Under the Hood: The Silent Invasion — The Mediterranean Oak Borer’s Threat Grows
By Tim Carl
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The Spotlight
Welcome to “Under the Hood,” our exclusive Saturday series for Napa Valley Features paid subscribers. This week we delve into the growing threat posed by the Mediterranean oak borer, a silent invader that is devastating Napa Valley’s iconic oaks. With no effective chemical treatments and little regulatory oversight, arborists like Juan Esteban Restrepo and experts such as Bill Pramuk are racing to slow its spread before it reshapes the landscape and ecology of the region. What can be done, and what’s at stake?
Additionally, we’re diving into the latest data from our readers’ polls and providing insights from our economic dashboard, covering local Napa Valley, U.S. and global markets.
In addition, we feature "What We Are Reading," a section with a handpicked list of recent articles that provide a variety of viewpoints on issues important to our community and beyond.
“What We Are Reading” quotes of the day:
"If you can prove that there is a market, [growers] are fans of innovation." – Anne Kettaneh, in "Bordeaux gets alcohol-free wine shop as times change," Decanter.
"Even with all the [undocumented] immigrants, we’re still short on labor in our vineyards, fields and wineries."– Rolando Herrera, in "Sonoma, Napa county vineyard owners fear deportation of workers amid Trump’s immigration crackdown," The Press Democrat.
"As a collective culture, we’re moving away from drinking alcohol." – Keena Hanson, in "Could cannabis sales cannibalise wine?" The Drinks Business.
Decanting the Data:
The Silent Invasion: The Mediterranean Oak Borer’s Threat to Napa Valley’s Iconic Oaks
By Tim Carl
NAPA VALLEY, Calif. — On the outskirts of Calistoga, arborist Juan Esteban Restrepo pressed his palm against the rough bark of an ancient valley oak. He didn’t need to look up to see what was happening — he already knew. The tree’s canopy was thinning, its upper branches brittle and dying. Small holes, no wider than a pencil tip, dotted the bark, each one an entry wound for a slow-moving catastrophe. At the base of the trunk, a fine sawdust-like substance, known as “frass,” had begun to accumulate — a clear sign of borer activity, the remnants of their destructive tunneling inside the tree.
“This is what we’re up against,” Restrepo said, shaking his head. “People don’t know it, but many of these trees are dying from the inside out.”
For years Restrepo has been caring for Napa Valley’s trees. His journey to becoming one of the region’s most sought-after arborists, however, was anything but conventional. Born in Medellín, Colombia, at a time when the city was synonymous with violence and upheaval, Restrepo found solace in nature. As a child he spent weekends in the countryside working alongside his grandmother in her garden. When he immigrated to the United States at 18, he pursued various jobs before marrying Bettina Rouas, owner of Napa’s Angele restaurant, and discovering his true calling — tree care and preservation.
Now, as the owner of Von Greiff Tree Care — a name chosen to honor his grandmother’s last name — Restrepo is facing what may be the greatest challenge to Napa’s oak woodlands yet: the Mediterranean oak borer, an invasive beetle threatening one of the key symbols of the region’s ecological and agricultural health. Beyond working with clients for fire mitigation, forest regeneration and landscape management, he has become an early advocate for protecting Napa’s oaks from this rapidly spreading pest.
An Unseen Killer
The Mediterranean oak borer (Xyleborus monographus) is a tiny ambrosia beetle, barely an eighth of an inch long, yet its impact is anything but small. First detected in Calistoga in 2019, the beetle has spread quickly, infesting valley oaks (Quercus lobata) and, to a lesser extent, blue oaks (Quercus douglasii).
Consulting arborist Bill Pramuk, who has been tracking MOB’s spread, says its presence is marked by dark, erratic tunnels carved into the tree’s inner wood. As the beetles burrow, they introduce symbiotic fungi that digest the wood, creating what is known as ambrosia — a food source for their larvae. But these fungi do more than provide sustenance. Some are pathogenic, accelerating the tree’s decline.
Like other ambrosia beetles, MOB follows a distinctive reproductive cycle. Females bore into trees, excavating intricate tunnel systems, or galleries, where they lay their eggs. Within these tunnels, the beetles cultivate fungi, which spread rapidly through the wood. Unlike typical wood-boring insects that feed on the tree itself, MOB larvae rely entirely on these fungi.
What makes the Mediterranean oak borer particularly difficult to control is its ability to reproduce parthenogenetically, meaning females can lay unfertilized eggs that develop into males and females. These siblings often mate within the natal gallery before dispersing, further accelerating population growth. This reproductive strategy allows a single female to establish a new infestation independently, even without mating, and potentially initiate multiple infestations within a single season. Such rapid and self-sufficient reproduction makes containment efforts exceedingly challenging, as a single beetle can give rise to widespread outbreaks in a short time frame.
“The first sign most people notice is canopy dieback,” Pramuk said. “A section of the upper branches will start to wither and turn brown. By the time the infestation is visible, the damage is often extensive.”
MOB vs. Sudden Oak Death: Understanding the Differences
While the Mediterranean oak borer is a growing threat, many in Napa Valley are more familiar with Sudden Oak Death, a devastating tree disease caused by the waterborne pathogen Phytophthora ramorum. Though both afflict native oaks, their modes of spread and impact differ significantly.
SOD thrives in moist environments, spreading via water droplets, particularly during rainy seasons. It primarily affects tanoaks (Notholithocarpus densiflorus) and coast live oaks (Quercus agrifolia), causing bleeding cankers and rapid decline. In contrast, MOB spreads through dry conditions, infested wood and human activity, targeting valley oaks and blue oaks. Rather than causing external lesions, it introduces fungi that compromise the tree from within.
SOD can be managed through quarantines and fungicidal treatments. MOB, however, presents a greater challenge. No chemical treatments have been proven effective, according to Pramuk, leaving proactive tree-health management and the strategic removal of infested material and not transporting firewood as the best available defenses.
How Did It Get Here, and How Does It Spread?
Native to the Mediterranean, the MOB has long coexisted with oak species that have developed natural defenses against it. But in California (and probably Oregon and Washington), where native oaks lack such resistance, the beetle has found a vulnerable new habitat. Scientists suspect it may have arrived through untreated wooden pallets, shipping crates or other solid wood packaging — one of the most common pathways for invasive pests.
Some have speculated that imported oak used in wine-barrel construction could be to blame, but Pramuk remains unconvinced.
“Barrel staves are heat-treated, which should kill any borers in the wood,” he said. “But shipping materials? That’s a different story.”
However it arrived, once established, the beetle’s containment has been hindered by inaction at the state and local levels. Unlike Sudden Oak Death, which is an “A-rated” pest in California, prompting extensive monitoring and containment efforts, MOB has no current state regulations to curb its expansion, as it is rated as a “B-rated” pest, meaning that management decisions are left to individual county agricultural commissioners rather than being subject to statewide regulation. Funding for bait-trap monitoring has been cut, according to Pramuk, and many utility companies and property owners unwittingly fuel the problem by leaving or transporting infested wood.
According to Restrepo, one of the biggest drivers of MOB’s spread is the movement of infected firewood.
“You see it all the time — someone cuts down a dead or dying tree and hauls the logs to another location,” he said. “They don’t realize they’re carrying the beetles with them.”
Pramuk added that improper tree removal has also accelerated the problem.
“Power line crews have taken down MOB-infested trees and just left the wood lying there [for people to grab],” he said. “That’s another way this beetle spreads.”
A Perfect Storm: Drought, Fire, Climate Change and the Edge Effect
Napa Valley’s oak woodlands are caught in a web of compounding environmental stressors that make them more vulnerable to threats such as the Mediterranean oak borer. Prolonged drought has significantly weakened these trees, disrupting photosynthesis and root growth. With depleted energy reserves and reduced defenses, oaks become easy targets for pests and diseases. Even after rainfall returns, full recovery can take years, leaving trees highly susceptible to secondary invaders such as ambrosia beetles and fungal pathogens. Symptoms of drought stress — wilting leaves, premature leaf drop and branch dieback — are often just the first signs of decline.
Adding to the crisis is the rapid increase in temperatures. Rising heat accelerates soil moisture loss and evaporation rates, further stressing already weakened trees. Warmer winters also allow pests like MOB to remain active longer and reproduce more frequently, expanding their range and accelerating infestations.
“In colder years, you’d expect natural die-off,” Restrepo said. “But with mild winters, the beetles just keep going.”
Fire compounds the issue. Healthy oaks typically act as natural firebreaks due to their thick bark and high moisture content. But drought-stressed or fire-scarred oaks become liabilities. Fire injuries create openings for fungi and pests, accelerating decay and mortality. Dead or dying trees left untreated after fires add to fuel loads, increasing the risk of future wildfires.
“Healthy oaks are very hard to burn, but dead oaks act as kindling, driving up fire risks and insurance costs,” Restrepo said.
The challenges are further magnified by the edge effect, a phenomenon where fragmented forests — such as those bordering vineyards, roads or developments — experience harsher conditions than interior forests. Oaks near forest edges face higher temperatures, stronger winds and lower humidity levels, which increase water loss and physical stress while making them more accessible to pests. Studies show that trees on forest edges suffer higher mortality rates due to these combined pressures.
Beetle Frass Puffs - Video by Bill Pramuk
The consequences of this “perfect storm” go far beyond aesthetics. The loss of mature oaks accelerates soil erosion, reduces carbon sequestration and disrupts habitats for countless species that depend on oak ecosystems. Rising temperatures, prolonged drought and pest invasions are creating a feedback loop of ecological instability.
Economically, the cascading effects threaten Napa Valley’s agricultural resilience while driving up costs for homeowners and businesses through increased fire risks and rising insurance premiums.
“When trees are stressed, they produce fewer defensive chemicals,” Restrepo said. “That makes them easy targets for pests like MOB and just more fuel for future fires.”
Technology in the Fight Against MOB
Restrepo relies on visual inspection coupled with advanced technology to assess tree health and detect MOB infestations before they become fatal. Using tools such as sonic tomography and resistographs, he can evaluate a tree’s condition without causing further damage.
“Sonic tomography works like an ultrasound,” Restrepo said. “It gives us a real-time image of the tree’s internal structure, showing areas of decay or weakness. Resistographs, on the other hand, measure resistance of the wood, allowing us to detect early-stage infestations.”
To minimize disturbance to the surrounding ecosystem, Restrepo employs specialized machinery designed for selective tree removal. Rotating grapple attachments allow precise handling of trees and underbrush, ensuring that nearby regrowth remains intact. Tracked woodchippers process felled trees efficiently, reducing fire risk, limiting pest spread and preventing erosion. Masticators selectively thin vegetation, lowering wildfire fuel while preserving the health of the remaining forest.
“We can’t just cut everything down,” Restrepo said. “That’s not management — that’s panic. Every tree and sapling we save is a chance for the forest to recover.”
A Call to Action: What Can Be Done?
There is no single solution, but landowners, policymakers and residents can take immediate steps to slow the spread and mitigate the damage. Pramuk and Restrepo recommend the following actions:
Early detection: Learn the signs of infestation-canopy dieback, small holes in the bark and frass at the base of the tree. If you spot these symptoms, contact a certified arborist immediately.
Proper disposal: Never store or transport infested wood as firewood. Instead, prune and chip the wood on-site or pile it in a sunny location and cover it tightly with a 6mm clear plastic tarp to contain the beetles.
Tree health management: Maintain tree resilience by reducing drought stress. Light irrigation at the drip line during dry months can help trees defend against infestation.
Stronger regulations and funding: Local and state officials must recognize the urgency of the issue. Without dedicated funding for monitoring, research and containment, MOB will continue its spread.
Community involvement: Report sightings of unusual or potentially invasive pests or plants in California by calling the CDFA Pest Hotline (800-491-1899), using the online Report a Pest Sighting Form and by contacting the Napa County Agricultural Commissioner's Office, which offers resources and guidance.
Yet even with these efforts, the challenge remains daunting. Without increased awareness and action, the region’s oak woodlands will continue to decline, their loss reshaping the landscape in ways that extend far beyond aesthetics.
Restrepo surveyed a grove of valley oaks, their thinning canopies and brittle branches marking the silent toll of the invasion. At the base of several trees, frass had already begun to pile up.
“These trees have stood for centuries,” he said. “If we lose them, we don’t get them back in our lifetime. This isn’t just about trees — it’s about the future of Napa Valley and beyond.”
Today’s Polls:
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Under the Hood: How Political Perceptions May Impact the Wine Industry
Under the Hood: Joanne Dickenson DePuy, the Woman Who Helped Change Napa Forever
Under the Hood: Cook St. Helena Marks 20 Years of Local Flavor
Under the Hood: Jennifer Bennett on Napa’s Changing Restaurant Landscape
Explore all Napa Valley Features stories on our main page.
Tim Carl is a Napa Valley-based photojournalist.
Recent Poll Results
In "Under the Hood: Napa Valley’s Economic Reckoning," the economic vulnerabilities of Napa Valley take center stage. Once celebrated as a resilient region of perpetual growth, Napa is now grappling with stagnation, declining tourism and challenges such as climate change, public health concerns and economic over-specialization. Through a series of reader polls, Carl explored reader sentiment regarding the region’s future, revealing widespread concern about its economic trajectory and emphasizing the urgent need for data-driven solutions to address these challenges.
Question: What is your primary connection to Napa Valley?
Summary: Napa Valley connections. Most respondents (85%) identify as residents, while 8% are wine industry professionals. Visitors, investors and general observers account for a combined 7%. Total responses: 72.
Question: How concerned are you about Napa Valley's economic future?
Summary: Concerns about Napa Valley’s economic future. A majority of respondents express significant concern, with 38% extremely concerned and 47% moderately concerned. Fewer are slightly concerned (10%) or not concerned (3%), while 2% remain uncertain. Total responses: 60.
Question: "The challenges facing Napa Valley require urgent, data-driven solutions."
Summary: Opinions on Napa Valley’s challenges. Nearly all respondents strongly agree (47%) or agree (41%) that urgent, data-driven solutions are necessary. A smaller group (12%) is neutral or unsure, with no respondents disagreeing. Total responses: 59.
Question: What do you believe is the biggest challenge for Napa Valley’s future?
Summary: Napa Valley’s biggest challenges. Over-reliance on wine emerges as the dominant concern (63%), followed by economic mismanagement (13%) and declining tourism (10%). Climate change and public health concerns are each cited by 7% of respondents. Total responses: 60.
The Economy
Economic Pulse Indicators Local:
The unemployment rate in Napa County remained steady at 4.3% in December 2024, slightly higher than December 2023's 4.1%, compared to California's unadjusted rate of 5.2% and the national rate of 3.8%, with employment across industries showing minimal changes and a total workforce of 79,600 jobs. (Source)
As of Jan. 28, Napa County had 1,895 Type-02 winery licenses, which was up six from last week. (Source)
As of Jan. 28, California has 6,793 Type-02 winery licenses, which was up 15 from last week. (Source)
The Civilian Labor Force in Napa County was 70,051 people in November 2024, with a historical high of 78,776 in October 2009 and a low of 54,459 in January 1991, according to the United States Federal Reserve. (Source)
Employment in the Leisure and Hospitality sector for Food Services and Drinking Places in Napa County stood at 7,381.53 as of November 2024, reflecting a slight decline of 1.6% from its peak of 7,589 in June 2019. This flattening trend contrasts sharply with the rapid 62.8% growth from 2009 to 2019, when employment rose from 4,661 to 7,589, signaling a potential disconnect between prior growth expectations and the stagnation observed over the past five years. (Source)
The median listing price for housing in Napa County fell to $1,448,000 in December 2024, reflecting a 19% decline from the May 2023 peak of $1,788,500 and a 3.4% month-over-month decrease from November 2024 ($1,499,000). (Source)
The average home value in Napa County is $888,678 as of Jan. 28, reflecting a 0.1% year-over-year decrease and a slight 0.12% decline from $889,760 last week, with homes now going pending in approximately 39 days compared to 27 days two weeks ago. (Source)
Since 2020, Napa County’s population growth rate has decreased 3.93% (down 5,397 since 2020) and is expected to decline significantly in the coming years. (Source)
The population pyramid for Napa County, based on U.S. Census data, is skewed toward older age groups, with the largest populations in the 55-to-74 age bracket and a narrower base, indicating a smaller proportion of younger individuals under 20. (Source)
The median age in Napa County continues to rise. (Source)
Economic Pulse Indicators United States:
Wine production in the United States totals 806.1 million gallons, with California contributing 680.3 million gallons, making up approximately 84.4% of the nation's total wine production. (Source)
California’s overall population is declining but not as fast as Napa County’s. (Source)
Total nonfarm payroll employment increased by 256,000 in December 2024, while the unemployment rate remained at 4.1%, with notable job growth in health care, government, social assistance and retail trade. (Source)
In the week ending Jan. 18, initial unemployment claims rose by 6,000 to 223,000, while the insured unemployment rate remained at 1.2% and insured unemployment increased to 1,899,000, the highest since November 2021.(Source)
The most recent household and payroll survey increased slightly to 155,232. (Source)
In December 2024, the Consumer Price Index rose 0.4% seasonally adjusted, driven by a 2.6% increase in energy costs, while the all-items index increased 2.9% over the past year, with food up 2.5% and core inflation (less food and energy) up 3.2%. (Source)
In December 2024, the Producer Price Index for final demand rose 0.2%, driven by a 0.6% increase in goods, led by a 3.5% rise in energy prices, while services prices were unchanged; over the year, the PPI increased 3.3%. (Source)
The recent un-inversion of the U.S. 10-year Treasury yield curve, traditionally a recession indicator, signals reduced immediate recession fears, though ongoing economic challenges such as a tight labor market and global manufacturing slowdowns suggest risks remain. (Source)
Gas prices were down slightly from last week. (Source)
Credit card debt reached another all-time high of $11,089 trillion as of Jan. 8, surpassing the previous historic peak recorded in December 2024. (Source)
U.S. retail trade and food services totaled $729.191 billion in December 2024, up from $724.609 billion in November. This reflects continued growth following a year-over-year increase of 0.43% reported in September, based on the latest seasonally adjusted data. (Source)
The S&P Global U.S. Manufacturing PMI increased to 50.1 in January from 49.4 in December 2024, indicating slight growth in manufacturing as factory production and new orders rose modestly, employment grew at its fastest pace since July, and inventories fell sharply due to higher input usage. (Source)
Economic Pulse Indicators Global:
Global GDP is expected to remain sluggish in 2025. (Source)
The United States maintains one of the lowest unemployment rates in the world. (Source)
After a brief dip, container rates continue to rise and remain at historically elevated levels. (Source)
Japan's GDP grew by 0.2% quarter-on-quarter in Q3, slowing from a revised 0.5% in Q2, with a boost in private consumption (0.9%) and government spending (0.5%) offset by a decline in business investment (-0.2%) and a 0.4 percentage point drag from net trade as imports outpaced exports, reflecting a fragile recovery amid external challenges. (Source)
The United Kingdom's GDP grew by 0.1% quarter-on-quarter in Q3, down from 0.5% in Q2 and below forecasts of 0.2%, with gains in services (0.1%), construction (0.8%) and business investment (1.2%) offset by a 0.2% contraction in production, primarily due to a 2.7% drop in energy supply. On the expenditure side, net trade, household spending (0.5%) and government consumption (0.6%) also rose, while gross capital formation declined. Year-over-year, GDP rose by 1%, up from 0.7% in Q2. (Source)
Germany's GDP grew by 0.2% quarter-on-quarter in Q3, recovering from a revised 0.3% contraction in Q2 and surpassing expectations of a 0.1% decline with boosts from government and household consumption, though year-on-year GDP still shrank by 0.2%, marking six consecutive quarters without growth. The government anticipates a 0.2% contraction for 2024 due to challenges such as high energy costs and weak external demand but forecasts 1.1% growth in 2025 with expected improvements in consumption, exports and investment. (Source)
China’s annual inflation rate eased to 0.1% in December 2024, down from 0.2% in November, marking the lowest level since March and highlighting growing deflation risks despite government and central bank support; food prices fell (-0.5%), non-food prices rose (+0.2%) and core inflation (excluding food and energy) increased 0.4% year-over-year, the highest in five months. (Source).
China's producer prices fell by 2.3% year-over-year in December 2024, the 27th consecutive month of deflation but the smallest decline since August, as Beijing's stimulus efforts helped ease the contraction in production materials (-2.6%) and consumer goods (-1.4%); on a monthly basis, prices edged down 0.1%, and for the full year, factory-gate prices contracted by 2.2%. (Source).
Consumer Confidence in China fell to 86.20 points in November 2024 from 86.90 points in October, remaining below the historical average of 109.34 points and close to its record low of 85.50 points in November 2022. (Source)
China’s property market continues to falter. (Source)
What We Are Reading:
As cannabis sales continue to grow, with cannabis drinks projected to reach $117 billion by 2032, the wine industry faces challenges as consumers increasingly replace alcohol with cannabis, driven by changing cultural attitudes, health trends and the rise of "California Sober" lifestyles, though legal complexities and political uncertainties could impact future market growth. (Source)
Vineyard owners in Sonoma and Napa counties fear significant humanitarian and economic impacts from Trump’s immigration crackdown as farmworkers, many of whom are undocumented or have family members at risk, face deportation, exacerbating an already critical labor shortage in California’s wine industry. (Source)
The U.S. no-alcohol market is projected to grow at an 18% volume CAGR from 2024 to 2028 reaching nearly $5 billion by 2028 with no-alcohol beer as the primary driver while segments like RTDs and spirits see more than 20% growth; premium products dominate, millennials lead consumption and barriers such as price, taste and availability persist despite innovation and rising social acceptance driven by sober influencers. (Source)
A University of Sheffield study in the UK suggests that promoting the Drink Less app through mass media and primary care could prevent 4,500 deaths, reduce hospital admissions and save the UK's National Health Service approximately $630 million over 20 years by helping people reduce their alcohol intake. (Source)
California's hotel development slowed significantly in 2024 due to economic and construction cost challenges though casino hotels, particularly on tribal land, thrived with Napa County seeing no new openings but three hotels under construction and 35 planned projects totaling 3,575 rooms. (Source)
A new shop in Bordeaux, Belles Grappes, is dedicated to dealcoholised wine and alcohol-free alternatives, reflecting a growing trend as winemakers and consumers increasingly explore no- and low-alcohol options, with European policymakers and producers adapting to shifting market demands. (Source)
Satisfaction with U.S. democracy remains at dismal levels, with 61% dissatisfied, despite an increase among Republicans, who rose from 17% to 33% after Trump’s 2024 election victory, while Democrats dropped from 38% to 35% and independents rose from 27% to 34%. (Source)
By the end of the century, heat-related deaths in Europe are projected to surpass cold-related deaths, with a nearly 50% increase in temperature-related mortality overall, especially in Mediterranean regions, unless rapid climate adaptation and mitigation actions are taken. (Source)
A group of experts has outlined five steps to combat fraudulent research from paper mills that produce fake scientific papers to exploit publication metrics, emphasizing the need for education on spotting fake research, publishing best practices, collaboration among publishers and further research into how these mills operate. (Source)
According to Nature Medicine, medicine has transformed over the past 30 years with advancements such as genome editing, immunotherapy, AI and mRNA vaccines, but the future depends on addressing global health inequities, climate challenges and ensuring universal access to innovations with a proactive, personalized and preventive healthcare model envisioned by 2055. (Source)
COVID-19 provided key insights into the immune system, showing that T cells play a crucial role alongside antibodies, the innate immune system triggers body-wide defenses, the nasal lining is vital for immunity and vaccines must target specific immune environments to be most effective. (Source)
Swiss farmers have sued their government for failing to act on climate change, arguing that rising temperatures, extreme weather and government inaction threaten their livelihoods and violate their rights, marking a shift toward farmers advocating for stronger climate protection policies. (Source)
Another informative Sunday piece. Public awareness is key as a first defense in dealing with this threat. Thanks to this wake up call I now know what to look for with the oaks surrounding my home and in our community.