Caterpillars of the marsh fritillary butterfly Euphydryas aurinia can extend their life-cycle in Scotland

Abstract

The marsh fritillary Euphydryas aurinia is declining across Europe and is of high conservation interest. Its ecology has been defined and its conservation status assessed primarily from the affinities and populations of young caterpillars in the autumn, before hibernation and high winter mortality. The possibility that caterpillars of E. aurinia can overwinter more than once was investigated on the Isle of Islay, Scotland after caterpillars were found to occur at some locations in the spring despite a pre-hibernation absence. Closely-related species in North America and Northern Europe can prolong larval development by diapausing for a year as does E. aurinia in Scandinavia. Measurements of development and manipulations of distribution confirmed that some caterpillars do extend the life-cycle in Scotland and may occur in areas devoid of larvae in their first year. Caterpillars attempting this life-cycle develop slowly in spring, attain the normal penultimate spring instar and then enter diapause while other caterpillars are pupating. They moult just before diapause, construct highly cryptic webs and on emergence the following spring are 5–6 times heavier than larvae emerging in their first spring, or the equivalent of a month or so ahead. They attain a final, extra instar as larvae in their first spring reach the penultimate instar. Knowledge of this life-cycle is confined in the UK to Islay but its occurrence in this mild climate implies that it is more widespread. Implications for insect conservation Conditions that permit long diapause are probably precise and may not be reflected in recognised qualities of habitat. The species may also be present despite a perceived absence in autumn, the standard period for monitoring. Assessments of the prevalence of the life-cycle and its contribution to the persistence of E. aurinia are required. Populations of E. aurinia are known to fluctuate greatly and do occur below the observation threshold for long periods.