Toxicity of abamectin and difenoconazole mixtures to a Neotropical cladoceran after simulated run-off and spray drift exposure
Introduction
Pesticides applied to agricultural fields to increase the yield may contaminate adjacent watercourses via spray drift, run-off, drainage and/or accidental spills (Capri and Trevisan, 1998). Developed countries, situated in temperate regions, are shifting towards reduced pesticide use as a result of improvements in agronomic practices, whereas developing countries, most of which are in tropical regions, are increasing their use of pesticides and fertilizers as they become wealthier (Sanchez-Bayo and Hyne, 2011, Lewis et al., 2016). Brazil, for example, became the world's top pesticide market consumer in 2008, accounting for approximately 20% of the total world use (Albuquerque et al., 2016). Despite this high use of pesticides in tropical countries like Brazil, there is still relatively little knowledge about the fate and toxicity of pesticides in tropical aquatic ecosystems as compared to temperate systems (Daam and Van den Brink, 2010, Sanchez-Bayo and Hyne, 2011, Carriquiriborde et al., 2014, Diepens et al., 2014, Lewis et al., 2016).
In the absence of data derived under (local) tropical conditions, risk assessments in tropical countries often rely on temperate toxicity data, although it may be debatable whether the fate and effects of chemicals are comparable in geographically distinct ecosystems (Daam and Van den Brink, 2010). Sensitivity comparisons of tropical and temperate species to pesticides have not demonstrated a consistent greater or lesser sensitivity of tropical species as compared to their temperate counterparts, although such comparisons are based on a relatively small tropical dataset (e.g. Maltby et al., 2005, Kwok et al., 2007, Rico et al., 2011). On the other hand, edge-of-field waterbodies in tropical agroecosystems have often been reported to be especially prone to pesticide contamination through runoff resulting from intensive irrigation practices and tropical rainfall (Daam and Van den Brink, 2010, Lewis et al., 2016, Novelli et al., 2016). Furthermore, pesticides are often applied in close proximity to water bodies surrounding agricultural fields, resulting in relatively high levels of spray drift (Castillo et al., 1997, Daam and Van den Brink, 2010, Sanchez-Bayo and Hyne, 2011). Other frequently noted relatively important entry routes of pesticides in tropical countries are dangerous transportation and storage conditions, unnecessary applications and overuse, use of cheaper but more hazardous pesticides, and washing of application equipment in water bodies (Daam and Van den Brink, 2010 and references therein). Consequently, despite the absence of a clear difference in sensitivity, tropical freshwater organisms are likely to be subjected to higher (peak) pesticide concentrations and hence risks in real-world field conditions than their temperate counterparts.
The main Brazilian strawberry crop area in the municipality of Bom Repouso (Minas Gerais) has a tropical climate by altitude, and can be classified as a monsoon-influenced humid subtropical climate according to Köppeńs classification. It is an agricultural area with intensive use of pesticides and previous field studies in this area identified the insecticide/acaricide Kraft® 36 EC (a.i. abamectin) and the fungicide Score® 250 EC (a.i. difenoconazole) as the main pesticides intensively used throughout the year (Nunes, 2010, Nunes and Espindola, 2012). These pesticides are hence likely to occur simultaneously in edge-of-field water bodies in this region and this pesticide mixture may have greater toxic effects to aquatic life in these ecosystems than the individual compounds.
The aim of the present study was to evaluate the toxicity of Kraft® 36 EC and Score® 250 EC to the Neotropical cladoceran Macrothrix flabelligera, a species native and of common occurrence in Brazilian freshwaters (Güntzel et al., 2003, Moreira et al., 2014). Laboratory toxicity tests were conducted with the individual compounds to establish their respective toxicity thresholds. Mixtures of both compounds were also tested to evaluate their combined effect and its underlying mechanism. The potential risks related with exposure to both compounds, alone and in combination, likely to occur in the field through runoff and spray drift was also evaluated through semi-field testing.
Section snippets
Test organism and culture conditions
Macrothrix flabelligera Smirnov, 1992 (Crustacea, Cladocera, Daphnidae) was initially isolated from the Lobo-Broa Reservoir (Itirapina, SP, Brazil) and had been kept in stock cultures for more than 4 years at the Ecotoxicology Laboratory of the Federal University of São Carlos (Brazil). The culture is maintained under controlled temperature (25 ± 1 °C) and photoperiod (12 h light:12 h dark; light intensity ± 1000 lx) in reconstituted water prepared according to standard ABNT (2005) with pH 7.0–7.8,
EC50 values for abamectin and difenoconazole
The mean EC50-48 h for M. flabelligera obtained from the twenty reference tests with potassium dichromate (K2Cr2O7) was 54 ± 17 μg/L (mean ± SD) with a 95% confidence interval (CI) of 46–61 μg/L. All toxicity tests data met all the validity criteria laid down in the guidelines of the Brazilian Association of Technical Standards (ABNT, 2005). Physicochemical conditions of the test solutions used were (minimum – maximum value): pH 7.0–7.7, water temperature 24.5–25.6 °C, electrical conductivity 152–159
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgements
We thank the National Council for Scientific and Technological Development (CNPq). This work was supported by the Brazilian government through the Special Visiting Researcher program (MEC/MCTI/CAPES/CNPq/FAPs reference 402392/2013-2) and the Portuguese government (FCT) through a postdoc grant for the second author (SFRH/BPD/109199/2015).
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