Author: Richard Stouthamer and Akif Eskalen

Ambrosia beetles — now known to California avocado growers as the Polyphagous and Kuroshio shot hole borers (PSHB, KSHB) — have invaded numerous areas outside their native range, causing substantial damage to avocado and other tree species.

Ambrosia beetles are highly successful as invaders because of their:

  • Wood-boring habit
  • Small size
  • Haplodiplody (unfertilized eggs become males and fertilized eggs become female)
  • High number of females in their population
  • Tendency of females to mate with their brothers within their native gallery, thus they do not need to seek mates outside of their gallery
  • Fungus farming, which allows them to colonize a wide range of trees

Drs. Richard Stouthamer and Akif Eskalen and their team conducted phylogenetic research to identify cryptic ambrosia beetle species (species that are morphologically identical but cannot interbreed) in an effort to determine the native range of the species that have invaded California. By identifying the beetles’ native ranges, researchers hope to identify the beetles’ natural enemies and determine whether they could be used for biological control of the pests.

To distinguish cryptic species of the ambrosia beetles, the research team conducted DNA barcoding on specimens they collected from Australia, Israel, Japan, Papua New Guinea, Sri Lanka, U.S., Vietnam, Taiwan and Thailand. They also investigated the phylogeography of the specimens — studying the processes governing the geographic distribution of the beetles.

Phylogenetic analysis showed there are four major clades (groups of organisms that have evolved from a common ancestor). The researchers determined the species that invaded San Diego County belongs to clade 2 (Kuroshio shot hole borer), whose native range is islands in the East China Sea: Taiwan and Okinawa. The third major clade, identified as polyphagous shot hole borer, invaded the Los Angeles area, with a native range including Northern Thailand, Vietnam, China, Taiwan and Okinawa.

The researchers collected many specimens of KSHB and PSHB but did not find evidence of interbreeding.

For more information, read “Species Delineation Within the Euwallacea fornicatus (Coleoptera: Curculionidae) Complex Revealed by Morphometric and Phylogenetic Analyses” and “Tracing the origin of a cryptic invader: phylogeography of the Euwallacea fornicatus (Coleoptera: Curculionidae: Scolytinae) species complex.”