Influence of deer abundance on the abundance of questing adult Ixodes scapularis (Acari : Ixodidae)

Nymphal and adult Ixodes scapularis Say were sampled by nagging at 2 sites on a. barrier island, Fire Island, NY, and at 2 sites on the nearby mainlan d. Nymphal densities did not differ consistently between island and mainlan d sites, but adult densities were consistently lower on the island. We test ed whether lower adult densities on the island resulted from greater nympha l mortality on the island than the mainland, or whether adult ticks on the island were poorly sampled by flagging because they had attached abundantly to deer, which were common on Fire Island. Differential nymphal mortality on islands versus mainland did not explain this difference in adult densiti es because survival of nat and engorged nymphs in enclosures was the same a t island and mainland sites. Ticks were infected by parasitic wasps on the island and. not the mainland, but the infection rate (4.3%) was too low to explain the difference in adult tick densities. In contrast, exclusion of d eer by game fencing on Fire Island resulted in markedly increased numbers o f adult ticks in nagging samples inside compared with samples taken outside the exclosures. Therefore, the scarcity of adult ticks in flagging samples on Fire Island resulted, at least in part, from the ticks being unavailabl e to nagging samples because they were on deer hosts. Differences in the de nsities of nagged ticks inside and outside the exclosures were used to esti mate the percentage of questing adults on Fire Island that found deer hosts , excluding those that attached to other host species. Approximately 56% of these questing adult ticks found deer hosts in 1995 and 50% found deer hos ts in 1996. Therefore, in areas where vertebrate hosts are highly abundant, large proportions of the questing tick population can find hosts. Moreover , comparisons of tick densities at different sites by nagging can be potent ially biased by differences in host densities among sites.