The Mediterranean Oak Borer (MOB) is an invasive ambrosia beetle that infests several species of white oaks in California: valley oak, blue oak, and Oregon oak. The beetle is spreading rapidly in Northern California counties and there is considerable potential for the beetle's range to expand as they can be moved in infested wood and the females can fly.
Earlier this year, UC IPM hosted a webinar where Cal Fire Specialist Dr. Curtis Ewing and UCCE Forestry Advisor Dr. Mike Jones shared information about MOB, other oak tree pests, and their management. Below are answers to questions asked during the webinar about these pests. To watch the recording of this webinar, visit https://www.youtube.com/watch?v=npMeYYo5w8c.
Identification and Reporting
Q: What should you do if you find oak pests such as Mediterranean oak borer, goldspotted oak borer (GSOB), or invasive shothole borer (ISHB))?
A: Visit the links below to report suspected infestations.
MOB: https://ucanr.co1.qualtrics.com/jfe/form/SV_9zwapzQGP0m1HBH
GSOB: https://surveys.ucanr.edu/survey.cfm?surveynumber=4013
ISHB: https://ucanr.co1.qualtrics.com/jfe/form/SV_bIuyTZY7hkqiqod
Prevention and Management
Q: Is chipping infested wood alone useful, or is it necessary to chip and burn to control MOB?
A: Chipping infested wood to one inch is an effective control. You do not have to chip and burn. Burning is effective if you cannot chip the material. Chipped infested material can be used onsite, but in general we do not recommend moving the wood chips long distance.
Q: How long do MOB beetles remain in the wood chips?
A: It varies depending on the time of year and how quickly the material dries out. MOB beetles can likely remain in the wood chips for months if the weather isn’t too hot and the chips remain damp.
Q: Can MOB-infested wood chips be drenched or inoculated with composting organisms to encourage quicker decomposition?
A: Yes, composting is an effective control if it reaches a high temperature. Above140°F will kill the wilt fungus associated with MOB, above 160°F will kill the beetle.
Q: How far can you move firewood?
A: A general rule is fewer than 50 miles, but the less distance you move it the better. We recommend less than 20 miles and only after it has been aged 2 years or more.
Q: How long can MOB survive in firewood or other dead wood?
A: MOB beetles will typically remain in the material for 8-10 months and up to 2 years under the right conditions.
Q: Are there any recommendations for chemical control or any sort of treatments for trees?
A: Research is still being done on the best control options for MOB. Consult with an arborist for your situation.
Q: With these invasive tree pests in mind, how do you recommend selecting trees to plant?
A: Research potential pests and their distribution areas. Plant a variety of trees less susceptible to those threats: diversity is key.
Q: Are there any references on evaluating the costs of damage from these pests?
A: Since MOB is a relatively new pest in California, there are currently no references for evaluating costs associated with its damage. Economic impacts have been estimated for GSOB and ISHB. In the first ten years after their introduction to California, costs associated with the GSOB infestation exceeded $10 million of public and private funds for both direct costs and opportunity costs: infested tree mitigation (dead tree removal & infested wood disposal); research; public education/outreach; surveys, detection & mapping; and lost revenues (due to closures of parks and campgrounds dealing with hazardous dead trees). See the ISHB website for information about the economic impacts of ISHB.
Q: How infested or damaged must an oak with MOB be to justify taking it down?
A: It depends: if it is a single infested tree in a new area not known to have MOB, cut the tree down immediately. In general, once MOB is in the trunk or main branches close to the trunk, the tree will likely die within a year or so, and removing the tree is recommended. If you're in an area that is already known to be infested, like West Sacramento, pruning infested branches is an option. When pruning, cut the infested branch all the way down to where it meets the trunk. Do not cut or prune infested trees in the spring when the beetles are flying, cut them in the fall.
Q: Can the fungi associated with MOB and ISHB be spread by chainsaws or pruning tools?
A: More research is needed to determine how well tools can spread these particular pathogens, but similar pathogenic fungi are known to be spread by tools like bull saws, hand pruners, loppers, and climbing spikes. Good sanitation and maintenance of tools is crucial. It is unlikely for chainsaws to spread diseases because heat, friction, and bar oil are usually lethal for pathogens.
Q: Are there any commercially available traps for MOB?
A: Traps to monitor for MOB are available, but in limited supply. Contact Curtis Ewing at Curtis.Ewing@Fire.Ca.Gov for more information about how to obtain and use MOB traps for monitoring purposes.
Ecology
Q: Do oaks from the area MOB originated have any natural defenses, and are there natural enemies for MOB?
A: In their native range, MOB has natural enemies, but these don’t significantly impact their populations. Natural population control from predation, parasites, or disease is generally weak. Most of the population control in their native range comes from trees having natural resistance to MOB. Research is underway to determine MOB’s characteristics and infestation dynamics in California, and if our trees have any natural resistance.
Q: Does the Mediterranean oak borer play a beneficial role in its natural habitat in Europe?
A: In their native range, they may provide similar benefits to the bark beetles in our pine tree forests of California-- they remove the weaker trees and open the forest up. In Europe, MOB only attacks trees that are already dying, and as a result MOB has only one generation per year. In California, MOB infests both stressed and healthy trees and can have multiple generations per year.
Future Outlook
Q: For susceptible trees, will there be survivors 100 years in the future?
A: One of our objectives is to find trees in our forests with natural resistance to these pests. Researchers have recently found ash trees that are resistant to another invasive pest, the emerald ash borer, and have begun breeding these trees for replanting impacted areas. Though we are hoping to find similar resistance in our other native trees, there will most likely be negative impacts to our ecosystem due to these pests. Further research will help us determine how to manage these impacts going forward.
Q: Where will the Mediterranean oak borer, invasive shothole borer, and goldspotted oak borer spread in California?
A: The spread of these pests is being monitored across the state. For more information about the distribution in California and which tree species are potential hosts, visit the ISHB, MOB, and GSOB websites. More research is needed to determine which areas of California are more or less likely to be impacted by these pests as they spread.
Resources
- Mediterranean oak borer: https://ucanr.edu/site/mediterranean-oak-borer
- Invasive shothole borer: https://ucanr.edu/site/invasive-shothole-borers
- Goldspotted oak borer: https://ucanr.edu/site/goldspotted-oak-borer
- UC Oaks: https://oaks.cnr.berkeley.edu/