Research Article 
Corresponding author: Helmut Zwölfer ( h.zwoelfer@gmx.net ) Academic editor: Mark Shaw
© 2015 Helmut Zwölfer, Marc Böheim, Erwin Beck.
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Citation:
Zwölfer H, Böheim M, Beck E (2015) Eurytoma serratulae and E. robusta (Hymenoptera, Eurytomidae): complementary host exploitation strategies of coexisting parasitoids and their impact on the host Urophora cardui. Journal of Hymenoptera Research 42: 4762. doi: 10.3897/JHR.42.8847

Our study investigates the host exploitation strategies of Eurytoma serratulae and E. robusta (Chalcidoidea, Eurytomidae), two parasitoid species that cooccur 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 mediumsized 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, parasitoidinteractions, stimulation of gall growth
A competitive coexistence of generalists and specialists is a common and wellstudied feature of parasitoid guilds exploiting phytophagous hosts (e.g.
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 fieldcollected 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. creticumgalls 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 (
Host ranges and distribution of Eurytoma serratulae and E. robusta. Eurytoma serratulae is an endoparasitoid of Urophora cardui.
Both parasitoids have been reared from U. cardui galls on C. arvense in England (
Eurytoma serratulae is a koinobiont (
Eurytoma robusta is an idiobiont (
To assess the role of gall size for the parasitization pattern we use data from 260 gall samples with a total of 7,064 galls collected from 1976 to 1993 in eastern France, southern Germany and eastern Austria. The number of cells / gall ranged from one to 18 (mean + SE = 3.807+0.045, median = 3). A total of 25% of the galls had more than five cells and 10% more than 6 cells. Incidence of parasitism (% galls attacked) by E. serratulae increases steadily with increasing gall size, whereas attack rate by E. robusta reaches a maximum at intermediate gall sizes (Fig.
Average attack rate of galls by E. serratulae (black) and E. robusta (white) as function of cell numbers per gall. Analysed material: 260 gall samples with a total of 7,064 galls collected from 1976 to 1993 in eastern France, southern Germany and eastern Austria.
Average frequency of E. serratulae (black) and E. robusta (white) larvae as function of cell numbers per gall. Analysed material: 260 gall samples with a total of 7,064 galls collected from 1976 to 1993 in eastern France, southern Germany and eastern Austria.
Average content of galls as a function of cell numbers / gall. (In parentheses: number of galls examined).
The mean exploitation rate of the cells of a gall drops in both Eurytoma species with an increasing number of cells / gall (Fig.
Since E. robusta oviposits later than E. serratulae, we expected an asymmetric relationship between the two Eurytoma species. For a statistical test the effect of gall size on the rate of parasitization had to be removed. If galls occupied by E. serratulae without its congener are compared with galls jointly occupied by both species together (Fig.
Our material allows comparing the two Eurytoma spp. with regard to gall density / thistle patch, which is a rough estimate of the quantity of galls in a thistle patch (E. serratulae = 688 samples; E. robusta = 1093 samples). The analysed samples represent average densities of locally and temporally fluctuating parasitoidhost complexes (
The influence of gall density / thistle patch (= galls / 100 thistle shoots) on E. serratulae (black) and E. robusta (white). (In parentheses: number of galls examined).
Fig.
Average numbers of E. serratulae larvae / gall in U. cardui populations with different parasitization rates of E. robusta. (Sample sizes: 227, 332, 295, 209 gall populations).
Hyperparasitism by E. robusta masks in most gall populations the superior capacity of E. serratulae to use the cells of the multilocular U. cardui galls. In 119 gall populations with less than 0.011 E. robusta larvae / gall / sample an average of 27.9% of the available cells where parasitized by E. serratulae, whereas E. robusta (605 populations with low or without parasitism by E. serratulae) used only 21.8%. The difference is significant (Ttest: p < 0.001).
The parasitization rates of E. serratulae and E. robusta vary temporally and locally with the population dynamics of U. cardui (
Parasitization of U. cardui samples by E. serratulae and E. robusta in 3 different regions of western Europe. Sundgau (France, region of Belfort): 307 samples, 6463 galls (1970–2004); Upper Rhine Valley (region n.w. Freiburg): 78 samples, 1885 galls (1972–1992); North eastern Bavaria (region s.e. Bayreuth): 216 samples, 4131 galls (1978–2004).
In galls of U. cardui the number of E. serratulae larvae / gall is positively correlated with the gall diameter (r = 0,428, p < 0.001, N = 1407). But the variable “E. serratulae larvae / gall” is also correlated with the variable “cells / gall” (r = 0,481, p < 0.001, N = 2733), which in turn strongly affects the gall diameter (r = 0,674, p < 0.001, N = 2733). Together the two variables account for about 45% of the variation of the gall diameter (R² = 0.454). A multiple regression (Table
Direct and indirect effects between cells / gall, E. serratulae / gall and gall diameter. Path diagram: all three path coefficients (beta values) are significant at p < 0.001. Multiple R^{2} = 0.4546. U (coefficient of nondetermination) = 0.5456 (N = 2,733 U. cardui galls with E. serratulae).
The effect of cells / gall and E. serratulae larvae / gall on gall diameter. Multiple regression: dependent variable: gall diameter, N = 2.733, multiple R = 0.67115, adjusted R² = 0.45024, SE = 3.02533.
Variable  B  SE (B)  Beta  T  Sign 

cells / gall  1.15157  0.02496  0.6606  46.147  < 0.001 
E. serratulae / gall  0.18681  0.04095  0.0653  4.562  < 0.001 
(Constant)  9.00247  0.11135  80.851  < 0.001 
An independent additional test is the comparison of galls with and without parasitoids (Fig.
Average gall diameter in galls with E. serratulae (3512 galls), with unparasitized U. cardui (3449 galls) and with E. robusta plus empty cells (4288 galls).
In a subsample of 81 galls we assessed larval biomass of the host and E. serratulae, gall diameter and length, and cell sum / gall. The average fresh weight of 66 mature unparasitized third instar larvae of U. cardui was 13.465 mg (SE = +0.545 mg) and that of 15 full grown E. serratulae larvae (inclusive of the sclerotized host skins) 5.207 mg (SE = +0.452 mg). Gall length (mean 24.302 mm, SE = +0.88 mm) ranged from 14.8 to 35.2 mm. Gall diameters (mean 13.82 mm, SE = +0.408 mm) ranged from 7.9 to 20.3 mm. Correlations of larval weight of E. serratulae in sclerotized host skins with the gall parameters diameter, length, and cell sum produced positive correlations (Table
Correlation of body weight of unparasitized U. cardui and E. serratulae larvae within U. cardui mummies with gall parameters.
U. cardui (N = 66)  E. serratulae (N = 15)  

Weight (mg) vs gall length (mm)  
r  0.0892 (p = 0.464)  0.8452 (p = 0.0001) 
slope  0.048 (SE = 0.067)  0.2253 (SE = 0.0395) 
Weight (mg) vs gall diameter (mm)  
r  0.1701 (p = 0.1722)  0.6233 (p = 0.013) 
slope  0.1972 (SE = 0.1428)  0.4503 (SE = 0.1561) 
Weight (mg) vs cell sum/gall  
r  0.0679 (p = 0.588)  0.5633 (p = 0.0393) 
slope  0.2201 (SE = 0.4043)  1.1746 (SE = 0.5127) 
Mechanisms allowing a balance in multiparasitoid systems are compensations between intrinsic and extrinsic superiorities (
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 (
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 multichambered 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.
A particular feature of E. serratulae is its capacity to contribute to the growth of U. cardui galls (Table
It is interesting to compare E. serratulae with Eurytoma obtusiventris Gahan, the monophagous and highly specialised parasitoid of another gallinducing tephritid, Eurosta solidaginis (Fitch), on North American Solidago spp. (
Our study shows that the specialised E. serratulae in its closelyknit and evolutionarily stable parasitoidhost 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.