Understanding the genetic, demographical and/or ecological processes at play in invasions: lessons from the southern cattle tick Rhipicephalus microplus (Acari: Ixodidae).

The southern cattle tick, Rhipicephalus microplus, is the ixodid species causing the largest economic losses in tropical agrosystems because of its recurrent invasive success, explosive demography on bovine herds, vector competence for diverse pathogens and frequent development of acaricide resistance. Its ecology and the physiological bases of the acaricide resistances it developed, as well as alternative tick control measures, have been intensively studied for decades. By contrast, the tick population genetic structure and its remarkable ability to quickly adapt to new environments have not yet received much attention. We investigated such issues using population genetics analyses in the recently invaded island New Caledonia. In this paper we aim to describe some guidelines for acarologists willing to investigate the processes at play in Acari invasions. Particular emphasis is given to the accuracy of sampling designs and sampling scales for population genetics to be actually informative on the demographical processes of the species (i.e., its mating rules, the determinants of population limits, population sizes, the relationships between genetic exchanges and geographical distances and relevant ecological factors).

Accumulation of acaricide resistance mechanisms in Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) populations from New Caledonia Island.

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.