Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2005
Publication Date: 12/1/2005
Citation: Davey, R.B., Miller, J.A., George, J.E., Snyder, D.E. 2005. Effect of repeated spinosad treatments on cattle against Boophilus annulatus under south Texas field conditions. Southwestern Entomologist. 30(4):245-255. Interpretive Summary: When pastures in the U.S. are infested with Boophilus spp. and cattle are maintained on the pasture, the eradication program relies exclusively on the systematic, repeated treatment of all livestock with the organophosphate (OP) acaricide, coumaphos to eradicate the infestation. However, resistance of many Mexican tick populations to OP acaricides along with the ban of many OP chemicals from the marketplace in recent years creates a critical need in the eradication program for evaluating new classes of acaricides that may have potential for use in the program. A study was conducted to evaluate the effect of repeated treatments of spinosad applied at three week intervals as a whole-body spray to cattle infested with Boophilus annulatus to determine whether eradication of a field population of ticks could be achieved. It was demonstrated that eradication was not achieved using a three week treatment interval at two different spinosad concentrations (0.05 and 0.08% active ingredient). Results showed that at one week following a given treatment control remained relatively good. However, there was a substantial decline in the level of control by two weeks following a given treatment. At three weeks following treatment, although some females reached repletion, the activity of spinosad was still high enough to at least protect treated cattle from being heavily re-infested. Tick counts obtained from sentinel cattle placed with untreated and treated cattle at various intervals throughout the study showed that the tick population in the pasture where cattle were treated declined through time, whereas tick counts obtained from sentinel cattle held with untreated cattle increased through time. Thus, while this treatment regime would not be suitable in an eradication program, it might be applicable for use in a control program.
Technical Abstract: Repeated treatments of spinosad applied as a whole-body spray to pastured cattle infested with Boophilus annulatus were evaluated to determine whether eradication of a field population of ticks could be achieved. During the first 21 weeks of the study cattle were treated at a concentration of 0.05% active ingredient (AI) spinosad at total of seven times at three week intervals (Phase I). Subsequently, during the last 18 weeks of the study cattle were treated at 0.08% AI spinosad a total of six times at three week intervals (Phase II). While neither concentration resulted in the eradication, the higher spinosad concentration was more effective than the lower concentration. At one week following each of the 13 spinosad treatments (Phase I and II) treated calves were significantly fewer ticks than untreated cattle on 76.9% of the counting intervals. However, at two weeks following each spinosad treatment (Phase I and II) activity declined, allowing substantially more numbers ticks to survive to repletion, particularly at the 0.05% AI treatment level (Phase I). At three weeks following each of the 13 treatments (Phase I and II) treated cattle had significantly fewer ticks than untreated calves on 61.2% of the counting intervals, indicating that the spinosad activity protected cattle from being heavily re-infested with larvae on a majority of occasions. Tick counts from six sentinel groups of cattle placed with untreated and treated cattle at various intervals showed that while the tick population in the pasture where treated cattle were held was declining, the population in the pasture where untreated cattle were held was increasing, indicating that the spinosad treatments were reducing the field tick population. Thus, while this treatment regime would not be suitable in an eradication program, it might be applicable for use in a control program.