A Mexican strain (Cz) of the southern cattle tick Boophilus microplus (Canestrini) was challenged with increasing doses of permethrin to select for resistance. A maximum 90.4-fold resistance relative to a susceptible strain, or a 35-fold increase over its initial level of resistance, was achieved after 14 generations of selection. The Cz strain suffered severe fitness costs for several generations after resistance to permethrin was increased to a certain level, despite reduced selection pressure after F18. The fitness parameters evaluated include engorged female weight, egg mass weight, hatching rate, and engorged female mortality. The normal seasonal fluctuation of engorged female weight was abolished for 8 generations after resistance reached the maximum level at F14. Significant decrease in egg masse weight was observed strating at F18. Despite reduced selection pressure in the following generations, the egg mass weight continued to decline, and reached its lowest level in F21 before showing recovery. Full recovery of the mean egg mass weight did not occur in the last generation studied. A delayed, but significant decrease in the hatching rate of eggs was also observed in F21, and lasted for 8 generations before returning to a normal level. A dramatic increase in engorged female mortality was also observed after F18. Enhanced metabolic detoxification as a major mechanism of resistance to permethrin in this tick strain may have contributed to the observed fitness costs in this tick strain.
Species 1: Acari Ixodidae Boophilus microplus (southern cattle tick)
Keywords: pyrethroid, selection
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