Rhipicephalus microplus is widely distributed in tropical and subtropical areas of the world
where livestock is a principal activity with great veterinary and economic importance. In
Ecuador, this hematophagous ectoparasite has been observed between 0 and 2600 masl.
One of the main tick control measures is the use of acaricides, which have been indiscriminately
used worldwide and in Ecuador. In this country, no studies on acaricide resistance in
Rhipicephalus microplus have been published. The current study aims to characterise the
level of resistance of R. microplus against three main acaricides commonly used in Ecuador
i.e. amitraz, alpha-cypermethrin and ivermectin to determine the level and pattern of doseresponses
for R. microplus in 12 field populations (farms). The level of acaricide resistance
was evaluated using three different bioassays: adult immersion test (AIT), larval package
test (LPT) and larval immersion test (LIT), as recommended by the FAO. The predictive
dose-responses were analysed by binomial logistics regression of the larval survival rate
(resistance). In general, we found resistance of 67% for amitraz; 50% for alpha-cypermethrin
and from 25 to 42% for ivermectin in the twelve field populations analysed. Resistance
levels were studied in larval and adult bioassays, respectively, which were slightly modified
for this study. For larval bioassays based on corrected mortality i.e. high (above 51%),
medium (21±50%) and low (11±20%) resistance, percentages less than 10% were considered
as susceptible. For the adult test, two resistance levels were used i.e. high (more than
76%) and medium (51 to 75%) resistance. Percentages lower than 50% were considered as
susceptible. In this context, for larval bioassays, amitraz showed 21%, 38% and 8% for
high, medium and low resistance, respectively. Alpha-cypermethrin presented 8%, 4 and
38% for high, medium and low resistance, respectively. Ivermectin presented 8%, 25% and 8% for high, medium and low resistance, respectively. For adult tests with amitraz 50% and
17% of the field populations showed average and high resistance, with evidences of average
resistance to alpha-cypermethrin in 50% of the samples and average resistance against ivermectin in 25% of the farms. No statistical difference amongst the three bioassays was
found and acaricide resistance was confirmed by logistic regression analysis; hence resistance
(dose-responses) in each field populations differed, depending on the choice of the
acaricide, frequent usage, frequency of treatment and farm management. The effective estimated
dose needed to eliminate 99% of ticks is higher than dose recommended by the manufacturer.
In conclusion, amitraz showed the highest resistance followed by ivermectin and
alpha-cypermethrin and reveals differences on resistance in each individual field population.
This information is important in order to establish the monitoring of resistance on each farm
individually, contributing to the rational use of acaricides included in an integrated control
program for R. microplus.