Mediterranean oak borer
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Steven Swain and Barbara Robertson
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Ambrosia beetles like the Mediterranean Oak Borer are only a few millimeters long but their size belies their capacity for destruction. Curtis Ewing
Ambrosia beetles like MOB are only a few millimeters long but their size belies their capacity for destruction. They don’t eat trees. They use trees like farm fields or veggie gardens. They bore tunnels into trees and sow fungi into these rows of tunnels. The fungi digest the trees, and the beetles eat the growing fungi. MOB’s favorite trees are the majestic valley oaks (Quercus lobata).
Valley oak, one of North America’s largest oaks, was once one of the most widely distributed in California. As its name implies, it grows best in deep, rich alluvial soils like those in river valleys. Soils that are also prime farmland. Huge numbers of valley oaks have been felled for farmland since the 1800’s. Even so, iconic trees still stand in vineyards and farm fields in Chico, Napa, Sonoma, San Rafael, Ojai, and beyond.
MOB Beetles prefer large, stressed valley oak trees. They rarely attack those with trunks less than six inches in diameter. Photo: King of HeartsLook for these symptoms: MOB prefers large, stressed trees; it rarely attacks trees with trunks less than six inches in diameter. Trees dying from MOB typically have a thin upper canopy, but with some leaves hanging on (unlike leafless limbs of drought-stressed trees.) This is because a MOB-infested tree often dies so quickly leaves don’t have time to form abscission zones (the scar tissue on a branch that lets leaves drop easily). Beetles tunnelling in the large branches cut off water to the smaller branches in the upper canopy then move down into the main trunk killing the tree. Symptoms in early summer often include clumps of tiny leaves that never fully flush out. Other maladies can produce these symptoms; however, MOB tunnels in cut wood are distinctive.
MOB-infected valley oak trees will have clumps of tiny leaves in early summer that never fully flush out. Photo: Philip BouchardThere are still many large, old valley oaks in Napa and northern Sonoma County, so the presence of the beetle is not a death sentence for every tree, though. Currently, the best approach to save valley oak trees is prevention.
When MOB was first recorded in Napa County, it followed a period of low rainfall, and the beetle seemed to spread rapidly. When the next few years brought adequate rain, the incidence of valley oaks attacked by MOB was vanishingly low. Then, it seems likely that the drought of 2021-2022 fueled the beetle’s recent expansion, allowing the pest to colonize trees that otherwise might have withstood their attacks. Most surviving trees had access to summer water.
Valley oak trees, once the most widely distributed trees in California, are threatened with destruction by a tiny beetle. Photo: Belinda LoAs a rule, mature oaks don’t like summer water, especially routine summer irrigation, but they can die of drought. In drought years, providing oaks with a deep watering once per month should be enough to give them the water they need without keeping their roots so constantly moist that they develop root rot. And that might be enough to keep MOB at bay.