Evaluation of a pour-on formulation of fipronil against Boophilus annulatus (Acari : Ixodidae) under natural South Texas field conditions

A long-term field study (60 consecutive weeks) was conducted to determine w hether a Boophilus annulatus (Say) population could be eradicated through r epeated applications of a 1% pour-on formulation of fipronil on cattle held in an infested pasture. Animals treated repeatedly over time (seven applic ations) were infested with significantly fewer (p < 0.05) female ticks (5 m m in size) than untreated animals in all except one of the 57 weekly tick c ounts conducted after the first treatment was applied. As the number of tre atments increased over time, there was a progressive decrease in the number of instances in which engorging females were observed on the treated cattl e. Tick numbers obtained from sentinel cattle placed in pasture with untrea ted and treated cattle at various intervals throughout the study indicated that a single fipronil treatment had no observable adverse effect on the fi eld tick population. However, after two treatments had been applied, there were always dramatically fewer ticks obtained from sentinel animals placed with treated cattle than were observed on sentinel animals placed with untr eated cattle. Furthermore, tick numbers obtained from sentinel cattle place d with treated cattle over time indicated that the tick population in the p asture declined by stages. While a single treatment had no effect on the ti ck population, the application of two to four fipronil treatments at variou s intervals dramatically reduced the tick population, even though substanti al numbers of females were still present on the sentinel animals. Ultimatel y, the application of five to seven fipronil treatments resulted in an extr emely low tick population in the pasture, although total eradication of the population was not achieved during the study. The mean weight gain of trea ted animals was significantly higher (p < 0.05) than untreated animals duri ng the study, indicating that the fipronil treatments had a highly benefici al impact on weight gain production.