The three most medically relevant ticks in the United States are the lone star tick (Amblyomma americanum), dog tick (Dermacentor variabilis), and deer tick (Ixodes scapularis). Together, they are major vectors of several viral, bacterial, and protozoan diseases of humans, livestock, and wildlife. The geographic distributions of all three tick species are expanding. Although correlational evidence suggests that climate change is driving the range expansion of ticks, experimental evidence showing how ticks survive and behave in different climatic conditions is lacking. We investigated how humidity influences the survival and questing behavior of these three species of ticks. We hypothesized that climatic stress (dryer conditions, in our case) reduces the survival and questing frequency of ticks, but the tolerance for climatic stress varies by species. We placed 72 adult females of each species into individual plastic tubes (20 cm height x 2.5 cm diameter) and separated them into three experimental relative humidity treatments representing distinct climates: 32% RH, 58% RH, and 84% RH. Every day for 40 days we assessed the survival and questing behavior of each tick.
Results/Conclusions
In all three species, survivorship significantly declined in dryer conditions. Survivorship was lowest in 32% RH, intermediate in 58% RH, and highest in 84% RH. Mean survival times of Amblyomma were 11.5 days, 18.3 days, and 38.5 days. Mean survival times of Dermacentor were 20.3 days, 32.1 days, and 37.7 days. Mean survival times of Ixodes were 2.8 days, 3.8 days, and 5.2 days. Contrary to our expectations, the frequency of questing behavior increased significantly with dryer conditions in Dermacentor, but not in Amblyomma or Ixodes. The heights at which ticks quested also varied with climatic conditions in Amblyomma and Dermacentor, but not Ixodes. Our results demonstrate that humidity conditions affect the survival and host-seeking behavior in ixodid ticks. Dry conditions decrease probability of survival and likely constrain range expansion into arid areas. However, humidity conditions also alter the propensity to quest and questing height, which may have important implications for risk of disease transmission.