The eco-toxic effects of pesticide and heavy metal mixtures towards earthworms in soil

Earthworms are the key soil organisms, contribute to many positive ecological services that could be degraded by pesticides and other soil pollutants such as heavy metals. Chemicals usually occur as mixtures in the environmental systems which can lead synergistic effects. The assessment and characterization of soil pollutants that effects risks are very difficult due to the complexity of soil matrix, poor understanding about the fate and effects of chemical combinations like pesticide and metal mixtures in terrestrial systems, and scarcity of toxicological data on mixtures of pollutants. In this review we summarized the current studies on individual and joint effects of pesticides and metals on earthworms and indicate the mixture that cause the synergistic interactions. The review explores the methods and models used previously to evaluate the toxicity of chemical mixtures, and suggests the perspective approaches for a better knowledge of combine effects as well as research methods The summarized report indicates that pesticide and metal mixtures at all organization levels affect the earthworms negatively. Whereas, the combined pollution generated by mixtures of pesticides and metal ions could induce the DNA damage, disruption in enzyme activities, reduction in individual survival, production and growth rate, change in individual behavior such as feeding rate, and decrease in the total earthworm community biomass and density. Among the pesticides organophosphates were identified the most toxic pesticides causing the synergistic effects. The findings indicate the scarcity of toxicological data concerning the assessment of pesticide and metal mixtures at genome level; while the mechanisms causing synergism were still not sufficiently explored.