Abstract
Neonicotinoid insecticides are widely used agents in agriculture to control a broad range of insect pests. Although use of neonicotinoid pesticides has resulted in the widespread contamination of surface waters, sublethal toxicity data of these products in relation to non-target aquatic biota are still poor. Therefore, the objective of this study was to assess the effects of two neonicotinoid pesticides with widespread use on the basic physiological functions: the thoracic limb activity and heart rate of Daphnia magna, and to screen for their potential to affect the cytochrome P450 monooxygenase system (ECOD activity) of daphnids. The considered pesticides were the acetamiprid- and thiacloprid based products Mospilan 20 SG and Calypso 480 SC, respectively. The dose-dependent variation in the three biological endpoints considered were assessed following 24 h exposures. The two neonicotinoid formulations elicited significant depression on the thoracic limb activity and heart rate of daphnids at doses close to the immobility thresholds of formulations (48h-EC50: Mospilan 20 SG = 190 mg L-1; Calypso 480 SC = 120 mg L-1), an effect mainly attributable to the overall drop in the general health status of the organisms. The alterations in the physiological traits were significant at exposures to 190 mg L-1 for Mospilan 20 SG and 48 mg L-1 for Calypso 480 SC. The dose related variation in the ECOD activity of daphnids exposed to the selected neonicotinoid formulations followed a biphasic pattern, with starting effective doses for Mospilan 20 SG of 6.3 mg L-1 (=1/20 of 48h-EC50 for Daphnia neonates), and for Calypso 480 SC of 0.034 mg L-1 (=1/4000 of 48h-EC50). Maximal ECOD activity (2.2 fold increase vs. controls) was induced by Mospilan 20 SG in daphnids exposed to 114 mg L-1 product (=48 h-EC20), and by Calypso 480 SC (1.8 fold increase) at 5.2 mg L-1 dose (=1/20 of 48 h-EC50). Our results outlined significant alterations in the physiological traits and ECOD activity in exposed daphnids at concentrations below the immobility thresholds (48 h-EC50) of the products used as benchmarks to rate their toxicity risks to aquatic biota. Therefore, we think our findings might deserve consideration in the environmental risk evaluation of these products.
Highlights
Neonicotinoids are systemic insecticides widely used in crop production to control a broad range of insect pests
Neonicotinoids act on the nicotinic acetylcholine receptors in the central nervous system, through a mechanism highly conserved across many insect species, and on other, non-target aquatic invertebrates (Zhang et al, 2000; Tomizawa and Casida, 2003, 2005; Vehovszky et al, 2015)
In terrestrial insects it was already evidenced that differences in the sensitivity to neonicotinoids is primarily dependent on the differential affinity of neonicotinoids for the nicotinic acetylcholine receptor as the target site (Taillebois et al, 2014; Crossthwaite et al, 2017; Manjon et al, 2018)
Summary
Neonicotinoids are systemic insecticides widely used in crop production to control a broad range of insect pests (as reviewed by Morrisey et al, 2015). Owing to their high water solubility and proven persistence in soil and water, neonicotinoids became ubiquitous pollutants even in the aquatic environment, where they entered mainly through agricultural drainage (CCME, 2007; EFSA, 2008; Morrisey et al, 2015; Raby et al, 2018a). To better understand the real risks of neonicotinoids to cladocerans, knowledge on their effects on the basic physiological functions of these organisms and on the efficiency to metabolize this class of xenobiotics are essential. That even in the case of daphnids their low sensitivity to neonicotinoids is attributed to a low density- or lack in specific receptor configurations, and the xenobiotic metabolizing system of the organism will affect the toxicity potential of neonicotinoid pesticides
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