Toxicity of mixtures of λ-cyhalothrin, imidacloprid and cadmium on the earthworm Eisenia fetida by combination index (CI)-isobologram method

• A novel method was used to study the nature of interactions of three chemicals.
• Earthworm assays were conducted in both filter paper and artificial soils.
• The mixtures containing imidacloprid exhibited antagonism.
• The combination index method could more accurately predict combined toxicities.

Contaminants in the environment do not appear singly and usually occur as mixtures. We applied the combination index (CI)-isobologram method which allows computerized quantitation of synergism, additive effect and antagonism to determine the nature of toxicological interactions of two pesticides λ-cyhalothrin, imidacloprid, and heavy metal cadmium towards earthworm Eisenia fetida. In an artificial soil test, λ-cyhalothrin and Cd combination was slightly synergistic at low effect levels which turned into a slight antagonism above fa values of 0.6, while the binary mixtures containing imidacloprid exhibited antagonism. The presence of imidacloprid in the ternary mixture also resulted in an antagonistic effect to the earthworms. This behavior became more antagonistic in the ternary mixture in filter paper tests.