What Small Stowaway is Killing California’s Trees?

Have you ever heard of a little beetle named the Polyphagous Shot Hole Borer (PSHB)? Despite their small size, these little terrors (stowed away from Asia) are now packing a big punch, killing trees in droves in Southern California.

How Trees Die

Pregnant female beetles attack hundreds of tree species, boring under the bark and depositing fungi namely Fusarium euwallacae along the way to feed their young. The fungi cause a disease in some trees called Fusarium Dieback, which is lethal. The fungi growth disrupts the flow of water and nutrients to host trees, and as soon as the tree dies, expectant beetles fly away to look for a new host.

External signs include whole branch dieback, wilting, gumming on trees’ bark, staining and white sugary discharge surrounding the small (0.1 inch, about the size of the tip of a ball point pen), perfectly round entry/exit holes. Inside a tree, Fusarium euwallacae can cause a black to brown discoloration on the wood.

What’s On The Menu?

Interestingly, the PSHB has a pretty indiscriminate palate. The beetle may try dining on hundreds of species, but only about 30 are susceptible to Fusarium Dieback including some favorites like coast live oak, California sycamore and avocado.

What You Can Do

Become a Citizen Scientist! If you see a suspected tree infestation, report it on the iNaturalist website, under the SCARAB Project, the tool scientists are asking the public to use for documenting PSHB sightings. It’s easy to use: snap a close-up photo of the beetle exit hole, identify the tree or shrub if you can (you can also check the box “ID Please?” if you don’t know), describe what you saw, where it is and then click on “Save observation.” Easy.

To Learn More



By Jessica Jewell

Jessica is TreePeople’s Director of Marketing and Communications. A native Southern Californian, you can find her wandering local trails, hanging out with her sidekick, Penny the Rescue Puppy, on her yoga mat, reading some Phillip K. Dick, or in her kitchen cooking up vegan confections.

Jessica's professional background goes back to her time working at PETA where she worked on celebrity campaigns, and a stint in the entertainment industry, where she worked at Technicolor on their Global Marketing Team.