The Polyphagous and Kuroshio shot hole borers (SHB; Euwallacea spp. nr. fornicatus, Curculionidae: Scolytinae) are recent invaders of southern California riparian habitats. These ambrosia beetles have a broad host range including many native, dominant riparian woody genera (Platanus, Quercus, Salix, Populus). SHBs carry obligate fungal symbionts that are inoculated into host trees and block the host’s xylem, leading to branch dieback or mortality, and potentially causing extensive dieback of riparian trees. SHBs are present in seven southern California counties; however, their distribution and dynamics in their northern range are not well documented. To characterize SHB distribution in riparian systems, we deployed traps baited with quercivorol, an attractant specific to this species complex. We deployed 59 traps in Ventura County, 26 traps in Santa Barbara County, and five traps in xeric regions of San Bernardino County. Traps were monitored biweekly from September 2016 to December 2017, during which SHB flight activity was evaluated by tracking variations in trap catch. The effects of soil moisture and local SHB abundance on SHB attack rates were tested experimentally using potted Salix cuttings imposed with varying water regimes, and installed in wet vs. dry locations, and in sites with varying neighborhood infestation severity.
Results/Conclusions
SHB were detected in 29% and 15% of traps in 2016, and increased to 42% and 23% in 2017, in Ventura and Santa Barbara, respectively, indicating range expansion from initial detection locations in both counties from natural or human facilitated dispersal. SHB was not detected in San Bernardino County. Abundances fluctuated during the year, ranging from 0 to 89 beetles per monitoring period (mean=24), with activity spiking in November, May and July-August. No beetles were intercepted in December through March, suggesting low dispersal rates during winter. Irrigation had no effect on SHB attack rate (W=48, p=0.54), however, attack rate was higher in areas of higher neighborhood infestation severity (F(1, 30)=21.7, p < 0.001; R2= 0.42) and in areas closer to water (F(1,30)=10.57, p=0.003; R2= 0.26). Although trees nearer to water were more severely infested, SHB host preference was not dependent on wetter soil. Proximity to heavily infested neighbors may be a more important predictor of SHB colonization. As SHB’s range expands, monitoring is vital to tracking SHB dispersal and characterizing factors associated with host colonization. Surveys will be expanded northward into cooler latitudes and eastward into drier regions to understand SHB population expansion and limits to establishment.