Mediterranean oak borer

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].

Trees attacked in California are primarily valley oak (Quercus 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]. In Oregon MOB has attacked Oregon oak (Q. garryana) (ODF, 2022). Rabaglia et al. (2020) say they cannot predict the beetle’s impact in California.

In Europe the Mediterranean oak borer attacks other hardwood species. One widespread and important North American species of oak, northern red oak (Q. rubra), is a proven host [CDFA January 2020 and December 2020]. Other host species in Europe are related to tree species native to North America. These occasional hosts include European chestnut (Castanea sativa), Oriental beech (Fagus orientalis), several Eurasian oak species (Quercus), elm (Ulmus), maple (Acer), and walnut (Juglans). Rabaglia et al. (2020) made no attempt to discuss the insect-pathogen complex’ possible impact to these other genera.

Where MOB is Established

The first Mediterranean oak borer outbreak detected in the United States occurred in September 2019 in Calistoga, Napa County, California. Authorities soon realized that specimens collected in 2017 from the same area also belonged to this species. Authorities have concluded that the Mediterranean oak borer has possibly been present in Napa County since 2012 or earlier (Ewing and Jones, 2024). Extensive damage has been observed on valley oaks (Williams, 2024). By December 2020, Mediterranean oak borer was known to be present in four counties: Lake, Napa, Sacramento, and Sonoma [CDFA December 2020]. A specimen was trapped near a wood processing site in Richmond (Contra Costa County) in 2018 but apparently no infested trees have yet been detected [CDFA December 2020].

During this same period, individual beetles were being trapped in Oregon, but the species’ status was unclear. The first detection was of a single specimen trapped at Chinook Landing near Troutdale as part of an Oregon Department of Forestry (ODF) woodborer surveillance program [ODF fact sheet; Williams 2024]. Enhanced trapping efforts failed to detect additional beetles [ODF; Rabaglia et al. 2020]. Another MOB was captured in a trap near Woodburn (Marion County) in 2021. In 2022, authorities captured 21 beetles at seven sites in four counties. Fourteen of the captures occurred near the original site, Troutdale. In May 2023, authorities detected the first infested tree – an Oregon white oak (Quercus garryana) – in Wilsonville, near Troutdale (Clackamas County). Further searches determined that at least 30 Oregon white oaks were infested. Authorities are monitoring trees suspected to be infested in Salem and Troutdale and have expanded trapping (Williams 2024).

The California and Oregon outbreaks were introduced separately, from different origins. DNA sequences indicate California populations originated from a haplotype found in France and Belgium; Oregon populations originate from a haplotype found in France and Germany (Ewing and Jones, 2024).

The introductory pathways are unknown. However, ambrosia beetles are often associated with untreated wood, including packaging made from wood (e.g., crates, etc.). Another possible pathway might be imports of plants for the nursery trade (ODF fact sheet). Regarding MOB specifically, the ODF suggests that the insects might have been transported on oak wine barrel staves imported from Europe (ODF December 2023). [Barrels that are loaded with wine or spirits are not subject to the treatment requirements specified by ISPM#15. The reason is that the wood is believed to have been treated adequately to kill pests during the process of steaming and bending the wood and charring the interior (https://www.ippc.int/static/media/files/publication/en/2017/02/ISPM_15_ED_En_2017-02-10.pdf ).]

The Mediterranean oak borer behaves differently in California and Oregon than it does in its native range in Europe (Williams, 2024). Whereas in Europe the beetle attacks dead or dying trees, or harvested logs, in California it attacks living trees – including vigorous ones. In Europe the beetle attacks trees once per year; in California it attacks the same tree repeatedly over a period of three to five years. The beetle also supports more generations in California, three rather than one or two.

The tree’s response also differs. European trees are highly resistant to attacks by MOB; in California, they are not.

Life cycle

Adult females of ambrosia beetles – including the MOB – tunnel into trees. The females carry fungal spores which they inoculate into the tunnels. The beetle’s larvae feed on the fungus “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 with which the female can then mate to produce fertilized eggs. The result is that a single female – mated or unmated – can establish a new infestation.

Early detection of a MOB infestation is difficult because the first attacks occur in branches less than eight inches in diameter in the upper canopy. Also the entrance and exit holes are tiny. The galleries are tunneled down branch. When they eventually reach the bole, the galleries girdle the tree. This process takes 3 – 5 years [Ewing and Jones, 2024]

Damage

No source mentions hazards associated with structural damage to the tree. Attention is focused on the fungi being transported by the beetle. While most fungi associated with ambrosia beetles are not pathogenic to the host tree, some are. Fungal species isolated from both beetles and plant tissues in California have 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 – is the most common fungus isolated in Europe and is pathogenic to cork oak (Quercus suber) [Rabaglia et al. 2020; CDFA January and December 2020]. Williams (2024) reports that laboratory tests have shown Oregon white oak to be vulnerable to this fungus. Additional pathogenicity tests are under way in California [Rabaglia et al. 2020].

The California Department of Food and Agriculture (CDFA) has twice conducted analyses to determine the risk posed by MOB to the state’s agricultural and forest resources and the effort the agency should expend to counter it. The first analysis (see CDFA January 2020) noted that California forests are periodically under stress from drought, fire, and disease, so they might be especially vulnerable to attack by ambrosia beetles. The second analysis (December 2020) did not mention this as a factor. While the agency website does not document a formal decision, CDFA is apparently managing MOB as a pest ranked “B”; this means that the organism poses a recognized economic risk but choice of management action falls to the discretion of the agricultural commissioners of individual counties.

The ranking process listed a number of threats posed by MOB to the state’s natural and agricultural resources:

1. MOB is probably able to establish over much of California. One confirmed host, valley live oak (Q. lobata), is widespread in California. [The species is already under threat due to conversion of its habitat to agricultural uses [Beckman et al. 2019; Carerro et al. 2022].
2. There is uncertainty as to how many North American genera and species in the family Fagaceae (oaks, beech) might be hosts of the beetle, fungi, or both.
3. If the Mediterranean oak borer – like other ambrosia beetles – can mate with siblings and be parthenogenetic, it has a “high” reproduction and dispersal potential.
4. The second analysis downgraded the likely economic impact, largely because they had concluded that X. monographus prefers older trees so it is unlikely to pose a significant risk to tree nurseries.
5. Both analyses noted the importance of oaks – particularly valley oaks – as components of forests and woodlands in California. Consequently, they assigned a “high” ranking in the environmental impact category.
6. By the time of the second analysis, authorities realized the beetle was established in four counties in two types of ecosystems, the Coast Range mountains and the Central Valley. Consequently, analysts determined that the pest was widespread in California but not fully established in the endangered area.

The final score (rank) for MOB was dropped from “high” under the first analysis to “medium” in the second, largely because of the lower estimate of economic damage.

Both the January and December 2020 analyses noted three areas of uncertainty:

1. It was possible that X. monographus was more widely distributed in California than known. As of 2024, the website www.ucanr.edu/sites/mobpc/ says it has not yet been found in additional counties.
2. Ambrosia beetles (including a different Xyleborus sp.) are attracted to oak trees infected with the causal agent of sudden oak death, Phytophthora ramorum. link These beetle attacks appear to hasten the death of the tree.
3. The beetle’s host range could be much narrower or broader in California than has been observed in Europe. As noted, several genera reported as including hosts in Europe, e.g., chestnuts (Castanea), walnuts (Juglans), ash (Fraxinus), stone fruits (Prunus), and elms (Ulnus), are important in components of forests or agriculture in California.

California and Oregon authorities and academics are pursuing research on several questions important for managing Mediterranean oak borer infestations. These include (Ewing and Jones 2024 and Williams 2024)

• tracking spread of outbreaks and tree decline.
• seeking resistant trees.
• determining the most effective traps & lures, and best trap placement.
• testing management strategies, including systemic insecticides and repellants, and removal or pruning of infested.
• testing whether solarization or burial is an effective means of killing insects in oak wood.
• testing pathogenicity of the several fungi associated with Xyleborus monographus.
• determining potential pathways for introduction and spread
• determining adult emergence timing based on temperature

SOURCES

Beckman, E., Meyer, A., Denvir, A., Gill, D., Man, G., Pivorunas, D., Shaw, K., & Westwood, M. (2019). Conservation Gap Analysis of Native U.S. Oaks. Lisle, IL: The Morton Arboretum.

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. 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

Ewing, C. and M. Jones. 2024. The Mediterranean Oak Borer (MOB, Xyleborus monographus) an invasive ambrosia beetle infesting white oaks in Northern California and Oregon. Presentation to the Western Forest Insect Working Group April 2024

Oregon Department of Forestry (K. Ripley and W. Williams). 2022. Forest Facts: Mediterranean Oak Borer Xyleborus monographus (Fabr.)

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

Williams, W. 2024. Mediterranean Oak Borer A potential new pest of oak. Presentation to the Western Forest Insect Working Group April 2024