The interaction between Onthophagus binodis and cattle dung pH: Impacts on reproduction and offspring phenology

The environment surrounding invertebrates can influence the physiology of larval offspring. Dung beetles provide several significant ecological functions, including dung breakdown, fly control and nutrient cycling. Cattle diet influences the chemical and physical constituents of dung, of which pH is considered critical. Few studies have assessed this, though a pH of 6.3 is the lowest threshold for dung beetle reproduction. We investigated the effects of an introduced and widespread dung beetle (Onthophagus binodis) on cattle dung pH (7.3, 6.0 and 5.0) and pH on O. binodis reproduction, offspring phenotypic traits and development time. Dung beetle presence increased the Δ pH (more alkaline) within dung pads after 96 h. Dung beetles produced broods in dung with a pH of 5.0, though in fewer numbers compared with the other pH treatments. Larval development was delayed in pH 5.0 with an average of 50 days compared with 44 days in dung with pH 6, 7, and the control (7.3). Smaller broods (ellipsoid volume [mm3]) were produced in dung with a pH of 5.0 compared with pH 6.0 and 7.0, and offspring emerging from broods produced from dung with a pH of 6.0 were larger compared with the other pH treatments. Our results show that dung pH is important for brood production and progeny phenotypic traits of O. binodis, an agricultural ecosystem engineer and that there is no experimental evidence to support the suggestion that dung pH influences the provisioning of broods in this species. The presence of dung beetles made cattle dung more alkaline after 96 hours. Dung beetles were able to reproduce in cattle dung with a pH of 5.0, though in fewer numbers compared to other pH treatments. There were no negative impacts on emerging beetle physiology, though dung beetles emerging from pH 6.0 were larger compared to other pH treatments.