|JONSSON, NICK - University Of Queensland|
|KEMP, DAVID - Commonwealth Scientific And Industrial Research Organisation (CSIRO)|
|KNOWLES, A - Elanco Animal Health, Inc|
|ARDILA, A - University Of Queensland|
|VERRALL, R - University Of Queensland|
|ROTHWELL, J - University Of Queensland|
Submitted to: Veterinary Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2009
Publication Date: 4/19/2010
Citation: Jonsson, N.N., Miller, R., Kemp, D.H., Knowles, A., Ardila, A.E., Verrall, R.G., Rothwell, J.T. 2010. Rotation of treatments between spinosad and amitraz for the control of Rhipicephalus (Boophilus) microplus populations with amitraz resistance. Veterinary Parasitology. 169: 157-164.
Interpretive Summary: The cattle fever tick is a pest of cattle worldwide. Most producers control this pest through pesticide treatment of host animals. The emergence of resistance to multiple types of pesticides over the last 30 years has complicated the control of these ticks and is a major source of concern to USDA's APHIS Cattle Fever Tick Eradication Program. In this study we showed resistance to one type of pesticide, amitraz, can be significantly reduced by rotation with another type of pesticide, spinosad. This reduction in resistance appeared to be also correlated with seasonal changes. The development of resistance to older pesticides on the market, such as amitraz, reduces the tools available to producers and drives them to use newer and more expensive chemicals such as spinosad. This research will allow producers to use a newer chemical along with an older chemical, in rotation, which will save producers money and increase the time in which resistance will develop in the tick to the new chemical.
Technical Abstract: A farmlet study was conducted over 4 years in which three treatments were applied to six groups of Holstein dairy calves. Calves in each group were infested with equal numbers of N-strain (susceptible) and Ultimo strain (amitraz and synthetic pyrethroid resistant) tick larvae to establish self-sustaining populations with an initial, measurable level of resistance to amitraz. Standard counts of all ticks between 4.5 and 8.0 mm diameter, on one side of each animal, were made each week, and treatments were applied when tick numbers exceeded a threshold of 25 engorged adults per side. The three treatments were: 1, spinosad spray whenever tick numbers exceeded the threshold; 2, amitraz spray whenever tick numbers exceeded the threshold; 3, spinosad whenever tick numbers exceeded the threshold for the first 2 months, then amitraz for 2 months, with alternation every subsequent 2 months. Engorged adult female ticks were collected from each treatment group on 10 or 11 occasions during the study and tested using the larval packet test bioassay (LPT) for acaricide resistance. Spinosad 250 ppm provided effective control of amitraz-resistant tick populations in the field, using a similar number of treatments as in the amitraz and rotation groups. The initial infestations of all groups resulted in the establishment of populations with in vitro evidence of resistance to amitraz using the LPT. Treatment with spinosad, or with a rotation between spinosad and amitraz, every 2 months, resulted in reduced levels of resistance to amitraz according to the LPT. The animals treated with amitraz alone showed increasing resistance to amitraz according to the LPT each summer and autumn with a return to full or almost full susceptibility to amitraz in early spring in all years. This pattern suggests a relative lack of fitness of the amitraz-resistant ticks that might be exploited by using an acaricide rotation strategy.