When summer heat sets in across Sonoma County, your trees aren’t just fighting drought stress—they’re under threat from aggressive, fast-moving pests. From bark beetles and borers to invasive beetles like Mediterranean Oak Borer, these pests thrive in warm, dry conditions and can quietly destroy trees before symptoms become visible.
At Tree Pro, we’ve spent over 30 years helping homeowners in Santa Rosa, Petaluma, Sebastopol, and surrounding towns protect their trees from these “tiny invaders.” If you’re seeing early signs of decline—or just want to get ahead of problems—this guide is for you.
1. Bronze Birch Borer: A Common Threat in Residential Landscapes
- Host trees: European white birch, paper birch
- Where it hits: Landscaped yards in Santa Rosa, Rohnert Park, Mill Valley, and Tiburon
- Symptoms: Upper canopy dieback, sparse leaves, D-shaped exit holes
- Risk Factors: Age, drought stress, poor irrigation, or pruning
Birch trees are beautiful but ill-suited for Sonoma’s hot, dry summers. Bronze birch borers are especially active now, boring into bark and disrupting a tree’s vascular system. They often attack older or water-stressed trees, making them a common sight in residential neighborhoods from Kenwood to Novato.
Pro Tip: Look for top-down dieback and call Tree Pro before the damage spreads to lower branches or neighboring trees.
2. Bark Beetles: Fuel for Fire, Killers of Pines
- Host trees: Monterey pine, ponderosa pine, and other conifers
- High-risk zones: Forestville, Healdsburg, rural Petaluma, western hills
- Symptoms: Pitch tubes, boring dust, fading needles, sudden collapse
- Why it matters: Beetle-killed trees are the top contributors to fire fuel loads
Sonoma County’s pine trees are especially vulnerable to bark beetles, especially in wooded areas and properties on the edge of wildland-urban zones. These pests introduce fungi that disrupt internal water flow, and by the time you see visible damage, it may be too late to save the tree.
Tree Pro’s ISA-Certified Arborists can assess infested trees and recommend whether treatment, removal, or defensible space pruning is the right move. Give us a call today at 707-285-9330.
3. Flatheaded Borers: Hidden Threat to Native Oaks and Maples
- Host trees: Coast live oak, valley oak, maple, ash
- Where to watch: Urban landscapes (Santa Rosa, San Rafael), sun-exposed sites
- Symptoms: Dieback, D-shaped holes, bark discoloration, sudden limb loss
- When they strike: Peak activity in summer heat
Flatheaded borers are a serious threat to oaks across Sonoma County—especially in areas where trees are exposed to reflected heat from pavement or rooftops. Because the damage happens internally, these borers are often missed until canopy loss becomes obvious.
Local Insight: In towns like Oakmont and Graton, where mature oaks are part of the natural canopy, routine monitoring is essential.
4. Mediterranean Oak Borer
- Host trees: Valley Oak and Blue Oak
- Risk zones: Most of Napa and Sonoma Counties)
- Symptoms: Small round holes, black staining, branch dieback
- Bonus Threat: Carries Fusarium dieback, a lethal tree disease
The Mediterranean Oak Borer (MOB), Xyleborus monographus, is an invasive ambrosia beetle that has become established in parts of California, particularly in oak trees. It was first discovered in Calistoga (Napa County) in 2019 and has since spread to Lake County and other areas. It’s important to report any suspected infestations to local authorities and take steps to prevent their spread.
Prevent It: Avoid moving firewood across regions and schedule a tree inspection if you see warning signs.
Regional Snapshot: Why Sonoma Trees Are Especially Vulnerable
The diversity of Sonoma County’s landscape—from foggy oak hills to dry valleys and irrigated vineyards—creates a variety of pest pressure points:
- Bay laurels in shaded forest zones help spread Sudden Oak Death.
- Urban heat islands and reflective surfaces (e.g., patios, roads) increase borer activity.
- Vineyard proximity often hides stressed trees affected by shallow root zones or overwatering.
- Fire-prone areas face compounding risk from pest-killed trees becoming fuel.
What You Can Do
Inspect your trees weekly. Look for exit holes, canopy thinning, sap, or sawdust at the base.
Irrigate wisely. Over- or under-watering contributes to tree stress, which pests love.
Call for professional help early. Treatable pests can become irreversible if caught too late. Tree Pro can help you get ahead of the issue.
Call Tree Pro Before the Damage Spreads
At Tree Pro, we’ve helped protect Sonoma’s trees since 1990 with expert diagnostics, safe treatments, and practical prevention plans. Our professional team knows the region, the pests, and how to preserve the health and safety of your landscape.