Characterisation of Antennal Sensilla and Electroantennography Responses of the Dung Beetles Bubas bison, Onitis aygulus and Geotrupes spiniger (Coleoptera: Scarabaeoidea) to Dung Volatile Organic Compounds

SIMPLE SUMMARY: Insects, including dung beetles, rely on volatile cues to locate food and mates. However, the antennal responses of dung beetles to dung headspace volatiles have received minimal attention. To address this gap, we conducted a scanning electron microscopy study to examine the density distribution of three types of antennal sensilla in three introduced dung beetle species found in Australia: Geotrupes spiniger, Bubas bison and Onitis aygulus. The gross morphology of the antennal sensilla of these species is described here for the first time. Notably, we observed distinct patterns of sensilla trichodea, sensilla basiconica and sensilla chaetica on the proximal and distal surfaces of three lamellae in their antennal clubs. Furthermore, using electroantennography, we investigated the olfactory responses of these dung beetles to ten selected dung volatiles and mixtures of the same volatiles. The test chemicals evoked differential antennal responses in all three test species. The results are discussed in relation to the distribution and density of the antennal sensilla and the potential role of dung headspace volatiles in dung preference by these dung beetles. Overall, our findings indicate the possibility of using EAG-active compounds to attract dung beetles in the field. ABSTRACT: Locating sporadically distributed food resources and mate finding are strongly aided by volatile cues for most insects, including dung beetles. However, there is limited information on the olfactory ecology of dung beetles. We conducted a scanning electron microscopy study on the morphology and distribution of the antennal sensilla of three introduced dung beetle species in Australia: Geotrupes spiniger (Coleoptera: Geotrupidae), Bubas bison and Onitis aygulus (Coleoptera: Scarabaeidae). Three main morphological types of antennal sensilla were identified: sensilla trichodea (ST), sensilla basiconica (SB) and sensilla chaetica (SCh). Distinct variations of SB distribution were observed in B. bison and G. spiniger and on different lamellar surfaces in both sexes of all three species. Sexual dimorphism in antennal sensilla distribution or their abundance was not evident. To complement the morphological characterisation of sensilla, electroantennography (EAG) was carried out to construct EAG response profiles of the three species to selected dung volatiles. An initial study revealed that antennae of all species were sensitive to a mix of phenol, skatole, indole, p-cresol, butanone and butyric acid, common components of livestock dung headspace. In addition to these six compounds, dimethyl sulfide, dimethyl disulfide, eucalyptol and toluene were tested for antennal activity. All compounds evoked measurable EAG responses, confirming antennal sensitivity. Geotrupes spiniger exhibited significant responses to all the compounds compared to the control, whereas B. bison and O. aygulus only responded to a subset of compounds. A comparison of relative EAG amplitudes revealed highly significant responses to p-cresol in G. spiniger and to skatole in B. bison. Geotrupes spiniger displayed differential responses to all the compounds. Pooled EAG data suggest highly significant differences in responses among the three species and among compounds. Our findings suggest that a blend of volatiles may offer potential for the trapping of dung beetles, thereby avoiding the use of dung baits that are inconvenient, inconsistent and may pose a threat to farm biosecurity.