Finding Marsh Fritillary nests

Marsh Fritillary third instar larvae. Photo: Martin Warren

Martin Warren, ex Chief Executive of Butterfly Conservation, recently led a training day on counting the webs in which Marsh Fritillary caterpillars grow.

The Marsh Fritillary is one of the few butterflies that is feasible to count in the caterpillar stage. The reason is that this species lays unusually big batches of eggs (up to 350) and the caterpillars spin conspicuous whitish webs (also known as nests) as they develop during the summer. The webs are spun to protect themselves from predators as well as a parasitic wasp (Apanteles bignellii) that can cause huge mortality. The best time to count the webs is during mid to late August when they can spotted trailing over the food-plant, Devil’s-bit Scabious (Succisa pratensis). The optimum window for counting lasts about a few weeks before the caterpillars go into hibernation deep in a grass tussock during late August or mid September (depending on the season).

On 16 August 16 2021, the Dorset Branch held an extremely successful training event at Lankham Bottom, where 10 people learnt how to count these webs in a systematic way to help assess the population. The idea was that they could then go on to monitor other sites that are being managed for this rare and declining butterfly.

View of a hillside with uneven ground

Lankham Bottom. Photo: Martin Warren

On larger sites such as Lankham, it is not realistic to try to count every nest, so the survey method involves recording a sample of the site and extrapolating this up to the whole site. The method is very simple and involves walking a zig-zag route of known length (for example by drawing on a detailed map) and counting all the nests found within a two metre width (one metre either side). This is about as far away as you can reliably see all the nests on a slow walk. So, for example, if you do a count along one kilometre, you will have surveyed an area of 1,000 x 2m (2,000m2). This can be multiplied up according to the total area of the site to give an estimate of the number of nests on the whole site. At Lankham, to save us walking up and down the steep slopes, we each walked a wavy line, 10m apart, counting webs as we went.

The Marsh Fritillary breeds in two distinct types of habitat, damp grassland and chalk downland. In the former (such as Alners Gorse reserve), Devil’s-bit Scabious grows as large plants with big leaves on which the caterpillars feed. However, on chalk grassland they can breed in shorter vegetation provided there is a good density of Devil’s-bit Scabious. At Lankham, the chalk grassland is grazed not only by cattle but also by numerous rabbits, so the Devil’s-bit Scabious grows as rather short plants, often in dense patches. As a result the caterpillars have to move often as they consume each small leaf, leaving quite a long trail of webs over several plants.

Areas of web, like spider's webs, at base of a plant with purple flowers

When surveying, it is important to count only the webs occupied by the caterpillars and not all the old ones. The occupied webs can be identified from a few small caterpillars crawling around the outside. In contrast the old webs are full of frass and sometimes the old shed skins as the caterpillars have moulted. It is also important not to mistake Marsh Fritillary webs with spider webs, especially funnel-web spiders that can also spin extensive webs amongst the grass. Obviously spiders’ webs usually contain a spider if you look carefully inside, but the easiest way to tell them apart is that spiders’ webs are usually brilliant white, whereas Marsh Fritillary webs are brownish and full of either old frass or caterpillars.

Areas of web like spider's web over some plants, with some caterpillars visible

Marsh Fritillary larval nest at Lankham, Aug 2021. Photo: Martin Warren

After one hour of training, the volunteers were ready to survey in earnest. So we split into three groups. Three surveyors stayed and counted webs at Lankham, while two other groups of four each went to survey nearby Hog Cliff and Southfield Down, both National Nature Reserves managed by Natural England. The Marsh Fritillary used to be found on both these downland reserves but had only been seen in ones or twos in recent years. It was important to know whether they were still breeding.

View of two people, well apart, on hillside with rough grass

Marsh Fritillary web surveyors at Hog Cliff National Nature Reserve. Photo: Martin Warren

At Lankham, the three surveyors found 11 Marsh Fritillary webs. This is well down on previous years but shows that a healthy population still survives. Sadly, no webs were found after two hours of surveying at the other two sites, but some good potential habitat was found so it is possible that the butterfly will recolonise in future years. The Marsh Fritillary is famous for its boom and bust life cycle, being numerous for a few years and rare the next. In the bust years colonies can even become extinct on a site, only to recolonise a few years later.

Habitat quality is also a factor that determines Marsh Fritillary success and the Branch is now advising Natural England on the best types of grazing regime. The Marsh Fritillary likes some moderate grazing, especially by cattle, which create a diverse sward with tussocks where the food-plant can grow into a reasonable size and where there are tussocks for the caterpillars to overwinter. It does not usually do well on sheep grazed sites as these often preferentially graze Scabious areas and lead to smaller plants. They also remove flower-heads and potential nectar sources. Consequently, rotational grazing by sheep is usually the only suitable option, but hard to put into practice. By monitoring a good network of sites, we hope to better understand this rare and complex butterfly.

Further information on the web counting method can be found on the UK Butterfly Monitoring Scheme


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