Spatiotemporal dynamics of the Southern California Asian citrus psyllid invasion

The Asian citrus psyllid (ACP), a vector of the bacterial pathogen causing huanglongbing (HLB), poses a serious threat to commercial and residential citrus trees. In 2008, ACP first began expanding northward from Mexico into parts of Southern California. Using geo-referenced ACP occurrence data from 2008-2014, we sought to better understand the extent of the geographic expansion of this invasive vector species. Leveraging a suite of spatial statistics and epidemiologic methods, our objectives were to: 1) describe the spatial and temporal distribution of ACP in Southern California, 2) identify the locations of statistically significant ACP clusters and hot spots, 3) quantify the extent of anisotropic spread, and 4) assess patterns of seasonality in ACP across the study area. We found clear evidence that the spatial and temporal distribution of ACP in Southern California is non-random. Further, we identified the existence of statistically significant hot spots of ACP occurrence and described the anisotropic dispersion across the landscape. For example, the dominant hot spot surrounding Los Angeles showed rapid and strongly asymmetric spread to the south and east. Improved understanding of the spatial heterogeneities in the introduction, establishment, and spread of the ACP in Southern California is important to detection and management of this invasive vector. Our study demonstrates the feasibility of quantitative invasive insect risk assessment with the application of a spatial epidemiology framework.