Sunday, 15 June 2014

The prophetess ...

Marsh Fritillary (Euphydryas aurinia)

This delightful butterfly, a female pictured above, is one of our most threatened species and has suffered severe declines in recent decades. Although declining throughout Europe, the British Isles is considered one of the few strongholds for this species. However, the reclamation of wetlands and the ploughing up of downland has meant that E. aurinia has now disappeared from many counties and has also suffered severe declines in the British Isles; this butterfly is therefore a priority species for conservation efforts. Although still widespread in some parts of its range, this butterfly is reputedly declining by over 10% each decade. The deteriorating fortunes of this species are believed to be the result of inappropriate habitat management, coupled with the need for sufficient habitat for the butterfly to form meta-populations, where local extinctions can be reversed by recolonisation from neighbouring colonies. It is currently found in southwest England with a small population in northwest England, the islands of southwestern Scotland and the adjacent mainland, and northwest and southwest Wales. It is also locally widespread in Ireland.

On a positive note, aurinia is an easy species to breed in large numbers in captivity and one, which lends itself to reintroduction or establishment in suitable areas where its main foodplant, Devil’s-bit Scabious (Succisa pratensis), grows in some profusion. My late good friends, the respected entomologists Peter Cribb and Peter Taylor, both kept healthy stocks of this species going for well over 25 years and bred thousands of healthy specimens for release purposes in various counties in the south of England. In Sussex, the former Ditchling Common colony originated from this source with the original stock coming from just over the border in Surrey.

Colonies of E. aurinia are known to fluctuate wildly in population density. It may be present in good quantities one year, only for the population to crash dramatically the following season without any apparent reason - before recovering just as unexpectedly. It does not do well in adverse weather conditions and can suffer significantly from the attentions of parasitic hymenoptera during the larval stage. Adults, which typically emerge in the middle of May, reach their peak in early June. In northern Scotland they emerge slightly later. There is one generation each year.

Where it occurs, aurinia uses several different types of habitat, including chalk hillsides, heathland, moorland and damp meadows. A factor common to all habitats is that they are in full sun, their higher temperature aiding larval development. As previously mentioned, the primary larval foodplant is Devil’s-bit Scabious (Succisa pratensis). Field Scabious (Knautia arvensis) and Small Scabious (Scabiosa columbaria) are also used.

On emerging from their eggs the larvae spin a silk web, by binding together leaves of the foodplant, in which they live and feed. Larvae build new webs as they grow and even move to a new plant if necessary. In later instars, the webs can be quite conspicuous. Larvae will also bask on the outside of the ‘tent’ absorbing the sun's rays, where their increased temperature aids digestion. After the third moult the larvae build a dense nest of silk low down in vegetation in which they hibernate. Larvae will emerge from their nest with the onset of spring and can be seen basking in warm sun as early as February. The larvae eventually split into smaller groups, continuing to build silk webs where they bask communally to keep their body temperature relatively high, even on cool days. More-mature larvae tend to feed alone and are often found wandering across open ground looking for their next meal or, eventually, a pupation site. There are 5 moults in total. The pupa (pictured above) is formed head down, attached to a twig or plant stem by the cremaster. The pupa is essentially white, with a beautiful mix of black, brown and orange markings. This stage lasts between 2 and 4 weeks, depending on temperature.

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