Rhipicephalus (Boophilus) microplus has been pesticide-controlled for several decades in the pacific island of New Caledonia. Since 1996, pesticide-control has been based on either deltamethrin (Butox®) or amitraz (Taktic®) in herds harbouring deltamethrin-resistant ticks. In this island, the first R. microplus deltamethrin- and amitraz-resistances were detected in 1992 and 2003, respectively. Using LPT bioassays, we have undertaken to update data regarding the geographical distribution and the physiological diversity likely to be involved in these resistances. We confirmed that after 17 years of intensive use of deltamethrin, several resistances of moderate levels (<30-fold) have evolved and/or diffused in any part of the island. We also evidenced that amitraz-resistant phenotypes have recently evolved in diverse western tick populations, although none has reached fixation in any tick population yet. According to synergists bioassays, the physiological changes involved in amitraz-resistance in New Caledonia would involve target modification and detoxifying P450 cytochrom oxydase(s). It may also involve detoxifying esterase(s) although this later point will need confirmation on samples bearing higher frequency of resistant phenotypes. Results are discussed with regard to the local evolutionary dynamics of resistance.

Mating system plays a determinant role in the maintenance and distribution of genetic variation. Difficulties in applying standard methods of indirect inferences onto parasitic life-cycles partly explain the current lack of knowledge on parasite mating systems. The present study develops a combination of direct and indirect inference methods circumventing such difficulties, and illustrates in particular how such a combination modifies our understanding of the biology of the southern cattle tick, Rhipicephalus microplus. To directly assess how females and males pair in natural populations, we sampled 225 mating pairs, genotyped them at six microsatellite loci, and analysed the correlation between mating status and genetic relatedness. This analysis revealed pangamy, i.e. a random association of male and female genotypes. In addition, indirect methods inferred that sib-groups of ticks exploit distinct individual-cows, and hence that ticks probably move among cows during their parasitic lifetime. Altogether, these results negate the expectation of a high frequency of sib-mating, and show the coexistence of genetically diverging stocks within tick cohorts and populations. These results have several consequences for our understanding of R. microplus. For instance, while existing models of pesticide resistance management look perfectly applicable to this species, the epidemiological models of the micropathogens it vectors needs revision. More largely, the methods developed here would help clarify the evolutionary patterns of any dioceous parasite.

Nine microsatellite loci from the cattle tick Boophilus microplus were isolated and characterized within New Caledonia. This Pacific island was invaded by the cattle tick from a few immigration events dating from mid-20th century. A population survey involving 94 adult ticks indicated monomorphism at one locus, presence of null alleles at another, and high genetic diversity (0.61-0.72) at seven loci apparently suitable for population genetics studies. This opens the opportunity to dissect the populational mechanisms involved in the spectacular capacity of B. microplus to cope with local environmental heterogeneity during its recurrent invasions of tropical areas.

Since its immigration in the Pacific island of New Caledonia in 1942 (i.e. about 240 tick-generations ago), the cattle tick Boophilus microplus has experienced a remarkable adaptive diversification there. In order to better understand the population factors involved, we have investigated the B. microplus population structure on that main host-species, Bos taurus. This study was based microsatellite loci and confirmed that the island colonization came along with a significant bottleneck. Knowledge on B. microplus biology led us to expect B. microplus populations to be composed of highly inbred lineages irregularly dispatched among the individual hosts belonging to the same herds. Instead, this study evidenced a weak inbreeding level and an absence of genetic differentiation within herds. Complementarily, a significant signal of isolation by distance exhibited that human-traffic of cattle does not promote high tick dispersal within the island. Finally, the tick density was found to be about a few hundreds of reproducing adults per squared kilometre, for a gene dispersal range of about a few hundred metres per tick generation. Results are discussed with regard to the evolution of new adaptive changes.