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The Dung Beetles (Coleoptera: Scarabaeidae and Hydrophilidae) of Ayrshire, Scotland
Lisa Webb
Winner of the Hammond Award 2004
Dairy farming shapes the rural landscape of Ayrshire, where silage and grazing pastures provide a mosaic of green fields. In those fields where livestock dung is plentiful, a valuable resource is provided for the insects that utilise dung for all or part of their life cycle.
Fabre (1911) described dung beetles as: the dealers in ordure, the scavengers of the meadows contaminated by the herd
. This 'scavenging' of dung by beetles is extremely beneficial as it accelerates dung decomposition on pastures thus ultimately improving nutrient cycling. Although the endemic species of Britain are small in comparison to the larger dung-rolling African species, they nevertheless cause rapid degradation of dung when present in large numbers. With their many invertebrate inhabitants, dung pats provide bountiful feeding grounds for predatory beetles e.g. Staphylinids, Carabids and Elaterids, as well as for vertebrate predators.
An adult Aphodius fossor dung beetle
Dipteran species are among the first invaders of dung, with many different species using the dung for oviposition (Skidmore, 1991). Common dung-breeding species include the dung-flies (Scatophagidae) and lesser dung-flies (Sphaeroceridae). The dung beetles, accompanied by their parasitic mites, also arrive at dung when it is very fresh although there is an element of succession within the dung beetles themselves. Early successional species include Aphodius depressus (Kugelann), A. prodromus (Brahm), A. rufipes (L.), A. sphacelatus (Panzer), Sphaeridium lunatum (Fabricius), S. scarabaeoides (L.) and Cercyon species (Gittings and Giller, 1998, Hanski, 1980). Both A. ater (Degeer) and A. rufus (Moll) are mid-successional species and A. fimetarius (L.) and A. fossor (L.) are late successional species (Gittings and Giller, 1998). Once dung is in an advanced state of degradation, soil invertebrates, including earthworms, begin to move into the dung pat.
In cattle-grazed pastures in Ayrshire (Grid references NS32, NS42, NS43, NS52), ten species of Aphodius, six species of Cercyon and two species of Sphaeridium were recorded from April to July in 2002 and 2003. The dung beetle fauna was sampled using dung-baited pitfall traps. These traps are ideal for use by anyone who wishes to record dung insect fauna while avoiding the messier procedure of directly sampling dung. Incidentally, these baited traps were excellent at trapping yellow dung flies, Scatophaga stercoraria (L.), too.
A dung baited pitfall trap
The Aphodius recorded ranged in size from 4mm (A. pusillus) to 13mm (A. rufipes). The most common Scarabaeid species were Aphodius depressus and A. prodromus. The former species was abundant in all months from May to July and Jessop (1986) described it as the most common Aphodius species in Scotland. In contrast, A. prodromus displayed a strong seasonal pattern in abundance. It was frequently trapped in large numbers in late-April and May (sometimes as many as 1000 individuals in one trap!) but it occurred in far fewer numbers in June and July. A. sphacelatus, which is morphologically similar to A. prodromus, had the same seasonal pattern as A. prodromus although it was not as abundant. Both species are described as 'spring and autumn' species (Gittings and Giller, 1997), but this sampling period only encompassed the 'spring peak'.
One of the largest Aphodius species trapped was A. rufipes and this late-summer species occurred in highest numbers throughout June and July. The early summer species A. ater did not show any peaks in abundance from April to July although it may not have been recorded in high enough numbers to detect any seasonal patterns. A. rufus has a locally common distribution in Scotland (Jessop, 1986) and it was recorded in low numbers only in one pasture adjacent to woodland. Another species described as local is A. pusillus (Herbst), which was only observed in a few pastures in one area. In a Finnish study, A. pusillus exhibited limited dispersal ability and existed as a metapopulation (Roslin and Koivunen, 2001). Perhaps this might explain its restricted occurrence in my study area and it would certainly be interesting to study the spatial distribution of this species in more detail. Of the three remaining species recorded, A. fimetarius, A. fossor, A. lapponum Gyllenhal, all occurred in relatively low numbers. Both A. fimetarius and A. lapponum have been described as common species in Scotland and A. fossor as uncommon in southern Scotland (Jessop, 1986).
The Cercyon are a group of small, rather convex species ranging in size from 1.5 - 4.5mm. Of the species trapped here, Cercyon melanocephalus (L.) is perhaps the most distinctive with its reddish elytra and black triangular colouration over the scutellar area. C. melanocephalus (L.) and C. atomarius (Fabricius) were the most common hydrophilids, followed by C. lateralis (Marsham) and C. haemorrhoidalis (Fabricius). The two species C. lugubris (Olivier) and C. pygmaeus (Illiger) were recorded in relatively low numbers. The Cercyon did not show any distinct seasonal patterns of abundance although there was a slight increase in abundance over the summer, with most trapped in July. Sphaeridium lunatum and S. scarabaeoides occurred in low numbers in all months.
Some of these dung beetles rely on dung for all of their life cycle while other species are more general and will perhaps only adopt coprophagy at a particular stage of development. Aphodius adults are coprophagous and, depending on species, their larvae are coprophagous or saprophagous. For example, the larvae of A. prodromus and A. sphacelatus often develop in soil and feed on decaying plant matter (Gittings and Giller, 1997). Of the Hydrophilids commonly found in dung, Sphaeridium have been described as specialists and Cercyon as generalists (Koskela, 1979). Adult Sphaeridium feed on dung while their carnivorous larvae feed on fly larvae within the dung (Sowig, 1997). Cercyon can feed and develop in all types of decaying matter including dung (Hansen, 1987).
Dung availability is important for the sustainability of dung beetle populations because once a beetle has emerged it must then be able to locate dung in order to feed and reproduce. The 'suitability' of dung is also crucial because different species can have varied preferences for dung that are guided by their life cycle requirements. For example, the moisture content of dung is an important factor as A. ater, A. fimetarius, A. rufus and A. fossor all prefer to oviposit in the drier parts of dung (Gittings and Giller, 1997). Sphaeridium are able to exploit dung that is too wet for some species (Anderson, Merritt and Loomis, 1984) and the author has noted that Cercyon show a preference for moister dung.
Potential threats to dung beetle fauna include the use of avermectinbased anthelmintics in livestock. Avermectin residues remain insecticidally active once passed into the faeces of the treated livestock, therefore there is concern that exposure to residues may adversely affect dung beetle populations. The author is currently researching avermectin effects on dung beetles as part of a PhD funded by the Scottish Agricultural College and the Royal Society for the Protection of Birds. One of the research aims is to determine whether these residues have a significant negative impact on dung insect populations or whether the populations can remain resilient to any localised declines.
In conclusion, dung beetles are fascinating insects and they also have an important function in farmland ecosystems. Their role in dung breakdown and as valuable prey for vertebrate predators should not be underestimated. To quote Fabre (1911) once again: Notwithstanding their filthy trade, the dung beetles occupy a very respectable rank
.
References
Anderson, J.R., Merritt, R.W. and Loomis, E.C. (1984). The Insect-Free Cattle Dropping and Its Relationship to Increased Dung Fouling of Rangeland Pastures. J. Econ. Entomol. 77: 133-41.
Fabre, J-H. (1911). The Life and Love of the Insect. A&C Black, London. Gittings, T. and Giller, P.S. (1998). Resource quality and the colonisation and succession of coprophagous dung beetles. Ecography 21: 581-92.
Gittings, T. and Giller, P.S. (1997). Life history traits and resource utilisation in an assemblage of north temperate Aphodius dung beetles (Coleoptera: Scarabaeidae) Ecography 20: 55-66.
Hansen, M. (1987). The Hydrophiloidea (Coleoptera) of Fennoscandia and Denmark. Fauna Entomologica Scandinavica 18.
Hanski, I. (1980). Migration to and from cow droppings by coprophagous beetles Ann. Zool. Fennici 17: 11-16.
Jessop, L. (1986). Dung Beetles and Chafers (Coleoptera: Scarabaeoidea) Handbk. Ident. Br. Insects. 5, Royal Entomological Society of London.
Koskela, H. (1979). Patterns of diel flight activity in dung-inhabiting beetles: An ecological analysis. OIKOS 33: 419-39.
Roslin, T. and Koivunen, A. (2001). Distribution and abundance of dung beetles in fragmented landscapes Oecologia 127: 69-77.
Skidmore, P. (1991). Insects of the British Cow-Dung Community, Occasional Publication 21, Field Studies Council.
Sowig, P. (1997). Predation among Sphaeridium larvae: the role of starvation and size differences (Coleoptera Hydrophilidae) Ethology, Ecology &Evolution 9: 241-51.
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