Standardized laboratory tests with 21 species of temperate and tropical sepsid flies confirm their suitability as bioassays of pharmaceutical residues (ivermectin) in cattle dung

Veterinary pharmaceuticals excreted in the dung of treated livestock can have strong non-target effects on the dung organism community. We report results of ecotoxicological tests with ivermectin for 21 species of temperate (Europe, North America) and tropical (Asia, Central America) black scavenger flies (Diptera: Sepsidae), using standardized methods developed previously for the yellow dung fly and the face fly. Our study documents great variation in ivermectin sensitivity of more than two orders of magnitude among species and even populations within species: estimated lethal effect concentrations LC50 (at which 50% of the flies died) ranged from 0.05 to 18.55 μg/kg dung fresh weight (equivalent to 0.33–132.22 μg/kg dung dry weight). We also show that controlled laboratory tests can—within reasonable limits—be extended to the field or to laboratory settings without climate control, as obtained LC50 were roughly similar. In addition to lethal effects, our study revealed relevant sub-lethal effects at lower ivermectin concentrations in terms of prolonged development, smaller body size and reduced juvenile growth rate. Finally, oviposition choice experiments showed that females generally do not discriminate against dung containing ivermectin residues. We conclude that sepsid flies are well suited test organisms for pharmaceutical residues in the dung of livestock due to their ease and speed of rearing and handling, particularly in the tropics, where high-tech laboratory equipment is often not available.

► Toxicity of ivermectin was determined in a comparable way in 21 sepsid species.
► Sensitivity differed strongly within and between these 21 fly species.
► In choice tests females did not discriminate between dung with/without ivermectin.
► Sepsid flies are well suited test organisms for pharmaceutical residues in cattle dung.