The chemical basis for the ability of fed males of Dermacentor variabilis (Say) and D. andersoni (Stiles) to differentiate conspecific and heterospecific females was investigated using a neutered female bioassay. Male D. variabilis responded strongly in a dose-response manner to increasing concentrations of stearic and palmitic acids and an extract of the anterior reproductive tract (ART) of fed D. variabilis females. Mating responses were greatest at 1 microgram of fatty acid and 1 female equivalent (FE) of ART extract. D. variabilis males also responded strongly to D. variabilis females treated with a conspecific ART extract and a mixture of fatty acids simulating the extract; responses were moderate to D. andersoni females treated with the same extract or fatty acid mixture. They also responded moderately to conspecific females treated with D. andersoni extract. Male D. andersoni displayed moderate increases in mating behavior over a range of concentrations of stearic and palmitic acids and conspecific ART extract. D. andersoni males mated only with D. andersoni females treated with a conspecific extract or a mixture of fatty acids simulating the extract. Mating by D. andersoni did not occur in response to D. variabilis females treated with D. andersoni or D. variabilis ART extracts. Stearic acid alone or in combination with 20-hydroxyecdysone, oleic, or myristic acid did not account for the specificity of response of either species. D. andersoni males were much less responsive to fatty acids, alone or in mixtures, than D. variabilis males. These findings indicate that fatty acids are important in mate recognition by D. variabilis, but explain only part of that by D. andersoni. The possibility is raised that another compound is required for mate recognition by D. andersoni.

Chemical analysis and male bioassay responses were used to investigate possible components of the genital sex pheromone of D. variabilis and D. andersoni. Bioassays with D. variabilis indicated greatest responses to myristic, palmitic, stearic, arachidic, and behenic fatty acids as well as myristic and stearic methyl esters. Responses of D. variabilis decreased with changes in chain length, different functional groups and unsaturation. Bioassays with D. andersoni indicated greatest responses to palmitic acid as well as moderate responses to other fatty acids and methyl esters. Chemical analysis revealed the presence of myristic, palmitic, palmitoleic, stearic, oleic and linoleic acids and several unknown compounds in the vaginal lumen and on the external gonopore area. Total free fatty acids in vaginal extracts were 24.8 ng/female equivalent in D. variabilis and 297.2 ng/female equivalent in D. andersoni. Saturated free fatty acids ranging in chain length from C14 to C22 appear to serve as components of the genital sex pheromone of D. variabilis and D. andersoni. Most of these components are present in the lumen of the vestibular vagina and on the external gonopore surface.

There was no evidence of response to an assembly pheromone when male and female Dermacentor variabilis (Say) and Dermacentor andersoni Stiles were exposed in petri dish assays to filter paper circles contaminated by other ticks (either conspecific or heterospecific), hexane extracts of ticks, tick feces, or tick excreta. These ticks did not assemble in response to the presence of other living ticks. Significant responses to saline extracts occurred in both species. However, D. variabilis did not respond to conspecific saline extracts of the same sex and D. andersoni males did not respond to conspecific saline extracts of females. D. variabilis males and females responded to filter paper circles contacted by conspecific ticks of the same sex, but not to those contacted by any other types. D. variabilis females also responded to saline controls. Ticks allowed to climb rods of a simulated “meadow” did not cluster in response to the presence of other live ticks or rods previously contaminated by ticks. These results do not support the hypothesis of an assembly pheromone in D. variabilis or D. andersoni. Contrasts between the ecology of the two North American Dermacentor species and those ticks for which assembly pheromones have been reported are discussed.