Blera fallax -Overview
For the past 15 years the Malloch Society has worked on the ecology and conservation of these endangered hoverflies. They are included in the UK Biodiversity Action Plan process set up to conserve important elements of the British flora and fauna. The discovery of populations of these hoverflies on some of their Scottish reserves led the Royal Society for the Protection of Birds (RSPB) to become lead partners for their conservation. With Scottish Natural Heritage, the RSPB are providing financial and other resources to implement conservation in a 3 year programme of work (2003-2005) developed by the Malloch Society.
These are some of the very first attempts anywhere in the world to manage and conserve individual species of saproxylic Diptera.
Habitat, distribution and abundance
In Scotland the pine hoverfly is associated with mature Scots Pine (Pinus sylvestris) although in Europe in will occur in association with other conifer species Most historical records refer to native pinewood sites but the modern records are more closely linked to mature plantations with large trees where forestry operations are taking place.
Despite at least 10 years of survey work only two remaining populations are known, both of these occur in Strathspey, centred on the area between Aviemore and Grantown on Spey. In historical times it was recorded from Deeside, along the River Findhorn and in several other parts of Strathspey. In Europe Blera occurs in similar situations in coniferous woodlands.
Population levels in natural situations at the two sites remain relatively low, probably less than 250, and fluctuate in response to forestry operations. The number of larvae at one site has been boosted by management action over the last 5 years and similar management is now under way at the second site.
Pine hoverfly larvae develop in wet situations in pine stumps usually where there has been some softening or decay of the heartwood by the pine but rot fungus Phaeolus schweinitzi .. The pine stump needs to have a diameter greater than 40 cm. In order to support a large enough wet decay area. The larvae may emerge as adults after only one year if conditions are suitable, or if conditions are sub optimal due to a small area of decay or overcrowding they may remain as larvae for at least two years. The larvae leave the decay to pupate around the margin of the stump or in surrounding undergrowth. Each fresh stump can probably support the appropriate decay for a period of 8-10 years before the stump dries out completely, a continuity of stumps is therefore required. Adults had been seen feeding on raspberry flowers ( Ribes spp) but little else is known about their behaviour or dispersive abilities.
History of decline, contributory factors and current threats
The pine hoverfly was first known in Britain in the late 19th century when a Victorian collector found the first specimen buzzing at his hotel window in Braemar. From then on up to the first World War it was taken relatively frequently by the relatively few collectors active at that time. There were occasional records up until the 1940s but then a marked gap in the latter part of the 20th century apart from the discovery of a number of individuals, presumably from the one breeding stump near Loch Garten, in the 1980’s. After some ten years of searching the larvae were eventually found by the malloch Society in the late 1990’s and the understanding of the conditions which they required for development prompted a widespread survey for further sites. However despite this extensive survey work only two sites are currently known.
In a natural situation it is considered that the pine hoverfly larvae develop deep in the stumps of large pines which, weakened by an attack of the but-rot fungus, would snap off during storms. However, perhaps due to extensive felling over the last 100 years such old trees such large, snapped pines are very rare in Scotland. As a result Blera larvae rely, perhaps almost entirelly, upon stumps cut as part of felling operations. Evidence from Norway and Finland supports this position.
There is a medium term requirement to conserve this species whilst the presemt pinewoods mainly in protected areas continue to grow and mature.
There is no current threat to the populations in the conventional sense, the main issue is that at present our pinewoods do not have the extent of mature or over mature pines which this species requires. Ironically felling within native pinewoods during the early 20th century probably meant that pine hoverfly populations remained high, conservation efforts in recent decades have stopped this felling but with a consequential negative impact on the species. The other important factor is that at present population levels are considered so low and localised that any large scale colonisation events into any surrounding appropriate habitat may be unlikely.