The mechanisms that drive how parasites ultimately end up feeding upon different host species can have profound consequences for our expectations of parasite range expansion, extinction risk, and in disease risk. Optimal foraging theory suggests natural selection should favor parasites that selectively feed upon high quality hosts and avoid low quality hosts, but only if they can expect to encounter, and thus have a choice between, multiple hosts of multiple species. In this scenario, parasites will feed upon host species in proportion to their quality as a host. If host encounters are rare, the risk of starvation would outweigh the fitness costs of feeding upon a low-quality host, and the parasite should attempt feeding upon any host they encounter. Here, parasites will feed upon host species in proportion to their encounter rates with those species. Determining the underlying mechanisms of parasite host selection requires breaking this complex behavior into its component parts and empirically examining each part through precise and ecologically realistic experiments.

I found that while blacklegged ticks encounter hosts frequently enough for selective feeding behaviors to be beneficial, at least in theory, laboratory experiments examining tick responses and attachment rates to host species show that while ticks differentially attach to host species, it does not scale with the quality of the host species. Instead, the probability a tick attaches to a host may be related more to host qualities that impact the hosts ability to “sweep” up ticks from the environment, rather than ticks choosing to attach. Collectively, this work suggests that observed patterns of blacklegged ticks feeding upon different host species may be driven by encounter rates with different host species, modified by how effective the host species is at sweeping up ticks from the environment.

Here, I have broken the blacklegged tick host selection process into discrete stages — from encountering a host to climbing onto a host — to empirically evaluate how juvenile blacklegged ticks select (or perhaps do not select) their hosts. By breaking down the host selection process into discrete stages where ticks may or may not exhibit differential responses to host species, I can determine if the underlying mechanism driving observed patterns of blacklegged ticks’ feeding upon host species is variation in encounters with host species or tick preferences for high quality host species.