Trophic preferences mediated by olfactory cues in dung beetles colonizing cattle and horse dung

Scarab beetles colonizing vertebrate feces are considered polyphagous insects, because they often use dung of several species. However, recent works have provided evidence for strong feeding preferences in some dung beetle species. The influence of dung odors in the process of resource selection by beetles has not received attention. The objective of this study was to investigate trophic preferences in Mediterranean dung beetles colonizing cattle and horse dung. The pattern of resource use by insects was studied using series of pitfall traps baited with cattle or horse dung in three different sites of south-central France. A total of 4,276 insects belonging to 39 species were captured. Cattle dung attracted more insects than horse feces (2,570 versus 1,706 individuals). Twenty-four of the 39 beetle species collected had clear feeding preferences for cattle dung (13 species) or horse dung (11 species). None of these species seemed linked exclusively to one kind of dung. The behavioral responses of seven scarab beetles to volatile compounds emitted by cattle and horse dung were compared in laboratory olfactometer bioassays. Three insect species (Aphodius erraticus, A. scrutator, Onthophagus vacca) were more attracted to volatile compounds from cattle dung. Two other species (Euonthophagus amyntas, Bubas bubalus) orientated preferentially toward horse dung volatiles. Except for two species, beetles were thus attracted to volatiles from the dung type they preferred in the field. The reasons why coprophagous beetles show local feeding preferences for particular dung types are discussed.