First Investigation of the Semiochemistry of South African Dung Beetle Species

Excerpt

The inhabitants of cities and large towns are largely unaware of the constant battle that rural communities have to wage against flies. In rural areas, on the other hand, inhabitants do not always appreciate the crucial role that dung beetles play in controlling dung-breeding fly populations. Not only do dung beetles play an important role in the destruction of the habitat of many dung-breeding flies (Heinrich and Bartholomew 1979), but by burying and dispersing dung, the coprophagous fauna associated with mammals is also responsible for returning a large proportion of plant nutrients to the soil (e.g., Bornemissza and Williams 1970).

Serious problems have been experienced in the cattle farming areas of Australia where cattle were introduced without the associated insect fauna. The dung pats left by cattle become the habitat of dung-breeding flies, some of which are blood-feeding pests and carriers of serious diseases (Hughes 1970). This resulted in the uncontrolled increase of fly populations and the deterioration of pastures in parts of that continent. It has been estimated that cow pats are responsible for the reduction of pasture, albeit only temporarily, by about 20% per animal per year (Waterhouse 1974). During the 1950s, George Bornemissza hypothesized that the introduction of foreign dung beetle species that are able to remove and bury cattle dung would aid not only Australia’s soil fertility by recycling the dung nutrients back into the ground, but would also reduce the number of pestilent flies and parasitic worms that use the dung pats as a breeding resource. In 1965, this idea culminated in the establishment of the Australian Dung Beetle Project, and eventually 43 dung beetle species were imported from Africa and other continents in order to combat the fly problem (Wikipedia August 2012). The introduction of exotic dung beetles and their subsequent establishment in Australia was a highly successful venture and largely solved the fly problem in many parts of the continent.

The emergence of problems similar to those in Australia is now also being observed in other countries. The destruction of the habitat of large mammals by urbanization and modern farming practices invariably results in dwindling numbers of dung beetles and an increase in numbers of flies during the warm summer months. The importance of this process is vividly illustrated in rural areas in Africa where visitors to game reserves with normal herbivore populations are rarely bothered by flies during the summer when dung beetle activity reaches its peak, whereas in small settlements a few kilometers outside these reserves the indigenous people are plagued by swarms of flies.

Extensive research has been devoted to the ecology, ethology, and evolution of dung beetles, notably by Bornemissza, Halffter, and their coworkers. Recently, Simmons and Ridsdill-Smith (2011) edited a wide-ranging review of the literature on the ecology and evolution of dung beetles. Although it is estimated that there are more than 4000 dung beetle species in Africa alone, the olfactory ecology of dung beetles remains a largely unexplored yet potentially very fertile research field. Little research has been carried out on the chemical aspects of the ecology of dung beetles, and practically no information was available on the existence, the modes of operation, and the chemical structures of sex attractants of dung beetles before the work discussed in this chapter commenced in 1978.

On the basis of their nesting behavior, dung beetles of the subfamily Scarabaeinae can be divided into three groups: The paracoprids construct their nests under a dung pat by excavating tunnels in which the dung is packed; the endocoprids excavate a chamber in the dung pat itself, forming brood balls within this chamber; and the telecoprids detach a portion of dung from the pat, rolling it some distance from the dung source before burying it. The majority of dung beetles species in southern Africa are mainly crepuscular paracoprids (87%) (Halffter and Matthews 1966). The diurnal telecoprids are also numerous, but are represented by fewer species (12%) (Ferreira 1972).

Larvae and adults of the majority of species of the subfamily Scarabaeinae are coprophagous, and are morphologically adapted to feed on vertebrate excrement (Halffter and Matthews 1966). Adult dung beetles ingest only the liquid or colloidal constituents of the dung by squeezing portions of the moist dung between highly specialized membranous mandibles and ingesting the expressed juice. There are differences in the courtship, mating, and brooding behavior of the Scarabaeinae species. Telecoprids prepare a brood ball from fresh dung, roll the ball to an apparently carefully selected spot at a distance from the dung pat or dung midden, where the female deposits an egg in a chamber constructed at the top of the dung ball. The larva feeds on whole dung particles with the aid of its chewing mouthparts until eventually it pupates (Waterhouse 1974). The next generation, as well as the adult beetles that have overwintered in the soil, emerge from the soil after the first summer rains when temperatures are high enough for habitation and when the soil is moist enough for the beetles to break free from the brood chamber or from the hard soil.

Dung beetles are attracted to dung on which they feed, and after a feeding period of a few weeks the beetles that have overwintered in the soil and those that breed in their first season are ready to start breeding. Dung beetles utilize volatile compounds emitted by dung to locate a fresh source of a species’ preferred dung type, although dung beetles are apparently not linked exclusively to one type of dung (Dormont et al. 2004, 2007).

In areas with intact ecology, dung beetles of many species arrive in the thousands at a dung pat or rhinoceros midden. There is fierce competition for fresh dung between the beetles. Even in the case of the large volumes of dung voided by, for example, rhinoceros, dung beetles manage to dehydrate the dung or to bury or disperse it within a few hours. A photograph of rhinoceros dung voided during the night in the Mkuzi Game Reserve, South Africa, is shown in Figure 3.1a. At about 10:00 the following morning during a sampling period of 20 minutes, 720 dung beetles were recorded arriving at this resource. The entire dung heap appeared to be in constant movement from the activity of thousands of beetles, resulting in the formation of an almost uniform mixture of dung and dung beetles. By that stage, it was already too late for the larger species to gather enough of the already half-dried-out dung for the formation of a brood ball. In the foreground in Figure 3.1a, the last beetles that had managed to form a dung balls can be seen rolling them away. By 15:00 in the afternoon most of the dung was buried underneath the dung or next to it, or rolled away, and only relatively dry plant material was left at this spot.