Marsh Fritillary Reintroduction Success

It’s now 18 months since INCC staff and volunteers collected the first caterpillars from the wild to bring back to the rearing pens at the National Botanic Gardens in Carmarthenshire.

The last update, in July 2021, described how the next generation of caterpillars were hatching and starting to feed on the Devil’s-bit Scabious that is their foodplant.

Since then the caterpillars grew and moulted 3 times while devouring the Scabious leaves beneath their communal webs.

At this point the caterpillars were at the 4th instar (the stage they reach before hibernation) and ready for the big day of release in September 2021.

Marsh fritillary caterpillars in their larval web

It’s over 20 years since Marsh Fritillaries were seen on Llantrisant Common and a huge milestone for the project was this first release.

INCC staff, volunteers, project partners, local people and students from local schools and Bridgend College all helped contribute to a fantastic day placing caterpillars from the rearing pens, in their larval webs, onto Devil’s-bit Scabious plants in specifically selected areas of the Common.

Volunteers releasing marsh fritillary caterpillars at Llantrisant Common

All we could do now was hope that the caterpillars would continue to feed on the wild plants and then find safe places to hibernate through the winter within their webs.

We retained some caterpillars back in captivity as a safety net in case the released ones failed to survive the winter. We were able to keep an eye on these as they started hibernating and then monitored the webs in early spring for the first signs of them emerging into the weak sunshine.

As soon as we saw that we began visiting the common on the lookout for caterpillars emerging there. Initially, upon waking from hibernation, the caterpillars don’t feed but bask communally in order to warm up.

The black larvae huddle together to aid their warming up and therefore are warmer than the surrounding vegetation. Because of this we decided to try out a thermal imaging camera as a way of spotting caterpillars and we were delighted to immediately spot a group of caterpillars – a big relief that at least some had survived! Further surveys found emerging caterpillars near all the release sites on the Common.

Marsh fritillary larval webs at Llantrisant Common seen through a thermal camera

For the next couple of months the caterpillars on the Common and in the rearing pens continued to feed on the Scabious plants, undergoing another 2 moults before pupation.

Then in May, after the first butterflies had emerged in the rearing pens we excitedly visited the Common in anticipation and hope of seeing the first Marsh Fritillaries flying there since the 1990s.

We were absolutely thrilled when, on the 11th of May, we saw our first butterflies flying free on the Common!

Marsh fritillary butterfly flying at Llantrisant Common

In the coming weeks INCC staff, volunteers and local naturalists recorded Marsh Fritillaries flying in a number of locations on the Common. Exciting milestones came thick and fast as butterflies were seen mating and then subsequently laying eggs!

In June we saw the first of the next generation of caterpillars emerging, an event which was hopefully being mirrored on the Common.

As these caterpillars and their appetites grew we were extremely grateful to Carys Romney and local school children who had worked hard to collect Devil’s-bit Scabious seeds from nearby sites in September 2021.

These seeds were then grown on by horticulture students at Bridgend College to provide fresh Scabious plants to keep the caterpillars going.

children from Ysgol Cymraeg Castellau helping to collect devil’s-bit scabious seeds for the project.
Devil’s-bit scabious plants grown from seed by horticultural students at Bridgend College

In the last couple of weeks we have been undertaking surveys of the Common in order to try to find the larval webs of Marsh Fritillary caterpillars which would provide proof that they had successfully bred.

We have been delighted to find larval webs in a number of different locations across the Common, 180 so far with a big section of the site still to survey!

Volunteers Carys Romney and Ben Williams undertaking a marsh fritillary larval web survey at Llantrisant Common

At the time of writing we are gearing up to bolster the population with a second release of caterpillars reared in captivity and the cycle will hopefully continue as successfully as it has so far.

We are extremely grateful to everyone who has contributed to the project in any way; it’s early days but it’s been more successful than we could have hoped for and without our supporters, funders, volunteers, project partners and local schoolchildren and college students we wouldn’t have been able to get to this point.

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