Cumulative toxicity of neonicotinoid insecticide mixtures to Chironomus dilutus under acute exposure scenarios

Abstract

Extensive agricultural use of neonicotinoid insecticide products has resulted in the presence of neonicotinoid mixtures in surface waters worldwide. Although many aquatic insect species are known to be sensitive to neonicotinoids, the impact of neonicotinoid mixtures is poorly understood. In the present study, the cumulative toxicities of binary and ternary mixtures of select neonicotinoids (imidacloprid, clothianidin, and thiamethoxam) were characterized under acute (96-h) exposure scenarios using the larval midge Chironomus dilutus as a representative aquatic insect species. Using the MIXTOX approach, predictive parametric models were fitted and statistically compared with observed toxicity in subsequent mixture tests. Single-compound toxicity tests yielded median lethal concentration (LC50) values of 4.63, 5.93, and 55.34 μg/L for imidacloprid, clothianidin, and thiamethoxam, respectively. Because of the similar modes of action of neonicotinoids, concentration-additive cumulative mixture toxicity was the predicted model. However, we found that imidacloprid-clothianidin mixtures demonstrated response-additive dose-level-dependent synergism, clothianidin-thiamethoxam mixtures demonstrated concentration-additive synergism, and imidacloprid-thiamethoxam mixtures demonstrated response-additive dose-ratio-dependent synergism, with toxicity shifting from antagonism to synergism as the relative concentration of thiamethoxam increased. Imidacloprid-clothianidin-thiamethoxam ternary mixtures demonstrated response-additive synergism. These results indicate that, under acute exposure scenarios, the toxicity of neonicotinoid mixtures to C. dilutus cannot be predicted using the common assumption of additive joint activity. Indeed, the overarching trend of synergistic deviation emphasizes the need for further research into the ecotoxicological effects of neonicotinoid insecticide mixtures in field settings, the development of better toxicity models for neonicotinoid mixture exposures, and the consideration of mixture effects when setting water quality guidelines for this class of pesticides. Environ Toxicol Chem 2017;36:3091-3101. © 2017 SETAC.

Chronic toxicity of 6 neonicotinoid insecticides to Chironomus dilutus and Neocloeon triangulifer

“…The 96‐h median lethal concentrations (LC50s) for C. dilutus ranged from 1.6 µg L −1 for thiacloprid to 61.9 µg L −1 for thiamethoxam (Raby et al ). Similar 96‐h LC50 estimates exist in the literature, and range from 1.85 µg L −1 for clothianidin to 55.34 µg L −1 for thiamethoxam (Stoughton et al ; LeBlanc et al ; de Perre et al ; Maloney et al ). Neocloeon triangulifer was more sensitive than C. dilutus ; 96‐h LC50s for all 6 neonicotinoids studied were <10 µg L −1 (Raby et al ).…”

Neonicotinoid Insecticides in Surface Water, Groundwater, and Wastewater Across Land‐Use Gradients and Potential Effects

“…It is also unclear how co‐occurring neonicotinoids impact aquatic organisms. For example, neonicotinoid mixtures have been shown to cause interactions, but the effects have been both synergistic and antagonistic (Warne and Hawker 1995; Deneer 2000; Maloney et al 2017, 2018). Because neonicotinoid‐based products often contain mixtures of neonicotinoids or other pesticides (Hladik et al 2014; Main et al 2014; Sánchez‐Bayo and Hyne 2014), future research should address this gap in knowledge.…”

Acute Effects of Binary Mixtures of Imidacloprid and Tebuconazole on 4 Freshwater Invertebrates

“…Binary-mixture toxicity data were first assessed using MIXTOX. A description of MIXTOX can be found in Jonker et al (2005), and brief overviews have been summarized by previous studies employing the approach (Loureiro et al 2010;Pavlaki et al 2011;LeBlanc et al 2012;Rodney et al 2013;Maloney et al 2017). Two models, commonly used to describe cumulative (or additive) toxicity of mixtures, are included in MIXTOX: concentration addition and independent action.…”