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Gloucester Grazing Trial: Dung Beetle Results

The Foyle Gloucester 2022 grazing trial followed a group of calves as they completed daily rotations across two different areas: a herbal mix and a wetland grass mix. Whilst we focused on the weight gain which could be obtained from the different leys, we also tracked the dung beetles which followed the calves, attempting to identify the species and numbers as well as the type of dung they inhabited.

The Dung Beetle

There are over 6,000 species of dung beetle around the world, they inhabit all continents except from Antarctica and are grouped according to their behaviour. The most well-known group is the “rollers” (telecoprids) which live in the Southern hemisphere and can be seen in nature documentaries rolling dung balls which can be up to 50 times the weight of the beetle. Rollers do not live in the UK, here we see “tunnellers” (endocoprids) which move the dung from the surface to below the ground and “dwellers” (paracoprids) which live in the dung itself.

Dung beetles offer a multitude of ecosystem services and have been estimated to be able to save the UK cattle industry £367 million a year. Whilst dung is on the surface it continues to release methane into the atmosphere, quick movement underground prevents this and increases the available grazing area. The digging of tunnels by tunneller species improves soil infiltration and aerates the soil, and their moving of dung allows it to become accessible to other underground species such as earthworms.

Dung beetles fly between dung pats and use their sense of smell to locate dung within the environment with some specialised species only consuming the dung from a single species (or left within a particular micro-climate; e.g., in the sun or shade) and others more flexible. Dung beetles have a symbiotic relationship with phoretic mites (which cannot fly) and transport them from pat to pat, these mites do not harm the dung beetle and instead are hunting for fly eggs to eat. In consuming fly eggs, the mites increase the amount of dung which is available for the dung beetles and reduce the pest fly population within the cattle herd.

Dung Beetle Findings at Gloucester Grazing Trial

At least four different species of dung beetle were found to be residing in the area the trial was completed; these species matched available descriptions and known population distributions although with sub-species only identifiable via microscope it could be that this number was slightly higher. A common species of hydrophillidae, a water scavenger beetle was also found frequently (Cercyon pygmaeus) which was likely responsible for forging some of the tunnels then used by the beetles. The four dung beetle species identified were; Onthophagus similis, Geotrupes stercorarius, Aphodius fimetarius and Aphodius fossor which can be seen below.

A total of 238 dung pats were examined visually with small sections removed, checked and returned until the whole pat had been inspected and all dung beetles present recorded. The age of the dung pat was noted as well as its characteristics (large, disturbed, flat etc.), the presence/absence of soil/outer tunnels and the air temperature. Between April and September 979 dung beetle individuals were found in the leys at the farm in Sandhurst, Gloucestershire.

Dung beetles sculpt tunnels through dung pats but they are not the only species to do so and outer tunnels were not a guarantee of their presence. Of those pats with visible tunnel openings dung beetles were found in 75% and of those without in 59%. Visible outer tunnels can be seen once a crust has formed but dung beetles can access before this point as well as through cracks and crevices which form as denser dung falls to the ground. Several dung beetle species burrow underground underneath the pat (taking the dung below the surface) and in 85% of those pats which had evidence of soil tunnels dung beetles could be found. Whilst Onthophagus similis was found mostly within the dung itself, Aphodius fossor was found at ground level and visible within the entry point of soil tunnels. The soil was not disturbed to dig for dung beetles as whilst this could have confirmed larger numbers, with weekly investigations it was felt there would be too much ground disruption should holes be dug under all pats examined.

Dung beetles arrive at dung pats within a few hours, at this point there was no crust and even in the denser herbal ley the dung was soft. By 24-48 hours (weather depending) crusts formed and whilst not deterring dung beetles, it was previously suggested that if they have not entered a dung pat by around day four they would not (due to a lack of moisture and available nutrients). Rotational grazing at the site allowed for age calculation of dung with few pats over 10 days of age containing any dung beetles and dung under five days the most heavily populated with Onthophagus similis the first species to arrive, followed by Aphodius fossor.

The dung beetle species identified were all capable of flight and so likely endothermic (able to generate their own body heat) but may still rely on external temperatures. Few dung beetles were found when the air temperature was below 17^oC or above 24^oC with Geotrupes stercorarius only found in conditions between 17^oC and 22^oC. This summer was particularly warm and dry with temperature soaring to well over 27^oC and it was likely dung beetles at this point had tunnelled below the ground with just 16 individuals found during weeks where temperatures were this high. The optimum temperature for the species found appeared to be around 22^oC which is reflective of the UK average summer temperature.

With the calves moving between a herbal and wetland grass mix, dung varied in structure in each of the locations (it is also assumed the nutritional value will have differed); dung deposited in the herbal lay was (mostly) dense and solid whilst that in the wetland grass appeared runnier and more fluid. As air breathing insects dung beetles can drown in watery dung so it was not surprising that a larger number were found in the herbal ley (677 dung beetles compared to 280 in the wetland grass mix).  

During the trial, some of the calves showed signs of New Forest Eye (Infectious Bovine Keratoconjunctivitis) which was treated using an antibiotic eye ointment. With concern over it spreading, all cattle at the site were treated with Flypor, an insecticide pouron. Flypor uses Permethrin as its active ingredient (a synthetic product) which previous research by Bristol university showed to be less damaging than products containing macrocyclic lactones such as Ivermectin (which we know damages dung beetle populations). The number of dung beetles found dropped by around 50% the week after Flypor was used (this was less of an impact than anticipated) and post treatment dung pats did appear more sparsely populated suggesting that whilst the chemicals present had not killed all individuals (no difference in the number of dead dung beetles found) it may have limited the pats ability to sustain large groups.

Further investigation is needed into the factors which can affect dung beetle populations (both positively and negatively) as well as how they can be encouraged and promoted on farm.