Associate Professor Washington State University Pullman, Washington, United States
Abstract: Blacklegged ticks (Ixodes scapularis) are one of the most medically and economically important ectoparasites in North America. Each of their three life stages require a single bloodmeal from one of many potential host species during which they can acquire or transmit pathogens. Host species vary tremendously in their capacity to feed and infect ticks. Therefore, host community composition, and the probability ticks feed upon certain species, strongly regulate tick populations and tick-borne disease risk. Moreover, we might expect ticks to select and preferentially feed on more permissive host species. A key question, however, is whether host-seeking ticks often or rarely encounter potential hosts prior to dying. Using a novel camera trap system capable of detecting all potential host species, from shrews to bears, we directly estimated the number of hosts ticks can expect to encounter prior to starvation. We placed cameras at the Cary Institute of Ecosystem Studies, which has a long-term mammal trapping program on the property, in the Hudson Valley of New York in 2021 and 2022—two years that varied greatly in mammal abundance. We expanded the project to an additional site with different vegetation, the Huyck Preserve, in 2022 to see if landscape factors influenced host encounter rates. In a year with very low mammal abundance, encounters were rare, and in a year with higher mammal abundance, encounters were more common. However, even in the more abundant year a larval tick might expect only 3.7 host contacts prior to starving, on average. Potential encounter rates varied a great deal between sites, and locations within sites, so individual ticks likely have many more or fewer potential contacts. Moreover, contact with a host does not guarantee a tick successfully attaches to a host, and ticks may require multiple contacts before successfully attaching to a host. Collectively, these results suggest contact with host may be a limiting step in the life cycle of blacklegged ticks, with many ticks likely to starve before ever encountering a host. Additionally, drawing upon optimal foraging theory, these results suggest that ticks should not preferentially feed upon high quality hosts, as few individual ticks will encounter enough hosts to benefit from being selective. This work highlights the need for direct, empirical measurements of contact rates between parasites and hosts in order to better understand parasite life history.
