Nesting by the Horned Dung Beetle (Copris lunaris): 1 – Initial stage, male (left) and female (right) working the ‘dung cake’; 2 – Female alone, making brood-balls of the ‘cake’ for laying eggs. Illustration by V.A. Timokhanov (Almaty, Kazakhstan).
Waste disposal is a growing problem for any industrialized nation. The UK alone generates about 100 million tonnes of waste each year, the majority of which is still being disposed of through landfill. The present story is about dung beetles or scarabs (family Scarabaeidae) that are involved in processing and decomposing dung.
On average, about 40% of the food intake of mammals is either excreted as urine or passed out of the body as faeces. This waste is decomposed and returned to the soil by insects that use dung as food for themselves and for their larvae, thereby preventing it from building up. How this is accomplished is best known for cattle dung.
A cow’s fresh dung pat is colonized by a succession of dung-breeding insects, numbering several dozen species and often exceeding 1000 individual insects. A total of 275 species has been reported to occur in cattle dung in Britain. The majority of them are dung beetles that feed directly on dung. There are three main ecological groups of dung beetles. First, small-sized beetles (Aphodius species) usually feed in the main dung mass. Others, like the horned dung beetle, dig burrows beneath the pat and pack pieces of dung into them for feeding their larvae (see figure above). The third group includes beetles that make spherical dung balls, roll them away and bury them intact in shallow burrows. The Sacred Scarab is the most famous of the rollers. As well as dung beetles, the pat is colonized by dung-feeding fly maggots, predatory beetles which feed on eggs and larvae of other insects, small parasitic wasps, fungus-eating insects and mites, etc. At the advanced stage of degradation, soil invertebrates, including earthworms, begin to move into the dung pat. The natural rate of dung degradation depends on temperature, humidity, habitat and season of deposition. In Britain, the complete natural disappearance of a dung pat is achieved in two to three months.
Sculpture of the Sacred Scarab in the Natural History Museum in Stokholm, Sweden. © Dmitri Logunov, Manchester Museum.
It is known that each cow produces an average of 12 dung pats per day, or over 9000 kg of solid waste per year. It is estimated that each year approximately 200 million tonnes of waste are produced by livestock in England and Wales, and about 900 million tonnes in the USA. About third of this is recycled by dung beetles. In the USA alone, the annual economic value of this service is at least $380 million.
Unfortunately, the activity of dung beetles is severely disrupted by current agricultural practices, such as the treatment of livestock with persistent anti-helminth drugs given to kill parasitic worms or helminths. Residues of these drugs can persist in the dung and are lethal to the beetles. As a result, the dung pats of animals treated with anti-helminthes remain biologically undegraded for months, fouling available grazing area. If left unprocessed, livestock wastes may present a health risk to humans, because they can contain some pathogenic microorganisms.
By recycling the nutrients locked up in dead organic materials such as dung, insects make these nutrients available to new life. As recyclers, they do an indispensable job for our planet. Without organisms breaking down dead organic materials and recycling nutrients in the wild, as well as in gardens and on farms, the planet would soon be piled deep with the waste products of its inhabitants, and potential spread of diseases would be unavoidable. Whether we like it or not, our own existence directly depends on insects and their ecological services. As M. Telfer (2004) put it: “Not everyone welcomes having ‘creepy-crawlies’ around but we should be grateful for what they do.”
In the following video, our special guest, Ms Roisin Stanbrook from the Manchester Metropolitan University, is taking about the ecological role of dung beetles in Kenya.
The presented story is based on: Logunov D.V. 2010. Nature’s recycling squad. Biological Sciences Review, 22(3): 22-25.
Further reading:
Berenbaum, M.R. (1995). Bugs in the system. Insects and their impact on human affairs. Helix Books.
Waldbauer, G. (2003). What good are bugs? Cambridge-London: Harvard University Press.
Entomology Manchester 