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Individual study: Provision of artificial breeding sites for the golden hoverfly Callicera spinolae at Thornham Walks, Norfolk, England

Published source details

Rotheray G.E. (2004) Autecology and conservation of Callicera spinolae the golden hoverfly (Diptera, Syrphidae). English Nature Research Reports, 581, 1-40

Summary

In the UK, the golden hoverfly Callicera spinolae (a large woodland fly) is listed as 'Endangered' and is a Biodiversity Action Plan (BAP) species. In Britain it is confined to eastern England; it is thought to be rare throughout its limited Western European range. The species is saproxylic (dependent on decaying wood), the larvae requiring wet rot-holes in trees. The loss of woodland and old rotting trees are the most likely causes for its rarity. Adult golden hoverflies are unusual among 'dead wood' hoverflies in having a flight period in autumn (adults are typically observed on ivy Hedera helix blossom in September and October), where as most other species fly during the spring.

In England prior to 1997, there were just eight localities where it had been observed and for most of these there were no recent records. As part of the BAP, all eight historical sites were resurveyed but no golden hoverflies were found. The sites included one near Cambridge where adults had been observed in the 1970s. Here, Dutch elm disease and the loss of many mature elm Ulmus sp. trees probably caused the demise of this population. However the species also appeared absent from other sites where mature trees with rot-holes still stood, so the reasons for these local extinctions are unclear. In 1998, one breeding population was found at a new locality, Thornham Walks (near Diss, Norfolk). As a way of providing additional breeding habiat, artificial 'rot holes' were erected at the Thornham Walks site.

Surveys revealed that golden hoverfly use a variety of broadleaf trees (including field maple Acer campestre, poplar Populus spp., ash Fraxinus excelsior, beech Fagus sylvatica and horse chestnut Aesculus hippocastanum) but that the rot-holes must contain mature decaying heartwood. The height of the rot-hole in the canopy did not seem to matter, but water pockets containing leaf mould were not used.

Artificial rot holes: Artificial 'rot holes' were made from clean 2 litre plastic milk bottles, measuring approximately 28 cm high x 11 cm wide. The top 8 cm was cut off to create an opening approximately 6 cm in diameter. This was the main access point for female hoverflies potentially attracted to the odour of the decaying wood placed within. Below the opening, five 0.5 cm diameter drainage holes were made to prevent flooding during heavy rain. Each bottle could thus hold between 1.25 to 1.5 l of wet decaying breeding medium. This amount was necessary as previous experience revealed that contents of smaller bottles could quickly dry out. Except for the opening and drainage holes, each bottle was covered in two layers of black plastic sheeting to ensure dark conditions inside.

Preparation of heartwood: The decaying heartwood that was placed in the bottles was prepared as follows. In March 2001, several small (about 7 cm diameter) fallen beech branches with decayed, soft and friable heartwood were collected at Newbattle Wood (Midlothian, Scotland), a site well outside the known range of C.spinolae. The bark was removed and the wood pulped by hand with water from a nearby river. Any invertebrates were removed. This wet wood was stored in three buckets, each with an additional 500 ml of wet decaying heartwood taken from beech tree-holes added. The aim was to add bacteria and other microbes within this material to promote decay, as decay odours probably attract gravid female hoverflies, and bacteria are the main source of food for their larvae. Many tree-holes exist at Newbattle but care was taken to remove only a small amount of material from each hole and to replace any insect larvae found.

Each bucket was covered and stored in the dark at room temperature for about five months, until August. At 4-week intervals, the contents were stirred and river water added to maintain wettness. After five months the material had a distinctive 'woody-decay' smell and was the consistency of porridge.

Installation of artificial rot-holes: On 25 August 2001, the buckets and the 20 bottles were taken to Thornham Walks and installed on the 25-26 August in an area of broadleaf woodland where golden hoverfly adults and larvae had been observed. Most bottles were positioned on mature to overmature trees between 3-8 m above ground in branch forks in shaded positions. Each bottle was filled two thirds full with the wet decaying material. Dried leaves and small pieces of bark were placed on top to a level almost to the top of the bottle to help prevent dessication and provide oviposition points for females.

Over-wintering and contents: The bottles were left until 6 October 2001 i.e. longer than the duration of the hoverfly flight period (September). They were then collected and returned to Midlothian. Additional river water was added as required, each bottle was covered with gauze and placed in cool, dark conditions to over winter. The contents were examined in July 2002 to allow time for any larvae to grow beyond the inconspicuous first instar stage. Larvae selected for rearing were returned to their bottles and checked every two months or so, rainwater being added as needed. Puparia were removed and placed on moist tissue paper in transparent 25 x 20 x 8 cm plastic boxes, until adults hatched.

Over 80 syrphid larvae were found in the 20 bottles; three bottles contained a golden hoverfly larvae. The rest were Myathropa florae, a common syrphid species. All three C.spinolae larvae pupated in June-August 2003. Two adult females emerged in September 2003. The third puparium died.

The results indicate that golden hoverflies will breed in carefully prepared artificial breeding holes and that they may thus have the potential to increase breeding habitat and be used to boost and maintain hoverfly populations.


References:
Rotheray G.E. (2004) Autecology and conservation of Callicera spinolae the Golden Hoverfly (Diptera, Syrphidae). English Nature Research Report 581. Download free from: www.english-nature.org.uk/pubs/publication/PDF/581.pdf


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