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The population of the marsh fritillary butterfly (Euphydryas aurinia ) in western Scotland fluctuates greatly and the species is difficult to locate during periods of scarcity. It was recorded on Tiree in 2014 for the first time in 64 years during a period of abundance. The nearest known populations occur 40 km to the east and its discovery represents either a recent colonisation event well beyond its known capability or it has escaped detection for a longer period. Surveys for the larval cocoons of two wasps that are parasitoids of E. aurinia and host-specific were undertaken on Tiree in spring and autumn 2017. The adult wasps are small with limited powers of dispersal and are unlikely to have followed E. aurinia to Tiree since 2014. Cocoons of one species Cotesia melitaearum were abundant in all concentrations of E. aurinia in spring 2017 but neither species was found in the autumn. The absence of the second species C. bignellii is inconclusive as it was also absent at known locations elsewhere in autumn 2017. The occurrence, distribution and abundance of C. melitaearum on Tiree indicate that E. aurinia had been present for some time before its discovery in 2014. Although continuous presence since 1950 cannot be extrapolated with certainty, the results highlight the propensity of E. aurinia to persist for long periods in numbers that fall below the observation threshold, even in areas of high wildlife awareness, and that it is not always an easy species to record. INTRODUCTION The marsh fritillary (Euphydryas aurinia ) is a species of high conservation concern and is protected in the European Union as it is still declining in large parts of the U.K. and Europe. In common with similar species, it is relatively immobile, and fragmentation of its habitat and the isolation of populations are the principal factors in its decline, limiting the recolonisation of habitat after local extinction (Ehrlich & Hanski, 2004; Schtickzelle et al., 2005). The species fluctuates greatly in abundance from year to year throughout its range, even at sites with apparently ideal habitat that are under recommended management. Although its biology is not understood fully despite its status, two parasitoids that are specific to E. aurinia are known to cause high caterpillar mortality in some years. The western seaboard of Scotland supports one of the principal concentrations of E. aurinia in the U.K. Here it is common in coastal Argyll and on the larger Inner Hebrides such as Mull, Islay, Jura and Lismore. Even so, understanding of its status in the region is incomplete. For example, it has been recorded in over 200 new 1 km squares in the last 20 years on Islay. Populations in Scotland utilise more varied habitats than elsewhere in the U.K. and occupy marginal agricultural land that is grazed or abandoned. The butterfly was recorded on Tiree in 2014 during a period when it was abundant elsewhere in western Scotland. This was the first record on the island since 1950 (NBN Atlas, 2016) and subsequent observations revealed both adults and caterpillars to be widespread (J. Bowler, pers. comm.). Tiree is 30 km west of the nearest land (Mull) that is known to support E. aurinia, and 40 km from the nearest known populations on the Ross of Mull and Mishnish. Tiree is a relatively small island (ca. 80 km2) and its fauna and flora, especially its birds, have been recorded intensively (e.g. Bowler & Hunter, 2007; Bowler et al., 2008). The appearance of E. aurinia presents a puzzle, therefore, as it represents either a recent long-range colonisation event far beyond its known capability or long-term persistence that has escaped observation in an area of high wildlife awareness. This study in 2017 aimed to establish if the hostspecific parasitoids of E. aurinia are present on Tiree. These are small braconid wasps with a wingspan of 2-3 mm and typically with more limited powers of dispersal than their hosts (Lei & Hanski, 1998; van Nouhuys & Hanski, 2002) and are unlikely to have tracked the butterfly to Tiree since 2014. Their absence would suggest that colonisation by E. aurinia is recent and their presence would confirm longer-term presence of the butterfly. THE STUDY SPECIES The marsh fritillary flies mainly in June in Scotland, and its eggs hatch in July. Caterpillars live gregariously and cryptically during the early stages of growth but construct prominent “webs” of silk on the foodplant devil’s-bit scabious (Succisa pratensis) towards the end of August, when they are 2nd instars (caterpillars moult between each instar, or stage of growth). Feeding webs of the 3rd instar are visible through to the moult to the 4th instar in September or early October when the caterpillars construct a web deep in the vegetation and hibernate. Caterpillars emerge from hibernation most usually in early-mid March and develop quite slowly to reach the 5th instar in late March or early April. These grow rapidly to reach the final 6th instar in mid-April and pupation by early May (pers. obs.). Two conspecific parasitoids Cotesia melitaearum and C. bignellii attack E. aurinia and are specific to it in Scotland. "C. melitaearum" is an aggregate of morphologically identical cryptic species each of which may be specific to one host within the tribe of butterflies (Melitaeini) which includes E. aurinia (Kankare & Shaw, 2004; Kankare et al., 2005). Cotesia melitaearum is widespread but C. bignellii was recorded for the first time in 2014, on Islay (pers. obs.). Elsewhere both species have up to three generations per generation of E. aurinia (Porter, 1979; Stefanescu et al., 2009). Three generations of C. bignellii do occur on Islay, but only two of C. melitaearum have been recorded so far: a third generation of the latter has yet to be recorded in Scotland (M.R. Shaw, pers. comm.). The spring generation of both species is evident when one or two wasp larvae emerge from caterpillars of E. aurinia , usually when 4th instars moult in late March and early April, and spin cocoons in which to pupate. The cocoons of C. melitaearum are relatively easy to spot in or around the feeding and moult webs of 4th instar caterpillars (Fig. 1), but those of C. bignellii are difficult to locate as caterpillars harbouring this species may disperse and hide before wasp larvae emerge (pers. obs.). The adult wasps that emerge from these cocoons attack 5th and 6th instar caterpillars of E. aurinia in April before they reach pupation. Affected caterpillars harbour many wasp larvae and these erupt in the summer, producing a mass of cocoons. This summer generation of wasps attacks early instar caterpillars of the next generation of E. aurinia and wasp larvae enter hibernation with E. aurinia , emerging in the spring to complete the cycle. Some of these wasp larvae develop early, emerge and spin cocoons before the hibernation of E. aurinia , producing a third, autumn generation of wasps. These attack caterpillars of E. aurinia just before or while they are in hibernation (pers. obs.). METHODS General Two visits were made to Tiree in 2017, in spring (9th 11th April) and autumn (8th 10th October). These were timed to coincide with respectively the spring moult of 4th instars when wasp cocoons, especially those of C. melitaearum, are most visible, and the autumn moult of 3rd instars and hibernation of 4th instars when cocoons of C. bignellii (and sometimes adults) can be found in late feeding webs and final moult webs. This information was obtained outwith Tiree through monitoring the development of caterpillars on Islay and mainland Argyll. Both visits were timed well, although the development of caterpillars on Tiree lagged slightly compared with elsewhere in 2017. Searches were made for webs in suitable habitat when it was encountered on the ground within 200 m x 200 m units of 1 km squares (therefore a 1 km square was composed of 25 such units). Fig. 1. A cocoon of the parasitoid wasp Cotesia melitaearum (arrowed) inside an empty spring moult web of the marsh fritillary (Euphydras aurinia ) on Tiree, Scotland, April 2017. Spring Surveys for webs in the spring focused on four lochs on the "sliabh" of Tiree, an expanse of wet, peaty ground in the centre of the island on which E. aurinia had been seen in 2014-2016 (Fig. 2). These were Loch na Gile (NM027481), Loch Riaghain (NM034471), Loch a’ Bhleoghainn (NM021456) and Loch Leana na Moine (NM028446), as well as areas beside the B8069. The location (10 figure grid reference) of all webs found was recorded using a GPS and the number of cocoons in each web was counted. Webs were broken and searched if caterpillars had vacated them. All cocoons were collected. Autumn Surveys were repeated within the spring search area but also expanded to encompass adjacent and new areas, especially around Loch Caol (NM030454) (Fig. 2), primarily because of the paucity of webs of E. aurinia . The locations of all webs that were located were recorded and all empty webs were broken and searched for cocoons. To provide a baseline for future monitoring, transects were walked during which the numbers of E. aurinia webs were recorded in standard fashion (counted 1 m either side of the observer and route length recorded). RESULTS Spring Eighty nine webs of E. aurinia were located in 11 of 35 survey units during spring 2017 (Fig. 3A). Concentrations of webs were found in three survey units, one each beside Loch Riaghain, Loch a’ Bhleoghainn and Loch Leana na Moine (these three units contained 67 webs or 75% of the total). Remaining webs were spread thinly and almost half of occupied search units contained two or fewer webs. Most caterpillars of E. aurinia were 4th instars in webs and were either approaching moult or were at moult. Small 5th instars occurred in and around these webs indicating freshly moulted caterpillars. Some larger 5th instars and several 6th instars occurred in some survey units, and these occupied a further seven units in the absence of any evidence of 4th

Parasitoid wasps indicate that the marsh fritillary butterfly (Euphydryas aurinia) has persisted on Tiree rather than re-colonised recently