San Jose could lose thousands of trees to invasive beetle

An invasive beetle killing a dramatic number of Southern California trees has found its way to San Jose — and it could threaten the urban forest that shades and cools Silicon Valley.

Santa Clara County officials aren’t sure when or how the species, known as the shothole borer beetle, arrived in San Jose. The non-native species was first detected in Southern California in the early 2000s. The beetle was identified locally in November last year, but may have been in San Jose undetected for several years. Now San Jose officials are trapping the bugs while county officials survey the city to determine the extent of the infestation.

Officials have tracked the bug around riparian areas like Coyote Creek and the Guadalupe River. But the beetle — which has already killed tens of thousands of box elders, California sycamores, valley oaks and more across seven counties in Southern California — is also infesting older and stately landscape trees in San Jose.

“These are large sycamores that have been here for years and years — and now they’re dying from this beetle,” Drew Raymond, acting agricultural commissioner for Santa Clara County, told San José Spotlight.

The pest could spell trouble for the region’s urban canopy, which cools surrounding neighborhoods. That’s a crucial resource for underserved areas across the county, such as East San Jose, where urban heat islands and a lack of shade make hot and sunny days more unbearable. San Jose’s tree coverage has dropped dramatically over the years. From 2012 to 2018, the city lost nearly three square miles of trees — a 1.5% overall drop in tree canopy.

After an internal audit found the city woefully mismanaged tree loss, the San Jose City Council in January 2023 voted to change the rules around tree removal permits.

County Supervisor Sylvia Arenas said the beetle presents a danger to the region’s tree shade.

“At a time when tree canopies can make such a difference in our neighborhoods, this invasive pest becomes a real pest for our community,” she told San José Spotlight.

The local shothole borer beetle discovery comes after urban foresters have already been monitoring another tree-killing beetle species, the gold spotted oak borer, for a couple of years since it was first found in San Jose. But so far they’ve stayed there, according to Jean-Paul Renaud, executive director of Canopy, a Palo Alto-based group dedicated to planting trees in the mid-peninsula.

“I know that there are traps laid out throughout Palo Alto and they’re regularly checked for it but nothing so far. We’re all holding our breaths,” Renaud told San José Spotlight. “This speaks to the importance of diversity in our tree species when planting in an urban setting so that one pest or disease doesn’t affect an entire tree inventory.”

Raymond said infested tree removal is already underway in San Jose to prevent the safety risk of a falling tree. But that will come with a heavy price tag.

“Because the shothole borer threatens larger trees, they would cost thousands of dollars to replace. It poses a threat to trees that have pretty substantial value. To replace a tree is not cheap,” Raymond said.

One can spot the early symptoms of a beetle attack through wet staining and discoloration on the bark of a tree’s main stem and branches. An infested tree will also show an entry or exit hole that is perfectly round and the size of a ballpoint pen tip. Other signs include gumming, insect waste, boring dust and sugary buildup around entry holes.

Researchers have confirmed the pest can kill 17 species of tree. But the beetle can live in dozens of other tree and shrub species, such as the California buckeye, coast live oak, avocado and acacia.

The beetle is tiny, with females comparable in size to a sesame seed and males being even smaller. Yet the insects have exacted enormous damage by tunneling into host trees and introducing a fungus that serves as food for the beetles and their larvae. The fungus leads to disease which prevents trees from transporting water and nutrients, damaging and killing them.

The bug can also spread through residential green waste collection. And there are certain steps to take to remove the pest so that doesn’t help it spread. Infested wood needs to be chipped down to one inch or less and further sanitized by composting or drying the wood out through a process known as solarization.

“Unfortunately there is no regulation on moving wood,” Raymond said. “Our biggest weapon honestly is education.”

Contact Brandon Pho at [email protected] or @brandonphooo on X, formerly known as Twitter.