Effects of combinations of the essential oils trans-anethole, thymol and carvacrol against larvae of the screwworm fly Cochliomyia hominivorax in vitro.

This study investigated the combined effect of trans-anethole, carvacrol and thymol on third-instar larvae of C. hominivorax. For this experiment, third-stage larvae of C. hominivorax were removed from a laboratory colony to perform an in vitro bioassay. The three volatile compounds used in this study, trans-anethole, carvacrol and thymol. Different dilutions of these volatile compounds were performed with the diluent acetone (99.5 %) to determine the associative effect (synergism, addition or antagonism). Three concentrations below the estimated LC50 were prepared (10, 20 and 100 μg.cm-2). These compounds were impregnated in filter paper discs, which were dried and transferred to Petri dishes. Ten larvae were added to each dish and incubated in climate-controlled chambers at 28 ± 1 ºC and 75 ± 10 % relative humidity for 24 and 48 hours. Mortality was assessed based on the absence or change in larval movement, as observed under a stereomicroscope. After 24 hours, trans-anethole (A) caused mortality rates in the range of 23-33.5 %, carvacrol (C) of 9-59 % and thymol (T) of 27-81.5 %, in the three concentrations evaluated. The combination of trans-anethole with carvacrol resulted in mortality rates between 11 % and 50 %, demonstrating antagonistic action. The combination of trans-anethole with thymol caused mortality of 12.5-55.5 %, and the combination of carvacrol with thymol generated mortality of 75.5-99.5 %, indicating synergism. After 48 hours, (A) produced a mortality range of 29-49 %, (C) of 10.5-63 %, and (T) of 37.5-86 % in the three concentrations, maintaining the same relationship of antagonistic and synergistic activity. The combination of trans-anethole with carvacrol resulted in mortality rates between 16 % and 56.5 %, while the combination of trans-anethole with thymol caused mortality of 27.5-62.5 % and the combination of carvacrol with thymol presented mortality between 83 % and 99.5 %. Hence, there was an antagonistic effect when trans-anethole was associated with carvacrol and thymol and a dose-dependent synergistic effect when carvacrol was associated with thymol against third instar larvae of C. hominivorax.