Trickle-down effects of the cattle parasiticide ivermectin on soil microfauna on a prairie grassland

Ivermectin is a veterinary drug that is used globally to control internal and external cattle parasites. Dung of cattle treated with ivermectin contains residues that may transfer from the dung pat into the soil to affect soil organisms. We examined the effects of ivermectin in cattle dung on collembolans, nematodes and five taxonomic groups of mites in soils of a native grassland in southern Alberta, Canada. The trial, conducted in 2016 and repeated in 2017, comprised three treatments: (i) dung spiked with ivermectin and placed on soil; (ii) dung without ivermectin and placed on soil; and (iii) bare soil. Soil cores were taken beneath and adjacent to dung pats 1, 2, 4, 8, 12 and 16 weeks after dung placement and their microfauna quantified. Both generalized linear modeling and path modeling showed that dung without ivermectin increased the abundances of all the taxa relative to bare soil. However, these increases were at least partially negated for nematodes, Mesostigmata and Oribatida if the dung contained ivermectin. Path modeling also revealed unidirectional positive associations between some taxa: from Prostigmata (non-Tydeidae) to Collembola, from Acaridae to Prostigmata (both Tydeidae and non-Tydeidae), and from Mesostigmata to Oribatida. Therefore, deposition of fresh cattle dung enhanced the abundance of all microfauna for up to 16 weeks post-deposition, probably in response to an influx of nutrients and moisture. However, ivermectin in the dung reduced this benefit for three of the seven taxa.