UC Santa Barbara

Invasive species threaten biodiversity and ecosystem processes, with varying impacts among introduced regions. The polyphagous shot hole borer (Euwallacea whitfordiodendrus) (Coleoptera: Scolytinae) is an ambrosia beetle native to southeast Asia that has become recently established in southern California, infesting riparian, agricultural and urban tree species. A large dieback event at the Tijuana River Valley in 2016 was attributed to the beetle and their symbiotic fungi (Fusarium euwallaceae), but it is unclear whether this is expected to occur in other riparian areas throughout southern California or if, instead, beetle populations are exhibiting a boom-and-bust type population decline. This thesis characterizes the spread and impact of E. whitfordiodendrus in a major river in southern California, the Santa Clara River (SCR), by evaluating regional expansion over three years, testing the effects of host water availability and host species on beetle preference, examining host susceptibility to beetles or their symbiotic fungi among riparian host species and tracking the progression of a local riparian infestation over four years. We found that after an initially rapid expansion, E. whitfordiodendrus dispersal has tapered off. Beetles exhibited no preference among hosts with differing water status or among different host species. We did, however, find that that host species differed in their susceptibility to beetle tunneling activity, which was explained in part by differences in wood density. Susceptibility to growth of the beetle’s symbiotic fungus also differed between species, with Populus fremontii being the least susceptible to fungal growth. Differences in host susceptibility may result in differing infestation severities in riparian areas which can vary in dominant tree species throughout a floodplain. Infestation rates at the SCR increased from 2017-2019 but were stabilized or lower in most plots by 2020. Salix lasiolepis was the most commonly and severely infested species in the field, however many infested individuals were resprouting by 2020. Overall, we did not find evidence to support a large-scale dieback event at the SCR and E. whitfordiodendrus populations may be entering a population ‘bust,’ yet more work is needed to confirm this.