Host-odour recognition in two tick species is coded in a blend of vertebrate volatiles

The questing behaviour of ixodid ticks serves for identification and locali sation of approaching hosts and is evoked by carbon dioxide, vibrations, vi sual and odour stimuli. In an olfactometer, we examined the specificity of the questing response of larvae of Boophilus microplus, a one-host tick whi ch develops mainly on cattle, and Ixodes ricinus, a three-host tick with a broader host spectrum. While all mammalian odours tested were equally stimu latory for I. ricinus, B. microplus was clearly more activated by bovine od ours. A phenolic fraction of bovine odour stimulated B. microplus only. Att ractive components of the host odours were identified by exposing the ticks to single chemicals and mixtures. Single chemicals stimulated questing res ponses only at levels higher than the levels detected in the bovine odour. However, an artificial odour blend of 37 pure chemicals, diluted to concent rations at which the individual components were inactive, proved to be as e ffective as natural host odour for both tick species. Further fractionation of the blend revealed that the combinatory effect was achieved by only 7 c ompounds in both species. Although B. microplus responded to the same syner gistic mixture of volatiles as I. ricinus, it showed significant higher sen sitivity to the cattle-associated compounds 1-octen-3-ol and 2-nitrophenol and this might contribute to its host-specificity.