in areas where it has demonstrated invasive characteristics (i.e., coastal regions where summer fog provides moisture for trees in California's otherwise long dry season). In our extensive reviews of the literature and outreach efforts to land managers across California, we also did not encounter any reports of measureable impacts on blue gum due to this insect, and as such, it does not seem to be relevant to the control of blue gum in areas where it is a concern in California.