Five facts about small tortoiseshell butterflies

One of incredible volunteers, Lorraine Preece, shares some fabulous facts about small tortoisehell butterflies!

Five facts about small tortoiseshell butterflies

Recently, fluttering against the inside windowpanes of my garden shed were a total of nine small tortoiseshell butterflies! Small tortoiseshells are one of the first butterflies to be seen in spring and are extremely welcome colourful visitors to our gardens. Spring has definitely arrived!

Small tortoiseshell butterflies don’t migrate

Instead of migrating like many butterflies, small tortoiseshell butterflies hibernate in their adult form, often in our houses and sheds. They emerge again in spring to feed on nectar and lay large amounts of green eggs on the underside of nettle leaves which they do right throughout June.

Small tortoiseshell caterpillars are fond of nettles

June is a great time to look for small tortoiseshell caterpillars as the species is abundant and widespread throughout Britain. The 2.5cm caterpillars feed on common nettle (Urtica dioica) and small nettle (Urtica urens) plants, they can be found near the tops of the stinging nettles where they spin silken webbing for predator protection.  Small tortoiseshells reach maturity towards the end of July forming metallic chrysalises on dead vegetation near the nettles. 

Small tortoiseshell butterflies hibernate during winter

In the Autumn, small tortoiseshells emerge as adults and are often seen in large numbers visiting garden flowers before they hibernate over winter as adults in places such as hollow trees, buildings and caves. If you are keen to attract Small Tortoiseshells to your garden you could plant buddleia (Buddleia) or iceplant (Sedum spectabile) for the adults, ensuring that there are patches of nettles for the larvae.

Interestingly, small Tortoiseshells hibernate together and take day trips out on sunny winter days.  In order to survive their hibernation they need to accumulate a lot of fat (20% body weight) so are very susceptible to predation. 

Small tortoiseshell butterfly numbers are declining

Sadly, the small tortoiseshell butterfly species population has seen a decline of 75% since 1976.  As part of the Big Butterfly Count run by Butterfly Conservation, we can play a role in increasing our knowledge of this decline by reporting sightings of small tortoiseshell in July and August. The reasons for this decline is not fully understood, however possibilities are climate change, pollution, pesticide use, and loss of habitat. One other suspect is a parasitic fly called Sturmia bella which lays its eggs on common nettle, these get eaten by the small tortoiseshell caterpillars which kill them before they can become butterflies. 

Butterflies are important biodiversity indicators

Butterflies act as important indicators of biodiversity, providing an early warning system for other potential reductions in wildlife.  Therefore when we conserve our butterflies we are protecting not only the butterfly but the whole ecosystem, preserving a biodiverse natural environment for future generations.