The Delights of Dung
For an insect, dung is a good thing to get into! It is available all year round and is an important and ubiquitous food source for many invertebrates (and vertebrates), supporting whole communities (with different dung types often supporting different communities). Some merely use the dung as a hunting ground and are predators as both adults and larvae, e.g. ‘Dung Devils’ (Ontholestes spp.) or the Hornet Robberfly, (Asilus crabroniformis), while others are dung feeders for only the larval part of their life-cycle, with the adults using another nutritional source, such as other invertebrates e.g. Common Yellow Dung Fly (Scathophaga stercoraria) and Sphaeridium beetles, or nectar and pollen such as certain hoverflies and species such the house-fly Eudasyphora cyanella.
Drab Dung Devil (Ontholestes murinus) lives up to its name, predating a hoverfly which has come to lays its eggs
Probably the most specialised element of the community are those species which are dung feeders at all stages and probably the most important of these are the dung beetles. In the UK there are over 60 species living either within the dung or tunnelling below it and these beetles perform a range of important functions. For one thing imagine the piles of dung that would build up if there were no dung feeding insects! We’d be neck deep in it in no time! Australian cattle farmers encountered a similar problem in the 1950s as antipodean insects had evolved to deal with dry marsupial dung rather than the wet pats of cattle. The accumulating dung was reducing the area of available pasture and providing a breeding ground for pestilential flies and many years were spent introducing dung beetles before the problem was solved!). It is estimated that about 80% of the nitrogen in the faeces remaining on the pasture surface would be lost without the actions of these beetles. Species such as Minotaur Beetle (Typhaeus typhoeus), Dor Beetles (Geotrupes spp.)
The Dor Beetle Geotrupes stercorarius. Larvae live in dung which the adults have buried. Image: Jessica Towne
and the very rare Horned Dung Beetle (Copris lunaris), which actually bury dung, recycle nutrients in the soil, improving grass growth and therefore the amount and quality of food available to grazing animals. They also break up the soil and improve drainage and aeration and their activities are also responsible for seed dispersal both vertically and horizontally. In addition, buried dung is no longer available to certain species of nuisance fly and this coupled with the fact that dung beetles also carry mites on their body when they travel between piles of dung, which then feed on the eggs of the flies, reduces their overall numbers.
The house-fly Eudasyphora cyanella
Their activities can also reduce the numbers of cattle parasites in the dung by speeding up the drying process (the parasites generally like it ‘wet and warm’). In addition, it has also been shown that dung beetles can reduce greenhouse gas emissions! Dung beetle tunnels in cow pats aerate the pat and the extra oxygen shifts the balance away from methane producing microbes (which don’t like oxygen), reducing the methane emissions from a single pat by up to 40%! The beetles are also an important food source for many other creatures including bats and birds.
Aphodius prodromus, one of a number of beetles whose larvae live within dung. Image: Gail Hampshire
However, in recent years several species of dung beetle have become increasingly uncommon, with over a quarter of UK species now being ‘Nationally Rare’ (found in 15 or less 10×10km squares) and four probably extinct! Changes in farming practises and the disappearance of livestock from historic pastures are a major factor in this decline in the UK, as are changes in the diet of modern livestock (particularly high protein diets) which make the resulting dung less suitable as a food source for the beetles. Ironically, considering their natural role in parasite reduction, one of the main causes of decline is the overuse of wormers such as Ivermectin, These chemicals come out in the cattle dung and while the adult dung beetles will still be attracted, they can be fatal to some beetle larvae as well as causing side-effects such as egg failure.