Evaluation of the attraction, lethal and sublethal effects of the faeces of ivermectin-treated cattle on the dung beetle Digitonthophagus gazella (Coleoptera: Scarabaeidae)

The attraction of the Digitonthophagus gazella dung beetle to the excreta of cattle treated with ivermectin (IVM) at 1% under controlled and field conditions (Phase 1) as well as the lethal (mortality) and sublethal effects (fecundity, emergence, and dung removal) of the excreta of bovines treated with IVM-1% and IVM-3.15% were evaluated (Phase 2). In Phase 1, an olfactometer was used to evaluate the attraction/repellency in the laboratory and field-controlled conditions using faeces of cattle recovered on 5, 14, 21 and 28 days posttreatment (DPT) from animals treated with IVM-1% (0.2 mg/kg) and untreated animals. Under field conditions, pitfall traps baited with faeces from animals treated with IVM-1% (5 DPT), and untreated animals were used to evaluate the attraction/repellency for D. gazella. In all three experiments, the faeces of treated cattle did not affect (P > 0.05) the attraction/repellency of D. gazella towards IVM-1% (P > 0.05). In Phase 2, six cattle were used: (1) three treated with subcutaneous IVM-1% 0.2 mg/kg and (2) three treated with subcutaneous IVM-3.15% (0.63 mg/kg). The collection of faeces was at −1 (control), 5, 14, 21 and 28 DPT. Each bioassay consisted of a terrarium with a pair of sexually mature beetles, where they received 50 g of faeces for 10 days with 15 replicates per group. The bioassays consisted of nine groups: control, IVM-1% at 5, 14, 21 and 28 DPT and IVM-3.15% at 5, 14, 21 and 28 DPT. There were no lethal effects on adults of D. gazella when exposed to faeces treated with IVM-1% and IVM-3.15%; however, faeces collected at 5 DPT from cattle treated with IVM-1% and IVM-3.15% showed a significant reduction in the average number of brood masses produced and in the average number of imagoes emerged (P < 0.05). There were no differences in faeces removal between treatments. In conclusion, D. gazella does not discriminate between faeces of cattle treated with IVM and untreated, and there were no lethal effects seen; however, fertility was significantly reduced at 5 DPT.