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Biology of Two Invasive Ambrosia Beetles, the Polyphagous and Kuroshio Shot Hole Borers (Euwallacea spp.), in California

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The polyphagous shot hole borer (PSHB) Euwallacea fornicatus and the Kuroshio shot hole borer (KSHB) Euwallacea kuroshio are two invasive species of concern in Southern California. Both are ambrosia beetles, small, wood-boring weevils that farm and feed on fungi inside of host trees. PSHB and KSHB attack living and apparently healthy trees, and their fungi are phytopathogens that, together with boring damage from the beetles, are responsible for the plant disease Fusarium dieback. PSHB and KSHB were first discovered in California in 2003 and 2014, respectively, and until recently many aspects of their biology were unknown. The goal of this dissertation was to expand the body of knowledge pertaining to these two species in their invasive range in California, using both applied and fundamental research. In Chapter 1, I describe a series of field experiments that were performed to assess the use of various lures and repellents for PSHB and KSHB, as well as the effect of trap modifications on beetle capture. I find that quercivorol is a suitable lure for both species, and that both species are repelled by verbenone and piperitone, which may prove useful in deterring beetle entry into naïve areas. In Chapter 2, I reared KSHB at a series of constant temperatures to determine its effect on their development and emergence. The upper and lower temperature thresholds for KSHB development were found to be about 32°C and 13°C, respectively, and optimal development occurs at about 28°C. I also predict the annual number of KSHB generations for invaded areas in California using climate data from the California Irrigation Management Information System. In Chapter 3, I performed experiments to determine if PSHB and KSHB could survive and reproduce on each other’s fungal symbionts. Both beetles could subsist on the others’ Fusarium symbiont, but reproduction was reduced on auxiliary Graphium and Paracremonium fungi, as well as non-ambrosial Fusarium species. Finally, in Chapter 4, I perform population genetic analysis of California populations of PSHB and KSHB to measure genetic diversity, admixture, and outbreeding. Rates of outbreeding and heterozygosity were higher than expected under the accepted hypothesis of strict sibling mating.

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