This is the final post in a six part blog series about rare insects in the Cairngorms. A new project launched last year to save six endangered invertebrates in the north of Scotland and project officer Gabrielle Flinn has been a closer look at each one of these species. This time, it's the turn of the pine hoverfly. The Rare Invertebrates in the Cairngorms project is a partnership involving RSPB Scotland, the Cairngorms National Park Authority (CNPA), Buglife Scotland, Butterfly Conservation Scotland and Scottish Natural Heritage (SNH). Keep an eye out for further blogs on the project and the work being done to help these insects.
A Pine Example of a Scottish Icon
Image by Ellie Rotheray
Of the six species receiving attention through the Rare Invertebrates in the Cairngorms project (RIC), the pine hoverfly (Blera fallax) is the scarcest and represents the decline of Scotland’s most treasured, ancient forests – the Caledonian pinewoods. The pine hoverfly relies on ancient woodland for its large gnarled trees and rot holes that can be found in them. It also relies on a naturally diverse, mosaic woodland that has open areas, where the flies can bask in the sun, and broadleaved trees – like rowan (Sorbus aucuparia) – where the flies can feed as adults. As a species iconic to the Scottish Caledonian pinewoods and an important decomposer and pollinator within the woodland, this species is a perfect champion for the outstanding habitat in which it lives.
Image by Andy Hay
Due to the decline of this habitat, many specialist vertebrate and invertebrate species have become endangered. This problem is shared across Europe and the pine hoverfly is considered globally endangered and critically endangered in the UK where it is known to only two sites – both in the Cairngorms National Park.
Pine hoverflies start life within rotting holes of large old Scots pine (Pinus sylvestris) trees (known to use other tree species outside of the UK). These rot holes are typically formed when a fungal growth creates a cavity within the trunk or stump of a tree. Water fills this hole and bacterial populations thrive in this nutrient-rich environment. The female hoverfly will find such a hole, after mating, and lay her eggs here. The larvae will then spend between one and two seasons filter feeding this bacterial soup and growing until they are ready to pupate. Adults emerge around June time and are known to feed on pollen and nectar from rowan trees and other floral species, such as wild raspberry, helping to pollinate these trees and plants.
Image by Gabrielle Flinn
Because of the rarity of this species and the likelihood that it spends most of its time above the height of humans, the adult is incredibly difficult to find. As such, from the date of writing this blog, an adult pine hoverfly hasn’t seen in Scotland for nearly five years.
We have, however, observed larvae in more recent years (and months) and this is entirely down to conservation efforts for this species. Due to a loss of naturally occurring rot holes – fewer old “granny” pines and naturally fallen trees due to human management – this specific habitat had to be supplemented to save the species. As such, rot holes were created on various sites within the stumps of felled trees. This is done using a chainsaw and once rainwater fills the hole, a perfect pine hoverfly nesting ground is created. The larvae have been found using these rot holes and the population of larvae is closely monitored by volunteers and experts. Through efforts of the RIC project and partners, in 2017 we found over ten times the number of larvae as was found in 2016, giving a glimmer of hope for the future of this species.
This success is all down to the teams involved in creating the stump holes as well as the incredible efforts of volunteers trying to protect this incredible species.
To get involved with the RIC project please contact gabrielleflinn@rspb.org.uk. To find out more about the project and keep up to date with what’s happening follow us on Facebook and Twitter.
You can catch up on Gabrielle’s previous blogs in the series here:
Blog 1 – Kentish glory moth
Blog 2 – shining guest ant
Blog 3 – northern silver stiletto fly
Blog 4 – dark bordered beauty moth
Blog 5 - small scabious mining bee