Highs and Lows of Marsh Fritillary Larval Web Counts

Marsh Fratillary by Amy Lewis

Marsh Fratillary by Amy Lewis

It was a dismal drizzly morning when Pauline met volunteers as well as representatives from Butterfly Conservation and Buglife at Cae Lynden to look for the caterpillars (larvae) of the Marsh Fritillary butterflies.
The caterpillars make it easier for us to spot them by spinning a web between the leaves and stalks of Devil’s-Bit Scabious, their food plant.

A web count is completed each year and gives us a better picture of the breeding success of Marsh Fritillary year on year. The counts of adults on the wing in May only reflect the success of the previous year’s breeding and survival through the winter.

Like all species there are years when they do well and others when numbers are low. These peaks and troughs can reflect a number of factors. Condition of the site, weather and parasites all play their part. Last year record numbers of the larval webs were counted on Cae Lynden, 65 webs were spotted.
The team spread out on one side of the path and walked forward, heads down, peering through the Molinia grass to find the base of the Devil’s-Bit Scabious. Eventually a shout ‘FOUND ONE!’ was heard. It was relatively small web and quite soggy but the caterpillars could clearly be seen and were moving around.

Apart from the odd shower the weather stayed dry as the team moved up and down the field, trying not to miss an area. In total 9 webs were found. A disappointing number but Russel Hobson from Butterfly Conservation explained that a number of parasites had been spotted last year so may be the high number of webs last year meant that a high number of parasites were about now. The larvae of the parasitic wasps and flies feed inside those of Marsh Fritillaries! Also it wasn’t ideal weather for them to be out feeding. They do like a bit of sun and warmth….don’t we all!

The team moved on to Ystradfawr. In spite of better weather and a good show of Devil’s-Bit Scabious in the field at Weavers Road no larval webs were found. However when we moved to Nant Gylais (Area 8 for those familiar with the site) 16 were found dotted throughout the field. This is the highest number ever found on this site.

It was a good note to finish on, and the sun was shining!