1a.Objectives (from AD-416):
In tropical and subtropical countries, ticks and tick-vectored diseases cause major economic losses to cattle production. The most popular method for the control of the tropical cattle tick Rhipicephalus (Boophilus) microplus is the use of acaricides. However, the continued use of these pesticides has resulted in widespread acaricide resistance problems. Pesticide mixture formulations have been shown to provide effective control to highly resistant populations of insect pests on crops, such as the white fly in Arizona. The finding of synergism between two currently used acaricides, amitraz and permethrin, has been reported by the PI at the USDA-ARS Knipling-Bushland U.S. Livestock Insects Research Laboratory in Kerrville, Texas. Enhanced control efficacy of such acaricide mixtures against R. microplus has been confirmed in a field trial at a dairy farm in New Caledonia. However, the efficacy of pyrethroids with synergists to control natural infestations of pyrethroid-resistant R. microplus populations has not been evaluated in the Mexican tropics. The agreement involves studies with both laboratory and field components in the State of Yucatan, Mexico.
The objectives of these studies are: (1) to determine the resistance ratio to amitraz and pyrethroid acaricides through bioassays; (2) to determine the synergism ratio when amitraz and/or PBO is used as synergists for the pyrethroids; (3) to determine the control efficacy of amitraz- and/or PBO-synergized pyrethroid acaricide formulations against natural tick infestations on cattle; and (4) to determine if the synergized pyrethroid formulations can reduce the frequency of sodium channel mutations in wild tick populations.
1b.Approach (from AD-416):
This project will consist of three phases:
1. Laboratory work to test the hypothesis on potential impact of the amitraz-synergized pyrethroid formulations on genotype composition of tick populations (year 1).
2. Field trial to compare the efficacy of synergized pyrethroid formulations with that of the traditional single pyrethroid formulation (year 2).
3. Implementation of a tick control/resistance management regime for ranchers that involve use of synergized pyrethroid formulations. This may require multiple treatments over a period of three months. Monitor efficacy and change of tick genotype composition over time (year 3).
A strategy to use laboratory bioassays to evaluate toxicities of synergist and acaricide alone and as mixtures at different ratios was developed. The cooperator in Mexico has conducted initial work using the bioassay recipes and synergist material provided by ARS scientists to evaluate the toxicological responses in a Mexican tick strain. Additional bioassays using formulated acaricide products are planned. Live and dead ticks from bioassays will be preserved and shipped to the ARS laboratory for genotyping tick larvae.