Two species, ubiquitous to the UK countryside, are the Small Tortoiseshell (Aglais urticae) and the Peacock (Inachis io) and they were once extremely common. The larvae of both species, feed almost exclusively on the Common Nettle (Urtica dioica) see image 1. The two species are quite easily identifiable, although both species, especially the Small Tortoiseshell, have suffered extremely drastic declines across the UK , which is explained further below. The Small Tortoiseshell is typically double brooded with a peak in April, after adults emerge from hibernation. There is then a long peak lasting two generations from late June to late August. The second brood adults then hibernate over the winter until spring. Whereas, the Peacock typically has one brood (one generation) a year, having a peak in April, after the adults emerge from hibernation. There is then a second peak in early August, as the first generation emerges, such adults will then hibernate through the winter until spring.
The decline in both species, but most especially the Small Tortoiseshell, has been most likely due to the natural colonisation of the parasitoid fly, Sturmia bella in the UK. This fly is a continental species and is known to use the Small Tortoiseshell, the Peacock and the Red Admiral (Vanessa Atlanta) as a host. Scientists from Oxford University and the Centre for Ecology and Hydrology, with funding from the Natural Environment Research Council (NERC), found that on average 26% of Small Tortoiseshell and 15% of Peacock caterpillars that were collected, were infected. The koinobiont parasitoid infects its host, through being consumed with the food plant, it then grows inside it host which is still developing, before emerging just after the last moult, eating its way out. It is thought to infect the Small Tortoiseshell more effectively due to its lifecycle. Unfortunately, this may not be the only reason for its decline. However do not despair, the species can still be seen.
The Small Tortoiseshell’s distinctive features are first the orange colouration on the upper surface of the wings, with a string of beautiful blue dots along the edges of the fore and hindwing, see image 2. The forewing, also has a band of colouring of effectively black then yellow patches along the leading edge. The wingspan is approximately 50mm. From below, this species has an effectively black hindwing, with a dividing line before getting slightly paler towards the edge, with only the forewing having pale yellow area, see image 3. The Peacock from above is a warm red with an eye spot on each wing tip. The eye spots are coloured with sky blue and egg yellow, with black surrounding them on the forewing, and yellow on the hindwing, see image 4. The wingspan being up to an impressive 70mm. From below, the Peacock appears mottled black and brown, with various jagged black lines across the wing, see image 5. Both species are found throughout the UK and have been recorded from the highlands and islands, including Shetland.
As noted, the species are associated with the Common Nettle (Urtica dioica), but as adults, they loves nectar rich flower species such as Scabious (Scabiosa sp.) and Buddleja davidii, where they use their proboscis to probe for nectar. The eggs of these species are often laid in clusters, on the underside of the nettle leaves. The eggs of both species being highly ribbed and barrel-shaped see image 6. Once hatched, Small Tortoiseshell larvae can be distinguished by the dense hairs/spikes along the body, and the pattern of black and yellow stripes and dots along the body, whereas the Peacock caterpillar is similar, having hairs/spikes, but is jet black, with red legs and white dots (pseudopods) see images 7 and 8.
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We thank photographers from UK Butterflies, W. Schö and Quartl for permission to use their images. Individual credits are given with each image.