Southern California is experiencing an unprecedented die-off of the trees greening, shading, cooling and feeding the cities in the region. During several years of observing forests and green spaces, botanists have watched insects and diseases eat up over 100,000 willows in San Diego.

Greg McPherson, a supervisory research forester with the U.S. Forest Service, found that the polyphagous shot hole borer beetle could kill as many as 27 million trees in Los Angeles, Orange, Riverside and San Bernardino counties, including parts of the desert. That’s roughly 38% of the 71 million trees in the 4,244 square-mile urban region (with a population of about 20 million people).

Beetle on the bark of a sycamore tree, one of many trees affected by the infestation

The polyphagous shot hole borer beetle on a sycamore tree in Craig Regional Park in Fullerton.
Mark Boster / Los Angeles Times

However, in San Diego, it was the polyphagous beetle’s cousin, the Kuroshio shot hole borer, that infested more than 144,000 willow trees in Tijuana River Valley Regional Park last year, officials said. These pests arrive almost daily via global trade and tourism, local transportation systems, nurseries and the movement of infected firewood.

An example of larvae wiggling inside infested trees in California

The larvae of a Pacific flatheaded borer wiggles and moves around in the caverns of an infested tree in Craig Regional Park in Fullerton.
Mark Boster / Los Angeles Times

Many of California’s trees, including ones that produce food, are exposed to much danger. The U.S. Forest Service has named a number of examples (including live oak, avocado, citrus, ash, almond, peach and sycamores) that are among the most endangered by this insect.

“Here’s the sad news about sycamores,” said Akif Eskalen, a University of California, Riverside plant pathologist. “If we cannot control the shot hole borer, it will kill all the sycamores in California. And when they’re done with sycamores, they’ll move to other trees.”

2 scientists and 1 research assitant observing trees

Dr. Akif Eakalen, plant pathologist at UC Riverside, Dr. John Kabashima and research assistant look at an infested tree in Craig Regional Park prior to it being cut down.
Mark Boster / Los Angeles Times

According to Eskalen, pathologists have known that the shot hole borer was transmitting a fatal fungal disease to 19 species of trees in Southern California since 2012. So far, scientists have identified 30 additional host species.

But the problem is much bigger than a pest. McPherson says that many of the trees grown evolved in temperate climates and can’t tolerate the stress of drought, water restrictions, higher salinity levels in recycled water and wind.

During windstorms in late March, trees in Niagra weren’t healthy enough to receive the wind with a steady build, so 26 were toppled due to age and disease; adding up to another 1,000 dead or dying trees at the Falls, according to City Administrator Nick Melson.