Mediterranean oak borer (MOB) is an ambrosia beetle native to Europe, western Asia (Iran to Turkey), and northern Africa (Algeria and Morocco). It has spread to Korea and the United States (California and possibly Oregon) [Ripley and Williams, 2022; Rabaglia et al. 2020]. Generally, it infests weakened or dying trees of a variety of oak and beech species that are already suffering from drought, other pests, or disease [ODF 2022].
Hosts attacked in Europe include: European chestnut (Castanea sativa), Oriental beech (Fagus orientalis), several Eurasian oak species, and the widespread North American species northern red oak (Quercus rubra) [CDFA January 2020 and December 2020]. Trees attacked in California are primarily valley oak (Q. lobata) and some blue oak (Q. douglasii). One black oak (Q. kelloggii) that was in very poor condition was also attacked [Rabaglia et al. 2020]. Additional genera of deciduous trees are also attacked. Oregon authorities are concerned that Oregon oak (Q. garryana) might be a host [Ripley and Williams, 2022]. No data are available about the vulnerability of other tree species native to eastern North America, such as oaks, beech, and chestnut. Rabaglia et al. (2020) say they cannot predict the beetle’s impact in California, and they do not discuss possible impacts in other parts of country.
Where MOB is Established
The initial (recognized) detection occurred in September 2019 in Calistoga, Napa County, California. Authorities later realized that specimens collected a year earlier from the same area also belonged to this species. By early 2020, infested trees were found throughout a 15-mile-long area in Napa County and in neighboring Lake and Sonoma Counties, indicating a well-established population in the area [Rabaglia et al. 2020]. By December 2020, Mediterranean oak borer was known to be present in four counties: Lake, Napa, Sacramento, and Sonoma [CDFA December 2020]. Authorities have concluded that the Mediterranean oak borer has probably been present for several years [CDFA December 2020; Rabaglia et al. 2020].
One specimen of Xyleborus monographus was also trapped in Portland, Oregon in 2018, but further trapping did not detect an established population [Ripley and Williams, 2022; Rabaglia et al. 2020]. One specimen was additionally found at a different site in 2021 and two were found in 2022 for a total of four; more trapping is necessary before a declaration will be made as to whether MOB is established in Oregon (Josh Vlach, Oregon Department of Agriculture, personal communication).
The introductory pathway of the California outbreak is unknown, but ambrosia beetles are often associated with untreated wood, including dunnage and other wood packaging. A specimen was trapped near a wood processing site in Richmond (Contra Costa County) in 2018 [CDFA December 2020].
Adult females tunnel into trees – often reported to be recently dead or stressed. The females carry fungal spores which they inoculate into the tunnels. The beetles’ larvae feed on the fungal “garden.” As they mature, newly hatched females can mate with their brothers before they leave their natal gallery to fly to a new tree to initiate gallery construction. The females can also be parthenogenic; an unmated female can lay unfertilized eggs that develop into males that the female can then mate with to produce fertilized eggs. The result is that a single female – mated or unmated – can establish a new infestation.
No source mentions hazards associated with structural damage to the tree. Attention is focused on possible pathogenic fungi being transported by the beetle. While most fungi associated with ambrosia beetles are not pathogenic to the host tree, some are (e.g., redbay ambrosia beetle and laurel wilt disease; polyphagous and Kuroshio shot hole borer and Fusarium). Ambrosial fungal species isolated from both beetles and plant tissues in Calistoga included Raffaelea montetyi, Paecilomyces formosus, Fusarium solani, undescribed species of Fusarium and Leptographium, and a yeast species, Saccharomyces microspore [Rabaglia et al. 2020]. One of these, Raffaelea montetyi, has proved pathogenic to cork oak (Quercus suber) [Rabaglia et al. 2020; CDFA January and December 2020]. As of the end of 2020, pathogenicity tests were under way in California [Rabaglia et al. 2020]; these tests included Oregon oak (Q. garryana) [Ripley and Williams, 2022].
The Risk to California Specifically
The first CDFA analysis (see below) noted that California forests are periodically under stress from drought, fire, and disease, so they may be especially vulnerable to attack by ambrosia beetles. The second analysis did not mention this as a factor.
The California Department of Agriculture has twice evaluated the risk that Xyleborus monographus poses to the state’s economy and environment. The first ranking was in January 2020 (full citation below); the revised analysis appeared in December 2020. The second analysis was prompted by the recognition of the beetle’s wider distribution which expanded to 4 counties and a need to re-assess. The second analysis changed the pest’s rank from “A” to “B”. A pest ranked “A” is defined as “A pest of known economic or environmental detriment and is either not known to be established in California or it is present in a limited distribution that allows for the possibility of eradication or successful containment.” Whereas a pest with a “B” rating is defined as “An organism of known economic importance subject to: eradication, containment, control or other holding action at the discretion of the individual county agricultural commissioner.”
The ranking process made the following determinations:
- Climate/Host Interaction: Both analyses noted that one confirmed host, Q. lobata, is widespread in California. In Eurasia, the Mediterranean oak borer is found in a variety of climates, indicating that it is probably able to establish over much of California. Therefore, both evaluations gave it a rating of “High” (3) in this category: likely to establish a widespread distribution in California.
- Known Pest Host Range: Both analyses noted that several genera in the family Fagaceae have been identified as hosts, but there is uncertainty about many of them. Therefore, both analyses gave Mediterranean oak borer a ranking of “Medium” (2) in this category: a “moderate” host range.
- Pest Reproductive and Dispersal Potential: Both analyses were based on an assumption that the Mediterranean oak borer mates with siblings and can be parthenogenetic. Consequently, a single female, mated or unmated, can establish a new infestation. Ambrosia beetles can be moved in infested wood and the females can fly. As such, both analyses gave the species a rank of “High” (3) in this category: has both high reproduction and dispersal potential.
- Economic Impact: This is the criterion on which the two analyses diverged. The first analysis included a concern that MOB might attack smaller trees. If so, it could impact production of oaks in California nurseries. In response to this concern, the first analysis gave the species the rank of “High” (3). In the second analysis, authorities concluded that X. monographus prefers older trees and so is unlikely to pose a significant risk to tree nurseries. As a reportable pest, it might trigger impositions of quarantines. Consequently, the second analysis downgraded this category to “Low” (1), with a focus on the potential loss of markets and quarantines.
- Environmental Impact: Both analyses stated that the beetle is known to attack living oak trees. In California, this beetle-fungus complex will meet naïve oaks, with the potential to cause severe damage. Oaks are important components of forests and woodlands in California; the known host valley oak in particular is widely distributed in California and an important component of oak woodlands. Some species of oaks in California are rare. Oaks provide habitat for threatened species, and large heritage oaks are important landscape trees and add to property values. Consequently, X. monographus was allotted a ranking of “High” (3) in this category. This ranking is based on the possibility that the species could cause the following impacts: criteria to occur (lettering taken from assessment process):
A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B. The pest could directly affect threatened or endangered species.
C. The pest could impact threatened or endangered species by disrupting critical habitats.
E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.
- Post Entry Distribution and Survey Information: Apparently this criterion focuses on the current extent of the infestation rather than the full area at risk (which is addressed in criterion 1, above). The first analysis was based on the initial detection in the Calistoga region, so a “Low” (-1) ranking followed. This meant that the beetle had a localized distribution in California, in one suitable climate region. By the time of the second analysis, authorities realized the beetle was established in four counties in two types of ecosystems, Coast Range mountains and the Central Valley. As a result, the beetle was given a rank of “Medium” (-2) in this category meaning that the pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
Final Score: Result of changed ranking re: economics was to reduce combined “Consequences of Introduction” ranking from High (13 points) to Medium (12 points).
While the California risk assessments state that the principal host, valley oak, is widespread, recent analyses of oak species’ conservation status rated valley oak as at risk [Carerro et al. 2022].
Both analyses noted three areas of uncertainty.
First, it is possible that X. monographus is more widely distributed in California. An extensive trap network targeting exotic wood boring beetles (in 11 counties, but not including Napa or Sonoma) had not detected the beetle in other parts of the state.
Second, research indicates that ambrosia beetles (including a different Xyleborus sp.) are attracted to oak trees infected with the causal agent of sudden oak death, Phytophthoraramorum. These beetle attacks appear to hasten the death of the tree. Therefore, X. monographus could have a synergistic impact with sudden oak death on California oaks.
Third, the host range of Xyleborus monographus could be much narrower or broader in California than has been observed in Europe. Thus the impact could be over- or underestimated in this proposal. The analyses do not mention the presence in California of several genera reported as hosts in Europe, [Rabaglia et al. 2020] including chestnuts (Castanea), walnuts (Juglans), ash (Fraxinus), stone fruits (Prunus), and elms (Ulnus). In other parts of North America beech (Fagus) are also native. Most of these species are under threat by other non-native insects or pathogens.
The second ranking process was concluded in February 2021 but there has been no report of a conclusion or further action.
California Department of Food and Agriculture. January 2020. California Pest Rating Proposal. Xyleborus monographus (Fabricius): Mediterranean oak borer. Coleoptera: Curculionidae. Current Rating Q; Proposed Rating: A.
California Department of Food and Agriculture. December 2020. California Pest Rating Proposal. Xyleborus monographus (Fabricius): Mediterranean oak borer. Coleoptera: Curculionidae. Current Rating A; Proposed Rating: B.
Carrero, C., E.B. Brun, A. Frances, D. Jerome, W. Knapp, A. Meyer, R. Mims, D. Pivorunas, D. Speed, A.T. Eberly, M. Westwood. 2022. Data sharing for conservation: A standardized checklist of US native tree species and threat assessments to prioritize and coordinate action. Plants People Planet, 1–17. Supplementary Information
Ripley, K and Williams, W. 2022. Forest Facts: Mediterranean Oak Borer Xyleborus monographus (Fabr.) Oregon Department of Forestry.
Rabaglia, R.J., S.L. Smith, P. Rugman-Jones, M.F. Digirolomo, C. Ewing, and A. Eskalen. 2020.
Establishment of a non-native xyleborine ambrosia beetle, Xyleborus monographus (Fabricius) (Coleoptera: Curculionidae: Scolytinae), new to No. Am. in California. Zootaxa, 4786 (2): 269–276 ISSN 1175-5326
Pest and Diseases Image Library , Bugwood.org