Abstract
The insecticidal properties of many anthelmintics pose a risk to dung fauna through the effects of drug residues in dung on the activity, oviposition and development of dung-dwelling invertebrates. Reductions in dung fauna numbers can inhibit dung degradation, which may impact biodiversity and nutrient cycling on farms. A simulation model was created to predict the impact of antiparasitic drugs on cattle dung fauna, and calibrated using published data on the dung-breeding fly Scathophaga stercoraria. This model was then tested under different effective dung drug concentrations (EC) and proportions of treated cattle (PT) to determine the impact under different application regimens. EC accounted for 12.9% of the observed variation in S. stercoraria population size, whilst PT accounted for 54.9%. The model outputs indicate that the tendency within veterinary medicine for targeted selective treatments (TST), in order to attenuate selection for drug resistance in parasite populations, will decrease the negative impacts of treatments on dung fauna populations by providing population refugia. This provides novel evidence for the benefits of TST regimens on local food webs, relative to whole-herd treatments. The model outputs were used to create a risk graph for stakeholders to use to estimate risk of anthelminthic toxicity to dung fauna.
Highlights
Anthelmintic drugs are widely and routinely administered to grazing livestock to control gastrointestinal nematodes and other parasites
We used a novel simulation to test the hypothesis that proportion of cattle treated (PT) had a greater impact on the population size of S. stercoraria than effective dung drug concentrations (EC)
The distribution of data predicted that populations of S. stercoraria were generally resilient and can maintain stable numbers up until a tipping point at which mortality becomes probable
Summary
Anthelmintic drugs are widely and routinely administered to grazing livestock to control gastrointestinal nematodes and other parasites. Anthelmintics are typically not fully metabolized within the host animal and residues of the drugs are often excreted in dung (McKellar et al, 1993) (and urine (McKellar, 1997)) and can exert non-target effects on invertebrate fauna which spend part, or all, of their life cycle in dung (Floate, 1998a, 1998b; Gover and Strong, 1995; Madsen et al, 1990; Sommer et al, 1992; Sutton et al, 2014) and on soil invertebrates (Scheffczyk et al, 2016) Such effects include inhibited motility, oviposition, emergence, and reduced dung pat colonisation (Floate, 1998a, 1998b; Gover and Strong, 1995; Suarez et al, 2003). Refugia from drug residues ought to be generated for dung fauna, supporting their populations; to date no systematic attempts have been made to evaluate this possibility
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