Assessing the potential risk of lone star tick, Amblyomma americanum, establishment in California

The lone star tick (LST), Amblyomma americanum, is an aggressive human biter and important vector of the causative agents of tick-borne diseases. As geographic ranges of multiple Amblyomma species rapidly expand due to habitat changes and human and other animal movement patterns, there are concerns for establishment of LST in US states with suitable climates, such as California, especially given a dearth in gaps in tick surveillance. Importantly, previous niche modeling confirmed habitat in California along the length of the coast and coast range mountains. We sought data on LSTs and LST risk from 30 different entities including companies that identify and test ticks, natural history and entomology museums and collections, experts, and large databases. These queries and published records yielded records of 76 LSTs from California as well as two Amblyomma cajennense sensu lato, one Amblyomma sabanerae, one Amblyomma maculatum, and five Amblyomma dissimile. Among LSTs, there were 18 adult females, ten males, 13 adults for which sex was not recorded, 22 nymphs, two larvae, and 11 with no stage data. LSTs were recorded only during late spring through late summer, with all three feeding stages superimposed temporally. Active surveillance was conducted at nine sites considered high-risk due to earlier records of LSTs or high probability of establishment on niche modeling, using tick flagging, monitoring ovine and cervid hosts, and circulating a flier; no LSTs were recovered in this activity. We assessed risk for establishment of LSTs in California using invasion theory, prior niche modeling, literature review of ecological constraints on LSTs within their current range, data acquired in retrospective and prospective segments of this study, and expert consultation. Elevated risk is expected particularly along coastal California. Although some of these ticks likely were transported from out of state endemic regions, the findings of 76 LSTs, including all three feeding stages, in multiple counties raises concern that establishment in the future could occur in California.