The early third instar larvae of Fannia canicularis were treated with various concentrations of Rumex dentatus, Protulaca deracea and Piper cubebae extracts. The Rumex dentatus was more toxic than Protulaca deracea and Piper cubebae. The extract mixture of Rumex dentatus and Piper cubebae induced a high synergistic action against the treated larvae, but the mixture of Protulaca deracea and Piper cubebae indicated an antagonistic effect. The reproductive potential of treated insects together with the biochemistry of their ovaries and testes were determined. The results indicated that the tested mixtures had delayed effects represented by the decreased the ovarian and testicular protein and the disturbance of their amino acid contents. This might be due in part to the interference of these mixtures with gonadal development, which affected negatively the fecundity and fertility of adults.