Lethal and Sublethal Effects of Ivermectin on Onthophagus landolti (Coleoptera: Scarabaeidae)

Previous work has documented toxic effects of ivermectin (IVM) on dung beetles from the Old World, but very little is known about this drug's effect on Neotropical dung beetles. Accordingly, we conducted a bioassay with dung spiked with IVM to assess its lethal and sublethal effects on the Neotropical dung beetle Onthophagus landolti Harold. The experimental design consisted of five treated groups G1, G2, G3, G4, and G5 receiving 0.001, 0.01, 0.1, 1.0, and 10 mg IVM/kg dung fresh weight, respectively, and two control groups (solvent control [CGA] and untreated control [CGU]). Adult survival and fecundity were measured throughout a 10-d period, and subsequent egg development and survival were monitored. Adult survival was only affected for treatment groups G4 and G5 groups (70 and 30%, respectively); groups G1, G2, G3 and both controls exhibited 100% survival. Fecundity was completely suppressed under treatment groups G4 and G5. Group G3 only had 1.7 and 2.1% brood mass production relative to CGA and CGU, respectively. Additionally, for groups G1 and G2 the proportion of adults emerging from brood masses was lower relative to CGA. Furthermore, development time for the second generation in groups G1 and G2 was 12.5% slower relative to control groups. Finally, dung removal by beetles from groups G3, G4, and G5 was significantly lower relative to control groups. In conclusion, toxic effects of IVM on O. landolti are associated mainly with reduced fecundity and lower dung-removal by adult beetles as well as reduced survival and slower development of offspring.