The insect life of Creag an Lochain

On the western flank of Meall nan Tarmarchan, Creag an Lochain is an area of broken crags which extends over a distance of 2km, overlooking the waters of Lochan na Lairige. A number of rare insects have been recorded here over the years; one of these is the mountain plume (Stenoptilia islandicus).

This delicate-looking moth was first found in 1953, although it wasn’t until 1988 that the specimens, held at London’s Natural History Museum, were identified correctly. In 1992 a small colony was found on Creag an Lochain and until 2013 this area of just a few square metres was the only place this moth was known in Scotland. In that year a second population was discovered on the opposite side of the loch, and subsequently a handful of other locations have been found scattered along Creag an Lochain, always close to the larval foodplant, mossy saxifrage (Saxifraga hypnoides).

Mountain plumes are also found in Iceland, Norway, Sweden, Finland and Russia, but there is one insect from Creag an Lochain that is currently known nowhere else in the world. This is the sawfly Euura arbusculae, discovered in 1941 and thought to be possibly extinct until it was re-found in 2018. Sawflies are in the order Hymenoptera, along with more familiar insects such as bees, wasps and ants. They differ most obviously from these in having a broad connection between the thorax and abdomen rather than a ‘waist’. Their name comes from the saw-like ovipositor which the females use to cut into plants to lay their eggs. Euura is a large genus of sawflies whose larvae feed on willows. The females lay eggs within the leaves, causing galls to form in which the larvae develop. These galls often take the form of a small red swelling, described as a bean gall.

Adult sawflies are difficult to identify, but usually each species of Euura lays eggs in a single species of willow. Euura arbusculae only induces galls on mountain willow (Salix arbuscula), which has its UK stronghold on Creag an Lochain. Searches in 2019 found galls in several patches of this willow on either side of of the loch, which is encouraging for the long-term survival of this rarity.

In 2000 the Trust fenced off 180 hectares of Creag an Lochain to exclude sheep and deer. Hundreds of willows were planted and tall herbs, such as wild angelica (Angelica sylvestris), meadowsweet (Filipendula vulgaris) and common valerian (Valeriana officinalis), previously confined to ledges, spread onto the slopes below the crags. We began monitoring the plants, bryophytes and lichens to see what effect our change in management was having, but the same level of information is not available for the invertebrates. Despite this, we have some good evidence that insect life has burgeoned within the exclosure.

In 2010 international experts gathered for the 14th International Sawfly Workshop and visited Creag an Lochain as well as several other locations in the southern Highlands. Over 400 species of sawfly are known from Scotland, but when experts in a relatively obscure group of invertebrates descend upon a rich habitat, new discoveries are almost inevitable! The meeting on Creag an Lochain didn’t disappoint – 5 species new to Britain were found, along with almost 80 other species. Some of the visiting entomologists also searched outside the exclosure and commented that far higher numbers of both species and individuals were found inside it.

Almost a decade later, in 2019, we hosted a visit from the Dipterists’ Forum. The Diptera are the true flies (which include hoverflies, bluebottles and midges), named as they have only one pair of wings. Again, our visitors were blown away by the richness of Creag an Lochain. Rob Wolton, chair of the group, wrote: ‘This National Nature Reserve is truly of exceptional importance for the conservation of flies, just as it is for so many plants and other animals. This reflects not only the base-rich rocks and wide range of montane habitats present, but also the National Trust for Scotland’s management of the reserve and the foresight of those who established the grazing exclosure at Creag an Lochain. I feel privileged to have visited and long to return!’

The group recorded over 170 species of Diptera from Creag an Lochain and Tarmachan, including several rare and scarce species such as the picture-winged flies Trypeta artemisiae, Campiglossa loewiana and Tephritis conura, the big-headed fly Dorylomorpha albitarsis, the dance fly Rhamphomyia morio, the dolichopodid Dolichopus maculipennis, the lauxanid Meiosimyza mihalyii, and an anthomyiid Pegomya rugulosa. As with the sawflies, very few true flies have common names!

We’ve all heard about the alarming declines in insect abundance, both in Britain and more widely, as part of the biodiversity crisis we currently face. Protecting our montane willows, allowing tall herbs to flourish and creating more structural diversity in the vegetation is one way we have fought against this at Ben Lawers NNR. And as we rely on visits from experts to record the more obscure invertebrate groups, there seems little doubt that there is still more waiting to be discovered!