Location: Tick and Biting Fly Research
Title: Efficacy of an Organophosphate Mixture Acaricide Applied as a Whole-Body Spray on Cattle Infested with an Organophospate-Resistant Strain of Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) Authors
Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 23, 2013
Publication Date: N/A
Interpretive Summary: Acaricide resistance in cattle fever ticks throughout Mexico poses an enormous risk to the continued success of the U.S. Cattle Fever Tick Eradication Program (CFTEP) if these pesticide resistant ticks become established in the U.S. Among the strategies that have been proposed as a means of preventing the establishment of acaricide-resistant cattle fever ticks in the U.S. is the use of pesticide mixtures. Pesticide mixtures have the advantage of exposing the pest to more than one pesticide simultaneously, thereby increasing the efficacy of the material because the pest is unable to cope with multiple pesticides at the same time. A study was conducted to evaluate the efficacy of the organophosphate (OP) mixture acaricide, Ravap® applied to cattle infested with all stages of a highly OP-resistant strain of cattle fever ticks at two different concentrations. Both treatment concentrations (0.15 and 0.3% active ingredient) had a significantly greater adverse effect than was obtained against an untreated group of ticks, but were not different from each other, providing 85.3 and 87.6% control, respectively. Furthermore, results showed that both dosages were most effective against ticks that were in the larval stage of development at the time of treatment (~97% control), while they were least effective against ticks that were in the adult stage at the time of treatment (~78% control). Although the results of the study showed that the mixture acaricide, Ravap® was more effective against OP-resistant cattle fever ticks than single non-mixture acaricides that have been evaluated previously, nevertheless, the mixture still failed to provide the 99% efficacy required for use in the CFTEP. Thus, a single treatment application with Ravap® against OP-resistant ticks would still pose a risk of establishing and dispersing ticks into un-infested areas. However, it is possible that multiple treatments at 7 or 10 day intervals could eradicate OP-resistant ticks in an emergency situation.
Technical Abstract: The efficacy of an organophosphate (OP) mixture acaricide, Ravap®, was evaluated as a whole-body spray treatment applied at 0.15 and 0.3% active ingredient (AI) to cattle infested with all parasitic stages of a highly OP-resistant strain of Rhipicephalus (Boophilus) microplus (Canestrini). Laboratory bioassays conducted prior to the study showed the tick strain used in the evaluation was 18.7-times more resistant to OP acaricides than an acaricide-susceptible reference strain. Overall results showed that both treatment concentrations had a significantly greater adverse effect on the ticks than was obtained in the untreated group in every measured parameter, except for female engorgement weight, where there was no difference. However, there was no difference between the two concentrations in any of the measured parameters, and the overall percentage control was 85.3 and 87.6% at 0.15 and 0.3% AI, respectively. Treatment with Ravap® was most effective against infested cattle in the larval stage, and least effective against the adult stage. The level of control against adults was 79.8 and 76.2% at 0.15 and 0.3% AI, whereas against ticks in the larval stage at treatment the control was 96.5 and 97.7%, respectively, with no differences between the two concentrations. However, while the control observed against ticks in the nymphal stage at the time of treatment was intermediate between the other two life stages (82.5% at 0.15% AI and 93.1% at 0.3% AI), nevertheless, there was a significant difference between the two concentrations. Even though this mixture acaricide provided relatively good control against a highly OP-resistant strain of ticks, the level of control was still well below the required 99% level necessary for use in the U.S. Cattle Fever Tick Eradication Program. Therefore, a single treatment application with this mixture acaricide against OP-resistant ticks would pose the risk of dispersing cattle harboring viable ticks to uninfested areas.