The light grazing and damp grassland on Ulva make it the perfect site for Marsh Fritillary butterflies. This rare species has an unusual life cycle, making them all the more rewarding when you see them!
Adults: The adult butterflies are distinctive with an orange, brown, cream and black chequerboard pattern. They hatch out in May and June, and as the females are heavily laden with eggs they do not fly far. After mating the eggs are laid on Devil’s-bit Scabious (Succisa pratensis).
Caterpillars: Their main food plant is Devil’s-bit Scabious and the eggs are laid within a clump of this plant. Once the eggs hatch the caterpillars congregate together and spin a web where they feed and bask in the sun. As they grow they will build new webs close to the previous one. In the autumn they will build a stronger web deep down in the vegetation and hibernate here until early spring. The caterpillars continue feeding in the spring, and gradually disperse before pupating.
Habitat: They favour open areas of grassland, especially damp grassland where there are tussocks of longer grasses among the vegetation. Light grazing by cattle helps to support this habitat which is why the species strongholds of the argyll islands are so important.
Similar Species: although other fritillary species are found on Ulva, the marsh fritillary is distinctive and not easily mistaken.
The species fluctuates in numbers from one year to the next, due to weather, habitat changes and parasitic wasps and flies. The species is in decline across the UK, but efforts are being made on the west coast of Scotland to help landowners maintain suitable habitat.
An article on the BBC website chronicles the hunt for the caterpillars in Wales.