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

Abstract

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.

Figure 1

(A) Aerial image of the field site illustrating the placement of the transect (blue line) (Charles Sturt University, Wagga Wagga, NSW, Australia). The direction of the prevailing wind is indicated by an arrow. (B) Image of a synthetic baited trap (for dung baited traps, vials are replaced with 50 g of fresh dung). Traps consisted of a tray half filled with water. Wire mesh covering the tray and water to prevent beetles from flying away and inhibit predation.

Figure 2

Rarefaction curve showing the cumulative number of dung beetle species collected (total of all treatments) with their total abundance in all four sessions. Line was produced at 95% confidence Interval.

Figure 3

Mean trap catch (± SE) in winter, spring, and summer (Kruskal–Wallis, Dunn ‘s test). For all four sessions P < 0.001 as determined by Kruskal–Wallis one-way nonparametric analysis of variance. Bars accompanied by the same letters are not significantly different as determined by Dunn's all pairwise comparison test. Refer to Table S3 for species composition. n = 4. C—bait free, D—dung bait, M1—skatole + indole + phenol + butyric acid + butanone + p-cresol, M2—M1 + dimethyl sulphide + dimethyl disulphide, M3—M1 + eucalyptol, M4—M1 + toluene, M5—M1 + dimethyl sulphide + dimethyl disulphide + eucalyptol + toluene, M6—dimethyl sulphide + dimethyl disulphide + eucalyptol + toluene.

Figure 4

Mean trap catch (± SE) of Omorgus sp. caught in dung and synthetic baits. Different letters indicate significance among bait types at P < 0.001 as determined by Kruskal–Wallis one-way analysis of variance followed by Dunn’s means comparison test. n = 4. C—bait free, D—dung bait, M1—skatole + indole + phenol + butyric acid + butanone + p-cresol, M2—M1 + dimethyl sulphide + dimethyl disulphide, M3—M1 + eucalyptol, M4—M1 + toluene, M5—M1 + dimethyl sulphide + dimethyl disulphide + eucalyptol + toluene, M6—dimethyl sulphide + dimethyl disulphide + eucalyptol + toluene.

Figure 5

Qualitative representation of total ion chromatograms (TICs) and retention times (min ± 0.05) indicating differences in major VOCs detected in cattle dung headspace used as field baits. All annotated compounds were confirmed by standards injections.