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Dec 2007 Update: are birds eating too many dung beetles?

An ecologist wrote to me with the theory that 2007 has been a heavy year for bush flies because certain birds had discovered that dung beetles made tasty snacks. She had seen ibises and herons digging in cow pads, finding beetles, and eating them. I asked James Ridsdill-Smith at CSIRO for his comments. He emailed me as follows:

It is interesting with the dung beetle program how people quickly forget how really bad the bush flies used to be. One would not expect the introduced beetles to be removing every scrap of dung produced, and you do not eradicate a major pest like the bush fly. The best you can hope for is to have an average level considerably lower than it was before. At times the flies will disappear and at other times there will be low numbers of flies.

The birds like the ibises have always moved into the irrigation areas in the south of the state in summer. In the early days around 1980 I remember seeing them walking along the front of irrigation water as it moved over the paddock, eating any insects that were forced to the surface. They included beneficial insects like dung beetles and predatory beetles, as well as pest insects like the African black beetle and spring beetle.

I don't think the situation is any worse than it was. People hope that fly control will just get better and better, but I don't think this will occur. We have a new balance in nature now with some winners and some losers.

Jan 1998 Update: the impact of dung beetles on bush fly populations

Following the release and redistribution of introduced dung beetles scientists have been investigating the impact of the dung beetles on bush fly populations. There is a common consensus that bush fly nuisance is reduced, and the experimental data that has been collected has all supported this. At one site in New South Wales a number of experiments were carried out in 1987/88 to measure the levels of egg to adult mortality of bush flies in cattle dung. Adding exotic dung beetles to cattle dung increased bush fly mortality from levels where populations could increase (85% mortality) to levels where populations decreased (97% mortality). The beetles caused populations potentially to fall on seven out of nine occasions that they were measured.

In one site in south western Australia around 1980 prior to the establishment of introduced beetles bush fly populations peaked each spring and early summer building up to around 20,000 flies/ha in December each year. Ten years later in 1990 peak bush fly populations were around 1000 flies/ha, considered to be a threshold at which they could become a nuisance. However, in one year out of the three when fly densities were labouriously measured, fly populations built up very early in the season in October, before the beetles became active, and flies became a nuisance. However, once beetles became active the fly populations were suppressed in this year also. These results illustrate two points. Firstly that the beetles consistently controlled bush flies during the important Christmas holiday period at this seaside holiday resort. Secondly that accurate timing of beetle activity is the key to successful control as the scientists predicted. Fly abundance and beetle abundance are affected differently by weather and there are likely to be some combinations of factors in a few years that allow the fly populations to escape for a period and increase, before the beetles bring them under control again.

Acknowledgements

The New South Wales data was collected by Marina Tyndale Biscoe and Bill Vogt of CSIRO. The early Western Australian data was collected by James Ridsdill-Smith and John Matthiessen of CSIRO, and the later data by Ian Dadour of AgWA.



James Ridsdill-Smith
5 Jan 1998




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