Development of a population dynamics model to investigate the displacement of Rhipicephalus decoloratus by Rhipicephalus microplus

Abstract

Rhipicephalus microplus is displacing Rhipicephalus decoloratus at an alarming rate. Besides being invasive, R. microplus is the only tick that can act as a vector for the substantially more virulent form of Babesiosis caused by Babesia bovis. There is no cross‐immunity between the indigenous Babesia bigemina and Babesia bovis. There is a need to understand better the reasons for the success of R. microplus at the cost of R. decoloratus. To address this need, we developed a stochastic population dynamics model with daily time steps. The model was cohort‐based for the eggs and questing larvae and individual‐based for all later life stages. The model parameters have been specified using the best available knowledge of the two competing species of ticks. Seven sensitivity parameters were identified that could influence the success of R. microplus. Different model simulations with varying values of the seven sensitivity parameters allowed the creation of a dataset of outcomes linked to the different sensitivity parameter combinations. The result measured was the proportion of R. microplus in the population after two years of simulation following an initial introduction comprising 30% of the total tick population. The sensitivity parameters assessed were (1) the development threshold for egg development, (2) the required degree‐days to complete egg development, (3) the delay of the development of the parasitic lifecycle of R. decoloratus, (4) the length of time males survived on the host, (5) the number of male partners of each female tick, and, finally, (6) the starting date of the simulation and (7) a slight change in the mean environmental temperature. R. microplus can displace R. decoloratus primarily due to its relatively faster development rate. In warm sub‐tropical climates, the stage of development that provides R. microplus with the most significant advantage is the development of the eggs, followed by the parasitic life stages. This effect is likely to diminish substantially in colder climates and winter since R. microplus has a higher temperature requirement to start egg development (specified as the development threshold). Although introductions of R. microplus into a herd are usually accidental, introductions just before winter will have a lower chance of establishing itself. There is a potential opportunity to eliminate already established R. microplus from mixed populations by aggressive dipping in autumn, followed by artificial infestations of R. decoloratus after the efficacy of the acaricide has waned. This may lead to disproportionately high numbers of R. decoloratus males in spring which could sterilize a high proportion of the R. microplus females tipping the balance in favour of R. decoloratus.