Extracts of four plant species used traditionally to treat myiasis influence pupation rate, pupal mass and adult blowfly emergence of Lucilia cuprina and Chrysomya marginalis (Diptera: Calliphoridae)

Ethnopharmacological relevance: Aloe zebrina, Clausena anisata, Erythrina lysistemon and Spirostachys africana are used traditionally in southern Africa to combat and/or treat myiasis, the infestation of the skin of mammals by larvae of a variety of fly species belonging to the arthropod order Diptera.The objective of the study was to establish the in vitro effect of extracts of these plant species on blowfly larvae to ascertain the potential use of these extracts and validate the traditional use.

Materials and methods: Chrysomya marginalis and Lucilia cuprina larvae cultured in the laboratory under controlled conditions were exposed to meat treated with acetone and aqueous extracts of four different plant species. In addition to solvent controls, the effects of the plant extracts were tested at concentrations of 10, 25, 50, 75, 100 and 150 mg/ml. Larval behaviour, larval development and emergence of adult flies were assessed after exposing the larvae to the treated meat. Ivermectin was used as a positive control while acetone and water served as solvent controls. Cytotoxicity of the extracts was determined using the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyl tetrazolium bromide] assay against C3A human liver cells, bovine dermis cells and Vero African green monkey kidney cells.

Results: An increase in the concentration of the acetone plant extracts resulted in a decrease in the ingestion of meat by larvae, pupal mass and adult emergence rates. The R(2) values for the correlation between dose and pupal mass was 0.92, 0.74 and 0.65 for A. zebrina, C. anisata, and E. lysistemon respectively. For C. anisata and S. africana extracts the increase in the concentration was also associated with the emerging adult flies being smaller. C. anisata was the most toxic plant extract, with an average IC(50) of 39 μg/ml, while E. lysistemon was the least toxic. Vero cells were the most susceptible cell type, with an average IC(50) of less than 100 μg/ml, while the human liver cells were the most resistant to the effect of the plant extracts.

Conclusions: The results suggest that the plants may contain compounds that interfere with larval feeding and the neuroendocrine control mechanisms in the blowfly. It appears that some of these plant species have the potential to deliver a product that can be used to control myiasis.