Field evaluation of a novel tick control strategy aiming a reduced selection for drug resistance: A case for "host-level chemical mosaic".

Widespread multiple acaricide resistance in Rhipicephalus microplus has challenged conventional strategic tick-control schemes using acaricides that prioritize short-term efficacy while largely overlooking how treatment structure affects resistance selection. Here, we propose and validate a novel protocol for chemical control designed to reduce pressure selection for acaricide resistance: host-level chemical mosaic (HLCM). This approach combines two initial uniform treatments with subsequent alternation of drugs between subgroups of animals within the same paddock. The first and second applications are given to the entire herd using different compounds. From the third treatment onward, half of the animals receive one active ingredient and the other half a compound from a different class; at the next application, the compounds are switched between the subsets. This approach was based on classical agricultural resistance-management frameworks, where mosaic strategies use the spatial deployment of different insecticides across distinct units. Here, we adapt this concept to the host scale, where individual hosts represent the spatially distinct selective units. A field trial was conducted in Rio Grande do Sul, Southern Brazil, using 80 Angus cattle allocated to four groups: untreated control, two conventional strategic protocols (C1 and C2), and a group managed under HLCM approach. All treated groups followed a strategic control framework triggered at the onset of the spring rise in tick infestation. C1 received two consecutive ivermectin (IVM) treatments followed by two consecutive fluazuron (FLZ) applications; C2 received one IVM, one FLZ and a mixture of organophosphate + pyrethroid (OP+P) thrice; while the HLCM group received one IVM followed by one FLZ application for the whole herd, and subsequently two treatments in which half the animals received IVM and the other half FLZ, with products switched between subsets at the last application. Tick counts on cattle and protocol's efficacy were monitored for 210 days. To monitor selection pressure for acaricide resistance, IVM susceptibility was chosen as model. It was assessed before and at the end of the season using the Larval Immersion Test. The efficacy of the treatments ranged between 80% and 96% for group C1, 58-97% for group C2, and 75-97% for group managed by HLCM. Baseline bioassays (before the trial) indicated a predominantly susceptible local population with a low-frequency resistant fraction. By the end of the study, ivermectin susceptibility in the untreated, C2 and HLCM groups remained similar to baseline (RR₉₀ < 4, overlapping confidence intervals), whereas C1 showed a marked shift (RR₉₀ ≈ 18.8), indicating intensified resistance selection. These findings suggest that the number, timing and the intra-herd distribution of treatments within the herd can substantially influence resistance trajectories without compromising field efficacy. Under the conditions tested, the host-level chemical mosaic maintained satisfactory tick control while limiting the increase in drug resistance, supporting its further use as a practical refinement of strategic tick-control programs in areas with emerging multi-resistance.