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Pine hoverfly

What action is going on for this species?

The following will give you further information on what work has been taking place through the Species Action Framework:

Species background

The pine hoverfly (Blera fallax) is a hairy, mainly black species with a bright red-tipped abdomen and a yellow face. It has a wingspan of 8.0-9.5 mm and resembles a small bumblebee.

Why is this on the Species Action List?

The pine hoverfly meets criterion 1a of the Species Action Framework, as a species for conservation action. It is restricted in the UK to only two sites in the central Scottish Highlands.  It is a UKBAP Priority Species and is included on the Scottish Biodiversity List.

It is also considered to be declining and under threat in Europe.

Habitat, distribution and abundance

There has been a documented decline in numbers and distribution since the early 20th century; population levels at the two remaining sites in Strathspey are low; and, unless management is undertaken this species remains under threat of extinction.  There is sufficient knowledge of the species' autecology and habitat requirements available for targeted action to be taken to help the species recover.

General ecology

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 butt-rot fungus Phaeolus schweinitzi.  The pine stump needs to have a diameter greater than 40cm in order to support a large enough wet decay area. 

The larvae may emerge as adults after only one year if conditions are suitable, but 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.

In a natural situation it is considered that the pine hoverfly larvae would develop in the stumps of large pines which, weakened by an attack of the butt-rot fungus, would snap off during storms.  However, given the lack of extensive areas of large, old pines in Scotland where this process could take place naturally the species relies, perhaps almost entirely, upon stumps cut as part of forestry operations.  Evidence from Norway and Finland supports this position. 

Adults had been seen feeding on flowers of raspberry 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.  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, presumed to be from one breeding stump, in the 1980s. 

After some ten years of searching, larvae of the hoverfly were eventually found in Scotland in the late 1990s and the understanding of the conditions which they required for development prompted a widespread survey for further sites.  Despite this only two sites are currently known.

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 present population levels are considered so low and localised that any large scale colonization events into surrounding appropriate habitat may be unlikely.

The 'Species Lead' at Scottish Natural Heritage

Iain.MacGowan@snh.gov.uk   Tel 01738 444177