Contrasting Volatilomes of Livestock Dung Drive Preference of the Dung Beetle Bubas bison (Coleoptera: Scarabaeidae)

Field evaluation of electrophysiologically-active dung volatiles as chemical lures for trapping of dung beetles

Dung beetles are economically important beneficial insects that process dung. To locate this source, they use volatile organic compounds (VOCs). The objectives of the study were to evaluate the attractiveness of ten electrophysiologically-active dung volatiles (phenol, skatole, indole, p-cresol, butanone, butyric acid, eucalyptol, dimethyl sulphide, dimethyl disulphide, and toluene) to dung beetles in the field and to investigate how the composition of volatile blends influences efficacy as lures for use in traps. Six combinations of the compounds were compared with field collected cattle dung bait and a negative control, across three seasons. Both dung and synthetic baits captured all exotic dung beetle species present in the study area. A six-compound mix (M1), comprising major dung volatiles, served as an attractive chemical mixture. The addition of dimethyl sulphide, dimethyl disulphide (M2) and toluene (M4) enhanced attractancy of M1 for dung beetles, while eucalyptol (M3) decreased the attractancy. The degree of attraction by various dung beetle species to synthetic baits varied, but baits proved to be effective, especially for summer trapping. The trap design used in this study presented a convenient and practical way to sample dung beetle and other associated scarabs from open pastures. The attraction of introduced dung beetle species to synthetic baits is documented here for the first time in Australia. In addition, necrophagous Omorgus sp. is reported here for the first time to be attracted to synthetic baits. They showed a significant attraction to the mixture containing dimethyl sulphide and dimethyl disulphide (M2). The current study represents a promising first step towards formulating a synthetic chemical lure for dung beetles, offering a consistent, standardised, and bio-secure trapping method compared to use of naturally occurring dung baits, especially as a multi-species lure.

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

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.