Good weather gives rare butterfly species a boost

A “treasure” of Northern Ireland’s countryside has had a bumper year, thanks to warm weather and conservation efforts.

The Marsh Fritillary butterfly used to be widespread across the UK and Ireland but plummeted in the last century as farming practices changed.

After years of work with landowners by the charity Butterfly Conservation, the latest survey of silken caterpillar nests has seen huge increases.

The charity said it showed the importance of habitat restoration, because good weather alone is not enough to reverse decades of decline.

The species depends on just one plant, Devil’s-bit Scabious, which grows across a range of grassland and heathland sites, most often grazed by cattle.

Much of this habitat was lost over the 20th century because of changes in farming practices.

As a conservation manager for Butterfly Conservation, Rose Cremin has worked with farmers across Northern Ireland to help them manage the land to support the species.

That can involve advising on what to graze where and when, and how to manage grass.

She said while weather is important, it is only a “short-term influencing factor”.

Because climate change could have good and negative effects on butterflies, the focus must be on habitat restoration.

“More importantly for the species survival, and for it to thrive, is really the appropriate management of land by farmers and the land managers to ensure that the habitat is right, that its food plant is there in abundance, and that the sward (grass) height suits for the female to lay its eggs.”

She added that many of the farmers she works with delight in walking the farm with the conservation volunteers to see how butterflies are faring on their land.

Every autumn, the charity’s volunteers carry out a survey of the sites where habitat is being managed.

They count the larval webs or nests that the caterpillars group together to form after hatching from eggs laid on Devil’s-bit Scabious.

Ms Cremin said there could be “20 to 50 or 100 individual caterpillars” in a single web.

“Basically they spin this around their food plant and eat the leaves of that, and when that’s exhausted, they move on to the next plant.

“So you sort of see this trail of old abandoned webs and plants as they move from one food plant to another food plant.”

Counting those abandoned nests gives a clear indication of how the species is doing.

The locations are not identified publicly to give the species maximum protection, but at one site this year the team counted 53 nests up from 24 last year – an increase of 121%.

Another farm recorded 139 caterpillar nests compared to just 24 last year.

At another, the number more than quadrupled, from five last year to 27.

Ms Cremin said the results were “great to see” and they proved that “the years of fantastic efforts by our volunteers and the farmers and landowners we work with are paying off.”

Like many butterfly species, the Marsh Fritillary is univoltine, meaning it produces just a single brood of offspring in its year-long life cycle.

Those offspring hibernate over winter as larvae – caterpillars – in a communal silken web called a hibernaculum.

Then in Spring, each forms a cocoon before emerging as an adult butterfly to lay its own eggs.

Producing just one brood a year means the species is vulnerable to anything that affects the food and habitat it depends on.

Between 1985 and 2019, its distribution dropped by 43%.

And butterfly populations generally are struggling with 80% in decline in either abundance (how many there are) or distribution (where they are found) since the 1970s.