Monitoring of resistance or susceptibility of adults and larvae of Amblyomma cajennense (Acari: Ixodidae) to synthetic acaricides in Goiás, Brazil.

Amblyomma cajennense or the Cayenne tick is a three-host ixodid tick species of low parasitic specificity that is the principal vector of Brazilian spotted fever. The primary objective of this study was to evaluate the possible development of resistance by adult specimens of A. cajennense to deltamethrin, a synthetic pyrethroid insecticide and the principal miticide/acaricide commercially available in the region. The second objective was to monitor the susceptibility and/or resistance of larvae of this species to 12 synthetic acaricide formulations selected from the principal pesticides available in Goiás for the control of ticks. Unfed male and female adult specimens of A. cajennense were collected from leaves of bushes along a nature trail in the municipality of Caldas Novas, Goiás, Brazil. They were submitted to immersion in the highest recommended dose of deltamethrin and subsequently, were placed in contact with filter paper impregnated with the substance. The toxicological effects caused by the insecticide were observed every 6 h over a 36 h period. To obtain larvae, engorged females of A. cajennense were collected from naturally infested horses that had been free of acaricidal residue for at least 45 days, in farms situated in five different municipalities in the state (Caldas Novas, Hidrolândia, Goiás, Terezópolis and Goiânia). The larvae were exposed to different concentrations of 12 commercially available acaricidal formulations using the larval packet test (LPT) method. The control groups were treated with distilled water alone. The bioassays were performed in quadruplicate at a temperature of 27 °C, relative air humidity > 80% and 12 h light/dark cycles. The mean percentage of mortality MX was 72.6% in the adult specimens after 24 h of exposure to the dose of deltamethrin recommended by the manufacturer, characterizing a status of resistance. MX of 82, 89, 89.6 and 90% of the larvae were obtained, respectively, for deltamethrin, cypermethrin + piperonyl butoxide (PBO), amitraz and permethrin, characterizing a status of probable resistance of the larvae to these acaricides. No significant mortality was found in the control groups.