The beetle and the bird

Dave Wood, Site Manager at The Oa nature reserve on Islay tells us why dung beetles are so important to our most stylish corvid – the chough, and what needs to be done to make sure there are enough dung beetles to support the chough population. 

The Scottish chough population is restricted to the Inner Hebridean islands of Islay and Colonsay. The population is in long term decline, from over 100 pairs in the mid 1980s to fewer than 53 pairs in 2014, with continued decline in recent years to just over 40 pairs currently. This has been attributed, at least since 2007, to poor first year survival, particularly during the post fledging period of July – November. This is thought to be caused by a combination of lack of suitable food and not much genetic diversity within the population, which can leave them vulnerable to diseases.  

 Studies of chough diet on Islay from 1988 were repeated in 2011-13 and dung beetle larvae, a key part of juvenile chough diet, were found to be much scarcer. Dung beetle larvae are an important food source for young choughs as they are a relatively easy and nutritious prey for the inexperienced juveniles to find. A major reason for the decline in abundance of dung beetles and other dung dwelling invertebrates are the insecticides commonly used in veterinary medicines to routinely treat livestock for fluke, worms, ticks and lice throughout the year.   

 As part of the Species on the Edge project, in 2021 we trialled small scale captive breeding of Aphodius dung beetles as a way of increasing numbers for choughs, particularly juveniles, to forage on in the period of July-October.  

 From past surveys and published research, we identified the species of dung beetles that had suitable life cycles, and were likely to be of most value to foraging juvenile choughs. The three best candidates were Teuchestes fosser, Acrossus rufipes and Aphodius pedellus. These are all found commonly on Islay, have relatively large larvae and are available at the right time of year. 

Acrossus rufipes, Aphodius pedellus and Teuchestes fosser

 Beetles of these three species were collected on the nature reserve by searching through dung pats, and 25 of each were placed into simple rearing cages with fresh cow dung. The fresh dung and cages reduced competition and predation from other invertebrates and birds, allowing the dung beetles to reproduce more successfully.  

 Shortly before expected emergence dates (when the adult beetles fly away from the dung!), rearing shelter lids were replaced with a wire mesh to allow the new adults to leave. Collection, rearing and release all happened at the same site, avoiding any risk from transporting adult beetles and any diseases they might be carrying between sites. 

The rearing cages.

Results 

Larvae were counted in half the cages after 35-40 days. Larvae were well grown at this stage but had not yet begun to pupate. The other cages were checked to see if similar densities of larvae were apparent but not accurately counted. This was done in case the detailed counting and disturbance of the larvae proved to be damaging to their development. This was assessed after pupation and didn’t appear to have had any negative effects. 

One T. fosser cage produced 206 well grown larvae. 

Two A. rufipes cages produced 383 and 402 well grown larvae. 

The other cages appeared to show similar abundance... except for the A. pedellus cage, where no larvae were seen. It appears that the beetles of this species chose not to breed!  

The numbers of larvae show that this method of captive rearing dung beetles has the potential to be very successful. Based on these larval counts, we could expect 8-16x increase in the number of adults emerging compared to the number placed in rearing cage.  

Some A. rufipes larvae were found at the bottom of the cage, so future cages will be deeper, to allow the larvae to tunnel deeper. 

Image of T. fosser larvae being counted and A. rufipes larvae in the rearing cage soil, at base of dung.

Further inspection of the rearing cages in October showed that the T.fosser cages had successfully pupated, with fresh adults being found in the cage. Some of the freshly emerged adults had red colouration, before the elytra (the outer wing case) had dried and hardened to the usual black colouration for this species.

Freshly emerged T.fosser with dung/soil pupation ball and pupal case remains.

The A. rufipes cages overwintered at the larval stage, with no sign of pupae or emerging adults through October/November. Checks in the following May showed some still at larval stage and some pupae, and in June numerous pupae and freshly emerging adults were evident. 

A. rufipes pupae in May 2022 and pupae, emerging adults and larvae at base of rearing cage in June 2022

Emerging adults and pupal cases of A.rufipes in June 2022.

Proposed future dung beetle farming as part of Species on the Edge project 

Dung invertebrate surveys carried out in 2021, together with the captive breeding trials, have shown that the expected dung beetle species are relatively abundant and widespread across the reserves (although with fewer larvae than were seen in previous surveys). 

 Advice from dung beetle experts suggests that alongside suitable grazing and habitats, populations are highly likely to increase naturally with the right grazing systems and carefully managed use of veterinary medicines for the livestock in the area.  

Therefore, captive rearing of dung beetles, on the scale proposed at least, is unlikely to have a significant impact on population levels across the wider landscape, without widespread changes in the routine use of veterinary medicines. However, due to the captive rearing process being very low tech and inexpensive, we plan to continue to captive rear at one site on The Oa. This will give us a better understanding of the species, and we can assess its impact on localised population levels following repeat surveys in four years.  

 Further work is planned through Species on the Edge to facilitate widespread uptake of veterinary medicine testing across Islay’s farms, in order to reduce usage across the wider landscape. With our partners on the Scottish Chough Forum and NatureScot, we hope this work can play a part in helping to reverse the decline in the Scottish chough population. 

Thanks to Bruce Thompson, who gave advice and support following successful trials of a similar nature in 2020 on his dairy farm in Ireland. 

 For more information on dung beetles and the important role they play for farmers and the wider environment check out https://www.dungbeetlesforfarmers.co.uk/ and https://www.dungbeetletrust.co.uk/ 

Main image: a chough with its beak open.  Martin Yelland