In the last 60 years the Lone Star Tick (LST, Amblyomma americanum) has rapidly expanded from its historical southeastern range as far north as Maine and Michigan and as far west as Oklahoma. LST carries a wide variety of pathogens and is the most common tick found attached to humans in the eastern United States. Although LST has been expanding westward, a recent correlative model predicts a stable western range boundary around central Texas and western Oklahoma due to the dry conditions found west of this region. Despite its generally dry climate, Southern California has several vegetation types, such as riparian woodland and chaparral, which create microclimates that are cooler and more humid than the surrounding areas, and may be suitable habitat for LST. We conducted two experiments to test how climatic factors influence the survival and behavior of LST and thus affect LST expansion into relatively dry areas like Southern California where LST expansion has been considered very unlikely. In the first experiment, we placed adult ticks at four different field sites to assess their survival rate in Southern California microclimates. In the second experiment, we placed adult ticks in questing arenas within environmental chambers set at 40, 35 and 30 ◦C to assess how temperature affects LST questing behavior.
In the field experiment, LST survived significantly longer in shaded sites than in sunny sites. During a heat wave, every tick at one sunny site died but every tick at a shaded site survived. We observed a significant difference in relative humidity between shaded and sunny locations, which likely explains why survival rates differed. In the laboratory experiment, LST questing frequency at 40 ◦C was significantly higher than at lower temperatures. LST may quest at a higher frequency at 40 ◦C because they are drying out and sense the need to find a host quickly. Questing height did not change with temperature. Currently we are assessing LST survival in a range of temperatures and relative humidities, simulating conditions similar to those in arid environments. Our experiments compel us to reevaluate the theoretical niche of LST and reconsider the potential of this important disease vector to establish in western North America.