11 • Parasitoids of European Butterflies

11 * Parasitoids of European butterflies MARK R. SHAW, CONSTANTÍ STEFANESCU AND SASKYA VAN NOUHUYS SUMMARY populations is both general and indisputable. Some of the parasitoids involved are extreme specialists but others are Parasitoids are insects (mainly Hymenoptera and Diptera) more generalist; however, mortality is often high even in the whose larvae develop by feeding on the bodies of other absence of specialists (e.g. Stefanescu et al., 2003b), and almost arthropods (mainly immature insects) and the adults are free certainly no European butterfly species altogether escapes. living. They are of immense importance in practically all In view of the considerable attention paid to European terrestrial ecosystems because of their impact on the popula- butterflies over the years, first by collectors and more tion levels of their hosts. In contrast to the considerable recently by conservationists, it is anomalous that we have attention paid to European butterflies their parasitoids have such poor knowledge of their parasitoids (Shaw, 1990). received very little scrutiny, and the recognition that some Apart from a very few, such as pest Pieris and Thymelicus reared parasitoids are in fact hyperparasitoids (parasitoids of species, and flagship conservation groups like the Maculinea parasitoids) has often been lacking in published records. A and Melitaeini species which have also attracted population review of the biology and taxonomy of European butterfly ecologists, almost none has been subjected to extensive parasitoids is presented, including a simple key to the various sampling to investigate parasitism. Such deliberate effort parasitoid families that include relevant species, followed by a would be needed firstly to establish which parasitoids regu- brief discussion of the authors’ current knowledge of their use larly attack a particular butterfly species, and in which parts of butterfly hosts. Quantitative case studies on the butterfly of its range, and secondly to investigate the effect these taxa Iphiclides podalirius, Pieris spp., Maculinea rebeli, Aglais parasitoids have on their host populations. urticae, Melitaea and Euphydryas spp. and Thymelicus lineola However, there is a complementary need for parasitoid- illustrate the range of parasitoid assemblages and the extent to based knowledge. Host–parasitoid complexes are not closed which parasitoids account for butterflymortality.Theneeds systems, and there is no a priori way of assessing whether a and difficulties inherent in moving from a simple understand- parasitoid reared from a particular species also uses related ing of what parasitoids are using a host population to a more hosts, or perhaps unrelated ones occurring in similar envi- sophisticated assessment of the impact they may have on the ronments. Each parasitoid species must be fitted into a host’s population dynamics are outlined. Parasitoids can be continuum between absolute monophagy at one extreme or expected to have many important effects on the ecology of using the butterfly only marginally as part of a diffuse or butterfly species and, on the evolutionary timescale, defence differently focused host range at the other. Therefore we against parasitoids has shaped aspects of the physiology of the need to establish comprehensive knowledge not only of immature stages and the behaviour of both larval and adult which species attack butterflies, but also of the host associ- butterflies. Butterfly researchers are encouraged to help build ations of each one. This is gradually accruing, at least for better knowledge about the host relations and effects of para- common parasitoid species, especially through small-scale sitoids. To better understand the place of parasitoids in the rearings involving a large number of host species. lives of butterflies there is still much to be gained from close We begin this chapter by explaining why so little is attention to natural history, careful experimentation and thor- known about parasitoids of butterflies, highlighting the ough taxonomic investigation. sources of error that undermine the knowledge base (see also Shaw, 1990). Next we summarise the general biology of INTRODUCTION parasitoids of European butterflies. A simple key, including Parasitism affects the egg, larval and pupal stages of butter- biological characteristics, links this with the following tax- flies and its importance as a mortality factor in butterfly onomically organised section on their life history and host Ecology of Butterflies in Europe, eds. J. Settele, T. Shreeve, M. Konvicˇka and H. Van Dyck. Published by Cambridge University Press. © Cambridge University Press 2009, pp. 130-156. Parasitoids of European butterflies 131 ranges. The chapter then examines the few existing case 1994). Thus an abnormal success in developing on a host studies of parasitoid communities associated with particular that is usually rejected (or unsuitable if accepted), or the butterfly taxa, the contribution of parasitoids to mortality occasional discovery of a suitable host that is usually absent and, to some extent, their potential roles in butterfly pop- from the parasitoid’s searching environment, will not alter ulation dynamics. the concept of host range for the parasitoid concerned. Importantly, if host range is expressed in the quantitative terms implied by the definition, then abnormal events and UNDERSTANDING AND IMPROVING misidentifications will gradually become marginalised. OUR KNOWLEDGE Many of the parasitoids of European butterflies are uni- The historical passion for butterfly-collecting was uncon- voltine and use univoltine hosts, or are plurivoltine in syn- ducive to a systematic study of their parasitoids, which were chronisation with plurivoltine hosts, or sometimes the generally seen as a troublesome scourge and too obscure to parasitoid may be plurivoltine and use a univoltine host be interesting. With the more recent decline of butterflies in generation for successive broods. In all these cases the para- Western Europe, collecting has decreased, and science- sitoid has a potential to be absolutely host-specific, but there based conservationists have become the more active group. are also plurivoltine parasitoids that depend on different Most effort focuses on getting the ‘bottom–up’ aspects of the host species at different times of year. This complicates the habitat (vegetation composition and quality, thermal proper- parasitoid’s population dynamics enormously, adds another ties of sites, etc.) right for the dwindling butterfly popula- dimension to the concept of host range and places an addi- tion, and (with some notable exceptions) parasitoids and tional habitat demand because representatives of more than other ‘top–down’ influences have generally been ignored. one set of hosts must be present (see also Shaw, 2006). In However, this is to neglect the strong possibility that the some cases the alternate hosts of parasitoids of European butterfly population will harbour specialist parasitoids even butterflies are not themselves butterfly species, but other more at risk than their host, that might also play important Lepidoptera. Gaining insights into host ranges in these roles in the host’s population dynamics. Thus, it is more terms is particularly challenging, but of fundamental enlightened to see the butterfly’s conservation in broader importance. terms, including the trophic level above it (Shaw & Many case studies (e.g. Shaw, 1982, 1990, 1993, 1994, Hochberg, 2001). Gradually our currently low knowledge 2002a; Askew & Shaw, 1986; Noyes, 1994) demonstrate that might then be enriched by conservation biologists as well as compilations of host–parasitoid records abstracted from the by the scientifically minded enthusiasts who contribute literature are so full of misinformation and ambiguity that important data on such a wide front. they are useless for understanding either the nature or the The difficulty of finding eggs, larvae and especially breadth of host ranges. In particular, misidentifications have pupae of most butterfly species in comparison with adults been rife, of not only the parasitoid but also the host (often has also kept knowledge on parasitism low. This contrasts an extraneous insect overlooked during rearing) and, addi- strongly with some groups of microlepidoptera whose early tionally, either the compilations are non-quantitative (giving stages are the most easily found, resulting in abundant data equal weight to rare and common occurrences) and/or they on the host associations of their parasitoids (e.g. Askew & are corrupted because authors have reiterated host records Shaw, 1974, 1986; Askew, 1994; Shaw & Horstmann, 1997). from already published sources without making it clear that One of the most important properties of a parasitoid is they are not giving new records (thereby leading to multiple how host-specific it is. In practice rather few parasitoids are scoring: see Shaw, 1993). The only way round these prob- absolutely host-specific (though locally they may be, if only lems is for the focus to shift from literature records to an one of the possible hosts occurs), but rather they usually assessment of extant specimens by competent taxonomists, have a host range that either comprises a group of phyloge- and for host data for parasitoids to be given in key works, netically related hosts or a group of ecologically similar ones, reviews, etc. in quantitative form (as ‘host mortalities’, scor- or some balance between the two. It is useful to define host ing gregarious broods as one) based on reared specimens range

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