Our study investigates the host exploitation strategies of Eurytoma serratulae and E. robusta (Chalcidoidea, Eurytomidae), two parasitoid species that co-occur in gall populations of the tephritid Urophora cardui on Cirsium spp. The endoparasitoid E. serratulae detects the host larvae before an externally visible gall is formed. It profits from large galls, as its parasitization rate increases with increasing numbers of chambers per gall. Oviposition by the ectoparasitoid E. robusta does not occur until a distinct gall with chambers has been formed. Its parasitization rate reaches highest values in medium-sized galls. Eurytoma robusta is the dominant parasitoid in host populations with low and moderate gall densities, whereas E. serratulae is the superior exploiter of host populations with high gall densities. Within single galls E. robusta is an important hyperparasitoid of E. serratulae, but E. serratulae has no adverse influence on E. robusta. Parasitism by E. serratulae induces host larvae to promote gall growth, an effect that is profitable to both the parasitoid and the remaining host larvae in the gall. Parasitism by E. robusta often leads to smaller galls, as cases of unsuccessful parasitization result in empty gall cells.

Urophora cardui galls, coexistence of Eurytoma, parasitoid-interactions, stimulation of gall growth

A competitive coexistence of generalists and specialists is a common and well-studied feature of parasitoid guilds exploiting phytophagous hosts (e.g. Smith 1929, Zwölfer and Kraus 1957, Fisher 1961, Askew 1968, Godfray 1994). A particular case is the coexistence of pairs of competing congeners, a situation found in Eurytoma species (Chalcidoidea, Eurytomidae) associated with gall forming tephritids: thus Eurytoma compressa (Fabricius) (= curta (Walker)) and E. robusta Mayr coexist in hosts living in Cardueae flower head galls (Zwölfer and Arnold-Rinehart 1993), and Eurytoma obtusiventris Gahan and E. gigantea Walsh in stem galls of North American Solidago spp. (Abrahamson and Weis 1997). The parasitoids Eurytoma serratulae (Fabricius) and E. robusta regularly co-occur in gall populations of the tephritid Urophora cardui (Linnaeus) on shoots of Creeping Thistle (Cirsium arvense). Information on the biology of E. serratulae and E. robusta has been published by Claridge (1961), Redfern (1983), Redfern and Askew (1992) and Zwölfer (1979, 1988). In view of considerable differences in its larval biology E. robusta has been placed in a separate species group of Eurytoma (e.g. Zerova and Seryogina 2006). Both species are solitary parasitoids.

Our study investigates the mechanisms that make the coexistence of the two Eurytoma species possible. The characteristics of the multilocular gall of U. cardui allow comparing the behaviour of the two Eurytoma species on the level of the microhabitat of individual galls and on the macrohabitat level of the gall populations. For both levels we compare the exploitation strategies of E. serratulae and E. robusta using our large basis of field-collected data that allow the statistical evaluation of “natural experiments”. We analyse the influence of gall size and gall densities in the field on the incidence, degree of parasitism and the exploitation pattern of the two Eurytoma species. In conclusion we compare the effect of E. serratulae and E. robusta on the growth of U. cardui galls and we discuss how the characteristic features of the multilocular galls of U. cardui and the specialized oviposition behaviour safeguard E. serratulae against an overexploitation of its resource.

Collections. For our study we combined cage observations on the behaviour of E. serratulae with the statistical analysis of field collected gall populations. For the evaluation of our field data we disposed of 693 gall samples (16 732 galls) of C. arvense and nine samples (374 galls) of C. creticum. The C. arvense galls were collected between the years 1969 and 2006 in France, Switzerland, northern Italy, Germany, Austria, Slovakia, southern England, and western Poland, the C. creticum-galls in 1984 and 1990 in northern Greece. Galls were collected from September / October to March / April. Urophora cardui populations frequently occur in the form of metapopulations (Eber and Brandl 1994), i.e. as a network of single demes (Saccheri and Hanski 2006). We treated a thistle patch with U. cardui galls as a “population” if it was separated from a neighbouring patch with galls by at least 200 m. We counted the number of thistle shoots and galls / patch in the field, estimated the total number of shoots / patch and dissected the galls under a stereomicroscope. We assessed the gall content (living or dead larvae or pupae of U. cardui, E. serratulae, E. robusta and other parasitoids, empty chambers, eggs of E. robusta) and gall size (cell number and gall diameter). All data were assessed in SPSS files by the program DATA ENTRY and analysed by IBM SPSS statistics 21. For the path analysis we followed Sokal and Rohlf (1981).

Host ranges and distribution of Eurytoma serratulae and E. robusta. Eurytoma serratulae is an endoparasitoid of Urophora cardui. Claridge (1961) showed that E. tristis Mayr, which Mayr (1878) and other authors recorded from galls of U. cardui, is synonymous with E. serratulae. All reliable host records of E. serratulae refer to U. cardui, a tephritid whose host plants are Cirsium arvense (Claridge 1961), C. creticum (Zwölfer 1988) and C. setosum (Frenzel et al. 2000). The ectoparasitoid Eurytoma robusta can so far only be identified as a morphospecies, the host records in the literature may actually pertain to a cluster of undescribed siblings. Claridge (1961) describes E. robusta as attacking “a wide range of tephritid galls on Cardueae host plants”. Zwölfer and Arnold-Rinehart (1993) list E. robusta from 14 Palearctic Urophora species on 22 Cardueae host plant species.

Both parasitoids have been reared from U. cardui galls on C. arvense in England (Claridge 1961), Switzerland, France, Germany and Austria (Zwölfer 1979, 1994), and Finland (Vikberg 2005). Additional records from U. cardui on C. arvense are from northern Italy, western Poland and Slovakia (H. Zwölfer, unpublished). The two Eurytoma species also coexist in U. cardui galls on C. creticum in northern Greece (Zwölfer 1988) and in U. cardui galls on C. setosum originating from the Ural Mountains (Frenzel et al. 2000). Thus, the two Eurytoma species attack Urophora cardui on all 3 host plants and in almost all regions where galls have been analysed. An exception is Denmark and Schleswig-Holstein (northern Germany), where Johannesen and Seitz (2003a, b) as well as we (H. Zwölfer, unpublished) found only E. robusta in U. cardui galls. There, the examined thistle patches with U. cardui may have been recently recolonized by U. cardui (Johannesen and Seitz 2003b). Parasitoids of minor importance in U. cardui galls are the ectoparasitoids Torymus chloromerus (Walker) (Torymidae) and Pteromalus elevatus (Walker) (Pteromalidae).

Mechanisms allowing a balance in multi-parasitoid systems are compensations between intrinsic and extrinsic superiorities (Smith 1929). Examples can be found in most parasitoid complexes where parasitoid species with different life histories competitively coexist (e.g. Zwölfer and Kraus 1957, Askew 1968, Godfray 1994). Another important aspect is the temporal niche divergence of coexisting parasitoids, i.e. the exploitation of different host stages (e.g. Pschorn-Walcher and Altenhoffer 1989) or different developmental phases of plant galls (e.g. Askew 1975). Simulation models (Hassell 1986, 2000) demonstrate that patchy host distributions, metapopulation structures and host refuges may stabilize parasitoid-host interactions and contribute to prevent overexploitation of host populations.

In our study we show that the particular characteristics of multilocular plant galls provide an additional stabilizing mechanism for competing parasitoid species, since the two Eurytoma species exploit the cells of an individual gall in different ways. Eurytoma robusta is intrinsically superior, i.e. where its larva comes into contact with an U. cardui larva containing a young E. serratulae, it eliminates both. An advantage is also the capacity of the E. robusta larva to feed as an inquiline on the nutritive tissues of the gall. For its oviposition E. robusta is dependent on galls with fully developed cells, where synchronisation problems can lead to mortality of the young host larvae and starvation of the E. robusta larvae. Frequent and often high superparasitism of E. robusta (Zwölfer et al. 2007) can result in a considerable waste of eggs. As an initiator of empty cells E. robusta is an important mortality factor of U. cardui (Fig. 8). Parasitism by E. robusta and empty cells due to host killing without successful development of the parasitoid larva can cause up to 100% host mortality of small U. cardui populations. In experimental U. cardui populations in the Swiss Jura (Zwölfer 1994) and in several of our monitored populations in Northern Bavaria (Zwölfer et al. 2007) an overexploitation by E. robusta resulted in the complete breakdown of local U. cardui subpopulations and the associated monophagous parasitoid E. serratulae. In such cases its relatively broad host range allows E. robusta to switch to other host species. With regard to its host U. cardui, E. robusta is a poorly adapted generalist parasitoid but nevertheless an important mortality factor.

The life history of its congener E. serratulae is perfectly integrated with that of U. cardui. The female of E. serratulae detects the young host larvae already before an externally visible gall is formed and is able to parasitize a greater proportion than E. robusta of the host larvae available in an individual gall. In this way E. serratulae profits from grown up U. cardui populations with large multi-chambered galls. The capability to induce a sclerotization of host cuticula, once the host larva is consumed, provides a certain protection of the hibernating E. serratulae larva, e.g. against hyperparasitoids or predators. The emergence of adults is well synchronized with the emergence of the host flies. The increasing parasitism by E. serratulae in thistle patches with higher gall densities (Fig. 6) indicates a host searching behaviour influenced by the concentration of the host resource. Within a local host population galls with more cells contain over proportionally more E. serratulae larvae. This relationship suggests that thistle shoots with larger clutch sizes of Urophora are better recognizable and more attractive for E. serratulae females. The positive density dependence on the macrohabitat and microhabitat levels relieves the host U. cardui at low densities and contributes to stabilize a host-parasitoid system (Hassell 2000). Within individual galls the exploitation rate of E. serratulae is inversely density-dependent, making possible an increasing host refuge for U. cardui with increasing numbers of cells / gall (Fig. 3). The reason for this inverse relationship may be egg limitation of the E. serratulae females leading to some sort of “predator satiation” and/or the difficulty to locate all the tiny host larvae of a large clutch. The survival advantage of U. cardui larvae in galls with higher cell numbers can be important for the population dynamics of the host; as such galls contribute considerably more to the reproductive fitness of U. cardui than small galls (Freese and Zwölfer 1996). Since the absolute numbers of E. serratulae / gall also increase, larger galls with higher cell numbers are ultimately profitable to both the parasitoid and the host. Tabuchi and Amano (2004) report a similar relationship for the gall midge Asteralobia sasakii and its parasitoids on Ilex.

A particular feature of E. serratulae is its capacity to contribute to the growth of U. cardui galls (Table 1, Fig. 10). Host manipulation by endoparasitoids is a common phenomenon, as parasites tend to alter their hosts in ways beneficial to their own fitness. A special type of this manipulation consists in the prolongation of the feeding stages of the host, which allows the parasitoid larva to accumulate additional food resources (Beckage and Riddiford 1983, Slansky 1986, Godfray 1994). We assume that E. serratulae larvae stimulate U. cardui larvae to produce additional gall growth in a comparable way. Since larger galls provide better protection and possibly also an improved nutritional situation for U. cardui (Freese and Zwölfer 1996) they are profitable for both U. cardui and E. serratulae. The life history of E. serratulae, and the fact that it was able to follow its host to all three host plants and to cover the whole distribution area of U. cardui, indicate that the parasitism of U. cardui by E. serratulae forms an evolutionarily ancient and stable association.

It is interesting to compare E. serratulae with Eurytoma obtusiventris Gahan, the monophagous and highly specialised parasitoid of another gall-inducing tephritid, Eurosta solidaginis (Fitch), on North American Solidago spp. (Uhler 1951, Abrahamson et al. 1989, Abrahamson and Weis 1997). Both parasitoid species are koinobiont endoparasitoids, both attack the young host larvae of a gall-maker before gall formation starts, both are more frequent in large host galls and both are exposed to congeneric hyperparasitoids (E. robusta and E. gigantea Walsh, respectively), which deposit their eggs later into the galls. An important difference between the two parasitoid-host systems lies in the gall structure. The multi-chamber gall of U. cardui can secure the survival of some host larvae, whereas the single-chamber gall of E. solidaginis, once attacked, provides no chance of a host-escape. Differences exist also in the exploitation strategies of the two parasitoids. No influence on gall growth is described for E. obtusiventris; its higher frequency in larger galls is interpreted as a mere result of interactions with other mortality factors of the Eurosta galls (Abrahamson et al. 1989). It might, however, be worthwhile checking whether E. obtusiventris larvae could manipulate Eurosta larvae to produce larger goldenrod galls in a comparable way as E. serratulae influences the growth of U. cardui galls.

Our study shows that the specialised E. serratulae in its closely-knit and evolutionarily stable parasitoid-host system with U. cardui follows a “sustainable” exploitation strategy, which is absent in the poorly adapted generalist E. robusta.

We gratefully acknowledge support by the German Research Foundation (Graduiertenkolleg No. 678 “Bedeutung von Wirk- und Signalstoffen für Tiere – von der Struktur zur Funktion im Ökosystem”). H. Zwölfer thanks Prof. Joe Shorthouse for a suggestion, made many years ago, that endoparasitoids may be able to stimulate gall growth. Two anonymous reviewers of a former version of the manuscript provided helpful criticism.