Influence of acaricide resistance on cattle-fever tick (Boophilus spp.) infestations in semi-arid thornshrublands: a simulation approach

Cattle-fever tick (Boophilus microplus and B. annulatus) populations that d evelop acaricide resistance become more difficult to control or eradicate. We used a simulation model to assess the direct and indirect effects of int eractions among season, habitat type, grazing strategy, and acaricide resis tance on the ability to eradicate Boophilus infestations in semi-arid thorn shrublands of Texas, USA. Season of infestation appeared to have the strong est effect, with infestations begun on 27 September (autumn) tending to die out sooner than those begun on 1 March (spring) and to remain undetected. Habitat type had the next strongest effect, with infestations surviving muc h longer as canopy cover increased from uncanopied buffelgrass (Cenchrus ci liaris) habitats to mesquite (Prosopis glandulosa)-canopied grass habitats. Acaricide resistance had a moderate effect; as expected, highly resistant tick populations survived longer than those with no acaricide resistance. T he importance of grazing strategy varied with changes in habitat type: as c anopy cover increased, infestation duration increased faster under continuo us grazing than under rotational grazing strategies. Importance of grazing strategy also varied with acaricide resistance: detected tick populations w ith no and slight acaricide resistance subjected to acaricide treatments te nded to survive longer under rotational grazing than continuous grazing, du e to reduced contact with a treated host. Populations with moderate and hig h resistance behaved more like untreated populations, tending to survive lo nger under continuous, rather than rotational, grazing, because they experi enced less mortality on a treated host. Assuming acaricide treatments at 2- week intervals and maintenance of cattle in infested pastures, results indi cate that, for each habitat type, infesting ticks have a threshold of acari cide resistance below which one can eradicate them faster with continuous g razing than with rotational grazing. As canopy cover increases, this thresh old appears to shift from high resistance (in grass) to slight resistance ( in mesquite).