Abstract
Among macrocyclic lactones (ML), ivermectin (IVM) and moxidectin (MOX) potentially affect all Ecdysozoan species, with dung beetles being particularly sensitive. The comparative effects of IVM and MOX on adult dung beetles were assessed for the first time to determine both the physiological sub-lethal symptoms and pre-lethal consequences. Inhibition of antennal response and ataxia were tested as two intuitive and ecologically relevant parameters by obtaining the lowest observed effect concentration (LOEC) values and interpolating other relevant toxicity thresholds derived from concentration-response curves (IC50, as the concentration of each ML where the antennal response is inhibited by half; and pLC50, as the quantity of ingested ML where partial paralysis was observed by half of treated individuals) from concentration-response curves. Both sub-lethal and pre-lethal symptoms obtained in this study coincided in that IVM was six times more toxic than MOX for adult dung beetles. Values of LOEC, IC50 and pLC50 obtained for IVM and MOX evaluated in an environmental context indicate that MOX, despite needing more time for its elimination in the faeces, would be half as harmful to dung beetles as IVM. This approach will be valuable to clarify the real impact of MLs on dung beetle health and to avoid the subsequent environmental consequences.
Highlights
Among macrocyclic lactones (ML), ivermectin (IVM) and moxidectin (MOX) potentially affect all Ecdysozoan species, with dung beetles being sensitive
Given that the majority of the MLs administered as veterinary medical products (VMP) are excreted through cattle dung retaining their insecticidal activity[12], a number of studies have been undertaken to assess the ecotoxicology of MLs on dung beetles
Fitting the % inhibition of antennal response data to log vs. normalised response models allowed for the interpolation of concentrations of both VMPs that inhibited the antennal response in S. cicatricosus adults by 50% (IC50) (P < 0.05; Table 1; Fig. 2)
Summary
Among macrocyclic lactones (ML), ivermectin (IVM) and moxidectin (MOX) potentially affect all Ecdysozoan species, with dung beetles being sensitive. Inhibition of antennal response and ataxia were tested as two intuitive and ecologically relevant parameters by obtaining the lowest observed effect concentration (LOEC) values and interpolating other relevant toxicity thresholds derived from concentration-response curves (IC50, as the concentration of each ML where the antennal response is inhibited by half; and pLC50, as the quantity of ingested ML where partial paralysis was observed by half of treated individuals) from concentration-response curves Both sub-lethal and pre-lethal symptoms obtained in this study coincided in that IVM was six times more toxic than MOX for adult dung beetles. Toxic effects of IVM and MOX would manifest in insects as a reduction in sensorial response of antennae, paralysis and irreversible ataxia of somatic muscles, and death Due to their actions on both GluCl and GABA ion channels, IVM and MOX potentially affect all Ecdysozoan species as both target and non-target organisms[10]. Following the same methodological guidelines proposed previously[19], both the sensorial response of antennae (sub-lethal effect) and irreversible ataxia of somatic muscles (pre-lethal effect) were examined by exposing a model dung beetle species (Scarabaeus cicatricosus (Lucas, 1846); Coleoptera, Scarabaeidae) to both IVM and MOX under different dose concentrations
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