The secret life of Marsh Fritillary caterpillars
Did you know some butterfly caterpillars live together in a silk web?
The endangered Marsh Fritillary butterfly (Euphydryas aurinia) of Europe is unlike any I had seen before.
Adult Marsh Fritillary butterfly.
These communal webs are for predator protection while they feed on the plant’s leaves. They will move from plant-to-plant gaining vital nutrition and building up energy stores because, before winter arrives, they will overwinter in a small web close to the ground within a dense grass tussock and enter into ‘diapause’. Diapause is analogous to hibernation in that it is a period of inactivity with slowed metabolism to endure harsh winter conditions, but unlike hibernation, diapause also pauses the growth and transformation of these young larvae (caterpillars) at a particular life stage. When spring arrives, they will emerge and continue their transformation into adult butterflies and the population’s cycle of life will continue. This means the ideal time to monitor numbers of these butterflies is while they’re in their webs in their larval stage because they’re easiest to find and not very mobile. It makes them one of the few butterfly species of the world where higher accuracy population counts can be obtained because of this, rather than the less accurate surveys of highly mobile adult butterflies that may only survive a short period.
The sad truth is Marsh Fritillary butterflies experienced a 66% drop in numbers between 1990 and 2000 in England and, unfortunately, Butterfly Conservation receive very few records from surveys in the UK to help monitor their numbers. However, they’re known to have a stronghold in Argyll, Scotland, so preserving this habitat is vitally important.
While visiting the rewilding estate in Tayvallich (in Argyll) last month, we learned that the presence of these butterflies in the area means it is recognised as one of ten Prime Areas for Lepidoptera (PALs) in Scotland by Butterfly Conservation Scotland. Our wonderful tour guides talked us through how to search for and identify them. Our visit coincided well with the ideal time to search for them. The butterfly’s foodplant, Devil’s-bit Scabious (Succisa pratensis) was easy to spot with its bright purple pom-pom flowers at the top of tall stalks. At the base of these plants you can sometimes find a dense web. If you look closely, you should see small, black larvae moving around. Although, in some instances, you might find a vacated web with discarded larval skins or a spider web instead.
Marsh Fritillary are threatened by overgrazing by sheep, which degrades their habitat. Ideally, grassland will be grazed by cattle or native ponies that leave behind uneven patchworks of short and long vegetation. Tayvallich currently has (non-dairy) cattle grazing in this area, which is clearly working well for these butterflies!
Image description: Marsh Fritillary larvae in a web in grassland.
Sources of info:
Source 1: Highlands Rewilding’s blog about surveying Marsh Fritillary butterflies in 2023 https://www.highlandsrewilding.co.uk/blog/marsh-fritillary-counting-at-tayvallich
Source 2: https://butterfly-conservation.org/butterflies/marsh-fritillary
Source 3: (PDF) https://butterfly-conservation.org/sites/default/files/habitat-marsh-fritillary.pdf
Source 4: https://scottishwildlifetrust.org.uk/species/marsh-fritillary/
Source 5: (PDF) https://ukbms.org/sites/default/files/downloads/UKBMS%20Ng2%20-%20Marsh%20Frit%20Webs%20guidance%20notes.pdf
Source 6: (PDF) https://butterfly-conservation.org/sites/default/files/marsh_fritillary-psf.pdf