Synergistic larvicidal effect and morphological alterations induced by ethanolic extracts of Annona muricata and Piper nigrum against the dengue fever vector Aedes aegypti

Background Phytopesticide combinations of different botanical sources are seldom reported. Annona muricata seed and Piper nigrum fruit ethanolic extracts enriched in acetogenins and piperamides, respectively, were synergistically used as larvicides against the dengue fever vector Aedes aegypti. Results Individual bioassays of A. muricata and P. nigrum indicated respective LC50 values of 93.48 and 1.84 mu g mL-1 against third-instar larvae. Five combinations of different proportions of plant extracts pointed to synergism between the extracts. The best A. muricata:P. nigrum extract combination was 90:10, which showed 5.12 times the amount of synergism, as confirmed by statistical equations and total concentration log versus combination proportions. Concerning the morphology, A. muricata caused larvae body elongation, mainly in the abdomen, along with the appearance of a cervix. Conversely, P. nigrum induced abdomen and whole body shortening. The morphological effects of A. muricata were prevalent in all of the combinations tested, irrespective of its proportion in the combination. Conclusion It is suggested that the different mechanisms of action of the larvicidal actives A. muricata acetogenins and P. nigrum piperamides explain the observed synergism. The combination of inexpensive botanicals and a low-cost organosolvent such as ethanol leads to a simple and efficient phytolarvicidal formulation.