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 LC were prepared (10, 20 and 100 μg.cm). 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.