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Effects of agricultural insecticides in aquatic invertebrate communities

Neonicotinoid insecticides constitute a group of systemic insecticides that are applied as foliar spray, granules or as seed-dressing in a broad range of agricultural crops worldwide. These insecticides are considered contaminants of concern due to their high toxicity potential to non-target terrestrial and aquatic organisms. Neonicotinoids have been recently banned for outdoor agricultural applications in Europe and have been included in the watch list of emerging contaminants that need to be regularly monitored in surface waters. This implies that water quality standards need to be developed by researchers to assess their risks for aquatic ecosystems.

A new study published by the Ecotoxicology group of the IMDEA Water Institute was dedicated to evaluate the sensitivity of aquatic invertebrates to the neonicotinoid insecticide imidacloprid and to an equimolar mixture of five neonicotinoids (imidacloprid, acetamiprid, thiacloprid, thiamethoxam, clothianidin) using outdoor freshwater pond mesocosms. The objectives of this study were three-fold. First, to compare the sensitivity of aquatic invertebrates to neonicotinoid insecticides under Mediterranean conditions with that reported by studies performed under different environmental conditions (e.g. North and Central Europe, sub-tropical regions). Second, to propose short-term and long-term threshold values for imidacloprid, and for neonicotinoid mixtures, for surface waters. Third, to test the concentration addition hypothesis for assessing neonicotinoid mixture toxicity risks for aquatic populations and communities, and to test the validity of several risk assessment methods to predict mixture toxic effects under semi-field conditions.

The study identified three taxa of micro- and macroinvertebrates (Cyclopoida, Cloeon dipterum and Chironomini) as highly sensitive to neonicotinoids, and found insecticide-related population declines at concentrations around 0.2 μg/L. The sensitivity of these taxa was found to be higher than that reported in previous studies performed under less warm conditions, suggesting that the high temperatures of Mediterranean ecosystems are a key factor influencing neonicotinoid toxicity. The study proposes 0.1 μg/L and 0.03 μg/L as threshold concentrations to assess short-term and long-term neonicotinoid exposure risks under Mediterranean conditions.  Moreover, as result of this study, we showed that the concentration addition model can be used as a plausible hypothesis to assess short-term mixture effects in aquatic ecosystems. Long-term mixture toxicity assessments, however, should consider the fate of the evaluated substances in the environment of concern. This study also demonstrated that novel mixture toxicity methods (such as the multi-substance Potentially Affected Fraction method) provide accurate estimations to predict risks for freshwater species assemblages, and therefore could be recommended for ecological risk assessments as part of the Water Framework Directive.

This is the first study conducted at the mesocosm research station managed by the Ecotoxicology group of the IMDEA Water Institute. Such mesocosm facility is one of the most advanced ones in South-Europe, and consists of 24 pond mesocosms, 9 artificial streams and a biodiversity lagoon. Such facilities can be used to assess the effects of chemicals (and other abiotic stressors) on aquatic population and communities, including invertebrates and primary producers. The mesocosm facilities will be used within the ECORISK2050 project (H2020-MSC-Innovative Training Networks) to assess how climate change and chemical stress affect freshwater Mediterranean ecosystems.

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The study has been published in Aquatic Toxicology