Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2010
Publication Date: 3/20/2011
Citation: Miller, R., White, W.H., Davey, R.B., George, J.E., Perez De Leon, A.A. 2011. Efficacy of spinosad against acaricide-resistant and -susceptible Rhipicephalus (Boophilus) microplus and acaricide-susceptible Amblyomma americanum and Dermacentor variabilis. Journal of Medical Entomology. 48(2):358–365.
Interpretive Summary: Boophilus microplus infest cattle throughout the tropical and sub-tropical regions of the world causing billions in losses through reductions in weight gains due to feeding and death or morbidity and due to the transmission of the causative agents of babesiosis and anaplasmosis. Generally, cattle ticks are controlled through the use of acaricides and, because of many environmental and economic factors, the use of acaricides is especially high in tropical regions of the world. As a result, high levels of resistance to acaricides in B. microplus has occurred in every tropical region of the world where chemical control has been used to control ticks on cattle. This study describes the susceptibility of several strains of acaricide-resistant R. microplus, collected from Mexico and Brazil to a relatively new acaricide, spinosad, as well as the activity profile comparing spinosad with other acaricides against multiple parasitic life stages of two additional Ixodid ticks, Amblyomma americanum and Dermacentor variabilis. We demonstrated that spinosad was efficacious against the larvae of many strains of single- and mulit- resistant B. microplus collected from Mexico and Brazil representing almost all known resistance mechanisms now known in B. micorplus. However, adults were not affected by treatment with spinosid. In order to reconcile these observations and further evaluate potential differences in efficacy that may have been dependent on parasitic life-stage, spinosad was evaluated and compared with amitraz, permethrin and chlorpyrifos against all three parasitic life stages of two multi-host ticks that readily infest cattle in the United States, A. americanum and D. variabilis. While spinosad was highly effective against immature stages of both ticks, activity was much lower against adult D. variabilis, as was the activity of amitraz. The data clearly illustrated differences in activity profiles for spinosad and the other acaricides that were dependent both upon parasitic life stage and tick species. Although not yet proven in field studies, spinosad should be able to eradicate acaricide-resistant B. microplus with a 1-2 week treatment interval at a concentration greater than 0.08% A.I. Additionally, future resistance monitoring should be accomplished as part of IPM programs using larvae as the test subjects as we found no mortality in the engorged female ticks after exposure to spinosad. The FAO-LPT or larval immersion tests (not used in this research) could be used for this purpose.
Technical Abstract: Various acaricide-resistant strains of Rhipicephalus (Boophilus) microplus, representative of the major resistance mechanisms found in Mexico and Brazil were exposed to spinosad using the FAO-Larval Packet Test and FAO-Adult Immersion Test (AIT). Larvae of all strains tested were found to be susceptible to spinosad. Conversely, spinosad did not show toxic activity towards engorged females used in the AIT. In-vitro tests against larvae, nymphs and adults of acaricide-susceptible Amblyomma americanum and Dermacentor variabilis revealed differences in activity that were dependent on tick species and parasitic life stage. Spinosad seemed to be a viable alternative to current acaricides available as there were no reports of resistance to this acaricide in R. microplus. The LPT should be used for future monitoring of resistance as the AIT did not provide useful information with this chemical. The potential benefit for the use of spinosad in IPM or eradication programs is discussed.