Population regulation in ticks: the role of acquired resistance in natural and unnatural hosts.

Attachment, engorgement and subsequent development of successive infestations of Ixodes trianguliceps larvae and nymphs on natural hosts, Apodemus sylvaticus, and unnatural hosts, laboratory mice, are compared. In laboratory mice, primary infestations above a threshold level of about 10 ticks elicit an immunological response which reduces, in a density-dependent manner, the rate of successful tick engorgement during subsequent infestations. In contrast, in A. sylvaticus successive infestations of larvae result in unchanged or slightly improved survival through to nymphs. The relevance of these results to the concept of host-parasite co-evolution and to tick population regulation is discussed.