Pest profile

Invasive Shothole Borers

Euwallacea fornicatus and Euwallacea kuroshio · Coleoptera: Curculionidae[2]

Invasive shothole borers are tiny ambrosia beetles that carry Fusarium dieback, which has killed tens of thousands of Southern California trees.[1]

Invasive shothole borer sign on a tree trunk, a tiny entry hole with dark wet staining, with a pen tip for scale
Invasive shothole borer sign on a tree trunk, a tiny entry hole with dark wet staining, with a pen tip for scale. (c) Jessleena Suri, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0

How to identify it

  • Females are 1/16 to 1/10 inch and brown to black; the smaller males can't fly.[1]
  • Entry holes are perfectly round, about the size of a ballpoint pen tip, often with wet staining, fine boring dust, gumming or sugary crust.[1]

Look-alikes

Look-alikeHow to tell them apart
Native ambrosia and bark beetlesMost native look-alikes attack only stressed, injured or dying trees, while invasive shothole borers colonize mostly healthy trees.[1]

Where it lives

By early 2023 they were in seven Southern California counties, from Santa Barbara and Ventura to San Diego and Riverside.[1]

US map: state covered by the cited source (seven southern california counties by early 2023).

Biology and life cycle

Females bore galleries, lay eggs and grow the fungus that larvae and adults eat; the beetles don't eat wood.[1]

Development takes 25 to 50 days, and models suggest 5 to 11 generations a year in Southern California.[1]

Damage and signs

  • Over 78 tree and shrub species are breeding hosts, including California sycamore, willows, cottonwoods, box elder, valley oak and Japanese maple.[1]
  • Effects range from branch dieback to tree death, depending on the host.[1]

Health risk

It attacks trees; UC IPM mentions no risk to people.[1]

Control

Prevention

Monitor host trees regularly for entry holes, and keep trees healthy with good cultural care.[1]

Sanitation

Prune infested branches promptly before beetles reach the trunk, and remove severely infested trees following proper wood-disposal rules.[1]

When to call a professional

Management choices depend on tree species, value, infestation level and hazard; UC IPM's matrix guides when pesticides may fit.[1]

Common questions

What are the tiny round holes with wet stains on my sycamore?
They may be invasive shothole borer entry holes, about the size of a ballpoint pen tip.[1]
Can invasive shothole borers be eradicated?
UC IPM says eradication is unlikely where established, but consistent management can contain infestations.[1]

Sources

  1. University of California Statewide IPM Program, “Invasive Shothole Borers (Pest Notes).” ipm.ucanr.edu/home-and-landscape/invasive-shothole-borers/ (accessed 2026-10-06)
  2. iNaturalist, “Euwallacea fornicatus taxon page (order and family).” www.inaturalist.org/taxa/885742 (accessed 2026-10-06)

Updated 2026-10-14. Built from sourced records. Spot an error? Tell us.