Dung beetles are possibly the most unsung of eco-heroes. Spending all of their lives, from egg to adult, in and around piles of dung, they hasten its decomposition, either by transporting it into the ground or consuming it. But which dungs are the most attractive to dung beetles and what attracts a beetle to its favourite deposit? ‘Many authors thought that these insects are just attracted by many kinds of faecal volatile compounds and that they were not selective, but when I was in the field I could see that I only found certain insects in certain kinds of dung, so I thought that this insect can choose or select dung probably by volatile compounds,’ explains Laurent Dormont from the CNRS Centre d'Ecologie Fonctionnelle et Evolutive, France. Curious to find out whether a dung beetle's dung preferences are hard wired or set by the environment they encounter during development, Dormont, Pierre Jay-Robert and Jean-Pierre Lumaret teamed up to test the French scarab beetle's dung preferences ().F1
Collecting freshly laid Agrilinus constans eggs from cowpats in fields around Montpellier, Dormont and Jay-Robert brought the eggs back to the lab and settled them either in fresh cowpats, horse, sheep or wild boar dung. After allowing the eggs to hatch and the larvae to develop into adults, Dormont and Jay-Robert tested the insect's preferences. Offering them a choice between cowpat and horse dung or cowpat and wild boar dung, the team were surprised to see that the beetles always chose to settle in the cowpat, regardless of which type of dung that they had been raised in. And the beetles also preferred sheep dung, even when they had developed in horse and wild boar dung.
‘This was a surprising result. We thought that the insect would prefer the dung volatiles in which they had developed, but when we did the experiment the insect preferred cattle or sheep dung even if they came from wild boar or horse dung,’ says Dormont. And when the team tested how the presence of other dung residents affected A. constans' responses, they found that the beetles avoided dung that had been colonised by other species but happily settled in dung occupied by their own kind.
Finally, Dormont teamed up with chemists Jean-Marie Bessière and Sylvie Rapior to analyse the volatile components from each type of dung to see if they could identify the compounds that the beetles found so irresistible. Dormont collected samples of each odour and analysed them with combinations of gas chromatography and mass spectrometry. ‘Jean-Marie Bessière is a flower scent specialist, so for him it was really new and he was very excited to identify dung odours. He knows plant odours well; however, the volatiles in dung have been transformed by digestion, so it was difficult for him to analyse and identify these compounds,’ explains Dormont. However, the team eventually found that all four odours shared nine components. Cow dung turned out to have the most complex odour, with 36 volatile compounds, while the wild boar was the least complex, with only 25 compounds.
Dormont and his colleagues are now keen to identify the volatile components that the beetles respond to with electrophysiology to understand why sheep and cow dung are the French scarab beetles' dungs of choice.
