A study has found that rare insects are thriving on harvested forestry sites.

The Forest Research survey – undertaken at Forestry and Land Scotland (FLS) sites in North Tummel Forest – looked at the importance of deadwood and early habitat on clear-fell forestry areas and found that a variety of spiders, moths, butterflies and beetles use these areas.

Amongst the rare moth species recorded were Northern Arches, Silvery Arches, Golden-rod Brindle and Cousin German, the latter is on the Scottish Biodiversity List as requiring conservation action.

Although very little is known of its life-cycle or habitat requirements in Scotland, traps caught extraordinarily high numbers of Northern Arches– the Scottish populations of which are important on the European scale.

At the start of Insect Week 2024, Forestry and Land Scotland Environment manager, Colin Edwards, said:

“The presence of some of our rarest moths only goes to show how important these changing or ‘successional’ habitats are.

“As conifer plantations are adapted they increasingly incorporate a range of natural habitats that offer a refuge for many protected species.

“These plantations also provide ecological connectivity between patches of native woodland and can help many species to increase their range.

“Most people tend not to think about beetles, bugs and spiders when it comes to biodiversity but these invertebrates are like the plankton of the forest – they pretty much underpin the entire pyramid of life.”

When a site is clear felled a host of ‘pioneer’ species move in to work alongside the forest specialists, but as the vegetation changes some of these ‘pioneer’ species move on as the new canopy closes over.

For example, increased structural diversity of ground vegetation promotes spider diversity; hoverfly diversity is increased by provision of wet areas; proximity to broadleaved woodland increases moth and butterfly diversity; and more native vegetation and foodplants in open areas help sustain a variety of larvae and provide nectar sources and sunlight for adults. This temporary habitat supports important populations of some of Scotland’s rarest and most threatened butterflies and moths including, Kentish Glory, Pearl-bordered Fritillary and Chequered Skipper.

Report author and entomologist at Forest Research, Katty Baird, said:

“The recording of four nationally scarce moth species including the Cousin German highlights the importance of recent clear fell and early-successional habitats in supporting some of our rarest moths.

“Clear-fell management provides important, though transient, open and scrubby habitats for 10-15 years post felling and is part of the mosaic of habitats needed to support a wide variety of insect species.”

Tom Prescott, Head of Conservation at Butterfly Conservation Scotland, said:

“Moths and butterflies play an important role in telling us about the health of our environment since they are widespread and found in many different habitats including harvested forestry sites.

“Their value as part of the food chain and in providing pollination services, as well as in providing cultural services, make them ideal indicator species.”

Habitats other than the forest within plantation landscapes are important for many species with clear-fell rotations along with other open areas such as rides and wayleaves contributing the overall biodiversity within woodland and the wider landscape.

Some of the other insects to benefit from a diverse forest landscape include spiders, beetles, wasps, bees and pine hoverfly.

Additional findings of the report are below:

• Spiders are an important indicator species when investigating differences in habitat and disturbance. The study found 51 species across the sample sites. Varieties of wolf spider were numerous. These hunt by pursuing prey over open ground. These species benefit from having more hiding places, protection from other predators and a range of suitable microclimates for them and their prey to utilise.
• Forty species of ground beetles (carabidae) were also identified with little variation between sample sites. The most abundant were varieties that are common and widespread open-habitat species in Scotland, not necessarily associated with forested landscapes. But as the canopy recovers, these species will reduce in number and forest specialist beetles move in.
• Providing a diverse forest structure also encourages a variety of natural enemies. This study found the longhorn beetle Rhagium inquisitor, a species native to the Scottish Highlands, typically inhabiting coniferous woodlands. When present in high numbers, its larvae can help control the larvae of other tree-feeding beetles such as the common pine shoot beetle (Tomicus piniperda) or Hylobius abietis.