Variation in selection pressures on the goldenrod gall fly and the competitive interactions of its natural enemies

Summary Larvae of the tephritid fly Eurosta solidaginis induce ball-shaped galls on the stem of tall goldenrod, Solidago altissima. Survival probability depends on gall size; in small galls the larva is vulnerable to parasitoid oviposition, whereas larvae in large galls are more frequently eaten by avian predators. Fly populations from 20 natural old fields in central Pennsylvania were monitored in 1983 and 1984 to examine the distribution of the selection intensity imposed by natural enemies, the parasitoids Eurytoma gigantea and E. obtusiventris, the inquiline Mordellistena unicolor, and the predatory birds Dendrocopus pubescens and Parus atricapillus. Mordellistena and E. obtusiventris are able to attack galls of all diameters while E. gigantea and the predatory birds preferentially assaulted small and large diameter galls, respectively. Eurosta in intermediate sized galls had the highest survivorship, hence selection had a stabilizing component. However, parasitoid attack was more frequent than bird attack, and the two did not exactly balance, thus there was also a directional component. The mean directional selection intensity on gall size was 0.21 standard deviations of the mean, indicating that larger gall size was favored. Interactions among the insect members of the Eurosta natural enemy guild are complex and frequent.     

Do parasitoids diversify in response to host‐plant shifts by herbivorous insects?

1. For herbivorous insects, the incorporation of a novel host into the diet, and subsequent formation of distinct host associations (races), is thought to be a significant early step in the speciation process. While many studies have addressed this issue, virtually nothing is known about the evolutionary response of natural enemies to herbivore host‐race formation. 2. The hypothesis that the parasitoid wasp Eurytoma gigantea (Hymenoptera: Eurytomidae) has formed host races in direct response to the host shift and subsequent host‐race formation by its host, the gallmaker Eurosta solidaginis (Diptera: Tephritidae) was tested. Emergence time, mating preference, and female oviposition preference were determined for parasitoids derived from galls of each Eurosta host race. 3. Male and female E. gigantea overlap broadly in their emergence times from each Eurosta host race, suggesting that there is no phenological barrier to gene flow. 4. In choice experiments, female parasitoids did not mate assortatively: females that emerged from one Eurosta host race were equally likely to mate with males from either Eurosta host race. 5. Oviposition behaviour experiments revealed that female parasitoids do not prefer to oviposit on their host race of origin and that there is no overall preference for one host race, even though fitness is higher when parasitoids are reared from Eurosta galls of the Solidago gigantea host race than when reared from Eurosta galls of the Solidago altissima host race. 6. These results suggest that E. gigantea has not diverged in parallel with its host in response to the herbivore host‐plant shift. Further studies are needed before the ubiquity of this diversification mechanism can be evaluated fully.     

MITOCHONDRIAL DNA PHYLOGEOGRAPHY OF HOST RACES OF THE GOLDENROD BALL GALLMAKER,EUROSTA SOLIDAGINIS (DIPTERA: TEPHRITIDAE)

We determined the phylogenetic relationships and geographic distribution of mitochondrial haplotypes of two host races of the tephritid fly Eurosta solidaginis, a gallmaker that attacks species of goldenrod (Solidago). We performed a preliminary survey by sequencing 492 bp from the 3′ ends of the mitochondrial cytochrome oxidase I and II subunits from a single individual from eight S. gigantea- and 10 S. altissima-associated populations across their range in eastern North America and from two outgroup species, Eurosta comma (two populations) and E. cribrata. Eurosta solidaginis haplotypes fell into two groups (“E” and “W” clades), which differed by four substitutions, one of which occurred within the recognition site of the DdeI restriction enzyme. We used the presence or absence of the restriction site to survey a total of 11 S. gigantea (20 individuals) and 20 S. altissima (43 individuals) host-race populations. All gigantea-fly haplotypes regardless of geographic origin carried the E-clade haplotype, whereas altissima-fly haplotypes were geographically partitioned. Altissima flies east of Michigan were of haplotype E, whereas those west of Michigan were of haplotype W, with mixed populations found in lower Michigan. These patterns confirm an earlier allozyme survey that suggested that S. altissima is the ancestral host for the gallmaker, but also suggest that the gigantea fly populations were derived from eastern U.S. altissima fly populations. The data support the conclusions of behavioral and ecological studies indicating that the shift to the derived host was facilitated by escape from natural enemies.     

Gall Flies, Inquilines, and Goldenrods: A Model for Host-race Formation and Sympatric Speciation

Host shifts and subsequent host-race formation likely play amore common role in the speciation of herbivorous insects thanhas generally been recognized. Our studies of the interactionsof goldenrod host plants (Solidago: Compositae), the gall flyEurosta solidaginis (Diptera: Tephritidae), and the stem- andgall-boring Mordellistena convicta (Coleoptera: Mordellidae)provide behavioral, ecological, and genetic evidence of insecthost races that may represent incipient species formed via sympatricspeciation. Eurosta solidaginis has developed genetically differentiatedand reproductively isolated host races that are associated withthe ancestral host Solidago altissima and the derived host S.gigantea. Conventional wisdom suggests that shifts even to closelyrelated host plants are limited by host preferences or the inabilityto utilize a chemically and developmentally distinct host. However,our preliminary work with Eurosta from S. gigantea implies thathost choice and gall induction do not deter a shift to S. canadensis.The galling of Solidago by Eurosta created a new resource thathas led to a subsequent host range expansion by the stem-boringbeetle. Mordellistena convicta from stems and galls are geneticallydistinct and likely shifted from stems to galls. Beetles fromS. altissima versus S. gigantea galls exhibit assortative matingand higher preference for and/or performance on their natalhost. The present-day distributions of the Eurosta host racesand their behavioral isolating mechanisms do not suggest thatgeographic isolation was required for their formation; ratherthese characteristics suggest a sympatric mode of differentiation.Our findings lend credence to recent assertions that sympatricspeciation may be an important source of biodiversity.     

The effects of drought on the<Emphasis Type="Italic"> Solidago altissima</Emphasis>-<Emphasis Type="Italic">Eurosta solidaginis</Emphasis>-natural enemy complex: population dynamics,local extirpations,and measures of selection intensity on gall size

Environmental catastrophes, such as severe drought, can reduce host-plant quality and/or abundance, which in turn decrease levels of herbivore populations. Such changes in herbivore populations affect populations of their natural enemies. As part of a long-term field experiment (1983–1991), galls of Eurosta solidaginis from 16 fields in central Pennsylvania were systematically collected from goldenrod ramets. Galls were dissected to compare the occurrence of E. solidaginis mortality caused by its natural enemies in 2 drought years (1988, 1991) with 5 pre-drought years (1983–1987) and 2 post-drought years (1989–1990). Gall diameters were significantly smaller in both drought years and early larval death significantly decreased E. solidaginis survivorship in the first drought year. Of the natural enemies, the parasitoid wasp Eurytoma gigantea caused significant selection for larger gall size in all pre-drought years, the 1991 drought, and both post-drought years, due to its differential attack of smaller galls. In spite of drought-induced small gall size in 1988, there was negligible selection on gall size by natural enemies. However, populations of E. solidaginis did suffer local extirpations at nine of the 16 fields during the first drought year and population recoveries of the gall inducer and natural enemies varied among fields in the post-drought years. As a consequence of reduced herbivore abundance in drought and post-drought years, some natural-enemy populations were absent. Drought therefore drastically reduced the abundance of E. solidaginis and natural enemies resulting in slow recoveries to pre-drought numbers. Received: 16 April 1998 / Accepted: 4 August 1998     

The effects of host race, gender, and host plant distribution on alighting behavior, mating, and oviposition in Eurosta solidaginis

Eurosta solidaginis Fitch (Diptera: Tephritidae) has formed host races on Solidago altissima L. and Solidago gigantea Ait. (Asteraceae), and reproductive isolation between these host races is brought about in part by host‐associated assortative mating. Any non‐assortative mating creates the potential for gene flow between the populations, and we investigated the conditions that favored non‐assortative mating. We hypothesized that the frequency of non‐assortative mating would be influenced by differences in the behaviors of the host races and sexes and by the presence and pattern of distribution of the two host species. To test these hypotheses, we caged flies on four combinations of 32 potted host plants: all S. altissima, all S. gigantea, and cages with both host species arranged in either two pure species blocks or randomly dispersed. We recorded the number of flies of each host race that alighted on each host species and the frequency of mating within and between the host races. Males of both host races were observed on plants more frequently than females. Flies of the host race from S. gigantea (gig flies) were observed on plants in greater absolute numbers, and they mated more frequently than flies of the host race from S. altissima (alt flies). In all treatments, gig flies of both sexes were found on non‐natal host plants significantly more frequently than alt flies, and gig females showed a weaker preference for their host species than did gig males or alt flies of either gender for their respective natal hosts. Assortative mating predominated in all treatments, and flies from each host race mated more frequently in cages containing their own host plant. The frequency of non‐assortative mating varied among treatments, with the matings between alt ♀ × gig ♂ being more common in the pure S. altissima treatment and the gig ♀ × alt ♂ being more frequent in the pure S. gigantea and random treatments. Matings between gig ♂ × alt ♀ were more common overall than the reciprocal mating, because gig males were more active in pursuing matings and in alighting on the non‐natal host plant than alt flies. Non‐assortative matings were more frequent in the random than in the block treatments, but this difference was not significant. Because of strong selection against oviposition into the alternate host, we hypothesized that host plant distribution would not affect oviposition preference. We tested this hypothesis by examining the oviposition behavior of naïve, mated females in two treatments in which both host species were present: either arranged in blocks or randomly dispersed. Females oviposited only into their natal host, regardless of host plant distribution.     

HYBRIDIZATION STUDIES ON THE HOST RACES OF EUROSTA SOLIDAGINIS: IMPLICATIONS FOR SYMPATRIC SPECIATION

We studied the inheritance of survival ability in host-associated populations of the tephritid fly, Eurosta solidaginis, to test predictions of sympatric speciation models. Eurosta solidaginis induces galls on two species of goldenrod, Solidago altissima and S. gigantea. The host-associated populations have been hypothesized to be host races that originated in sympatry (Craig et al. 1993). We found evidence for disruptive selection for host use, which is a critical assumption of sympatric speciation models. Each host race had higher survival rates on their host plant than on the alternative host. F₁ and backcross hybrids also had lower survival rates than the pure host-race flies on their host plant. Since assortative mating occurs due to host-plant preference (Craig et al. 1993) this would select for divergence in host preference. Low hybrid survival could have been due to strong genetic incompatibilities of the populations or due to host adaptation by each population. Strong genetic incompatibilities would result in poor survival on all host plants, while host adaptation could result in low overall survival with high hybrid survival on some host plants with particularly “benign” environments. High survival of F₁, F₂, and backcross hybrids on some plant genotypes in some years supported the host adaptation hypothesis. F₁ flies mated and oviposited normally and produced viable F₂ and backcross hybrids indicating gene flow is possible between the host races. A few flies developed and emerged on the alternative host plant. This demonstrates that genes necessary to utilize the alternative host exist in both host races. This could have facilitated the origin of one of the populations via a host shift from the ancestral host. The inheritance of survival ability appears to be an autosomal trait. We did not find evidence that survival ability was maternally influenced or sex linked.     

VARIABLE SELECTION ON EUROSTA'S GALL SIZE. II. A PATH ANALYSIS OF THE ECOLOGICAL FACTORS BEHIND SELECTION

We examined phenotypic selection exerted by natural enemies on the gall-making fly Eurosta solidaginis in an extensive field study of 16 populations, spanning four generations. Gall-makers that induce small galls are vulnerable to the attack of Eurytoma gigantea. This imposes upward directional selection on gall size. Insectivorous birds, predominantly the downy woodpecker, are more likely to attack larvae that induce large galls than small ones, and this imposes downward directional selection. We used path analysis to explore the relative contributions of these natural enemies to the net directional selection on gall size. The path models further examined several ecological factors that influence selection intensity through their effects on parasitoid and bird attack rates. Net directional selection varied more strongly with E. gigantea attack than bird attack. Competitive interactions among birds and the three parasitoid species, including E. gigantea, were evidenced by low winter bird attack rates in fields where a high proportion of galls contained the overwintering parasitoids. Eurytoma gigantea attack was heavier in fields where mean gall size was small and bird attack heavier in fields where mean gall size was large. Neither birds nor E. gigantea showed simple density-dependent attack. Data suggested a form of frequency-dependent attack by birds but not by E. gigantea.     

ONE HOST SHIFT LEADS TO ANOTHER? EVIDENCE OF HOST‐RACE FORMATION IN A PREDACEOUS GALL‐BORING BEETLE

Abstract.— We show that a predator, the tumbling flower beetle Mordellistena convicta (Coleoptera: Mordellidae), has formed host races in response to a host-plant shift and subsequent host-race formation by its prey, the gall-inducing fly Eurosta solidaginis (Diptera: Tephritidae). This fly has formed two host races, one that induces stem galls on the ancestral host plant, Solidago altissima (Compositae), and another that induces stem galls on the closely related S. gigantea . We found that subpopulations of M. convicta that attack E. solidaginis galls on the different host plants have significantly different emergence times and, although slight, these allochronic differences are consistent across a range of temperatures. More importantly, we found that beetles assortatively mate according to their natal host plants, and female M. convicta preferentially attack and/or their offspring have higher survival in galls on natal host plants. Our data suggest that subpopulations of M. convicta that attack E. solidaginis galls on S. altissima and S. gigantea have formed host races. This is one of the first studies to demonstrate that a host shift and subsequent host-race formation by an herbivorous insect may have resulted in subsequent diversification by one of its natural enemies.     

BEHAVIORAL EVIDENCE FOR HOST-RACE FORMATION IN EUROSTA SOLIDAGINIS

We report behavioral evidence that Eurosta solidaginis, a stem-galling tephritid fly, has formed host races on its two goldenrod hosts, Solidago altissima and S. gigantea. Previous work has shown that flies from each host plant differ electrophoretically at the level of host races. The two host-associated populations were truly sympatric and were frequently found on host plants of the two species growing interdigitated with each other. Each host-associated population demonstrated a strong preference for ovipuncturing its own host. The S. gigantea–associated population emerged 10 to 14 d earlier than the S. altissima–associated population, contributing to the reproductive isolation between populations. Partial reproductive isolation is also maintained by a preference for mating on the host from which the fly emerged. The populations meet the criteria established for host races, suggesting that they may be in an intermediate stage of sympatric speciation.     

Can there be an escalating arms race without coevolution? Implications from a host-parasitoid simulation

Summary Under a restricted set of conditions, predator-prey or parasite-host systems may exhibit an escalating arms race over several generations that is not coevolutionary. Preconditions for such a process include high correlation between prey/host quality and defensive capability, and phenotypic plasticity in predator/parasite-counter defenses that responds to quality. We present simulation models based on the parasitoid waspEurytoma gigantea, which lays its egg in the goldenrod gall induced by the flyEurosta solidaginis. For the parasitoid to successfully lay an egg, the gall walls must be thinner than the parasitoid's ovipositor is long. Wall thickness is highly correlated with gall size, so probability of successful attack declines with gall size. However, since the parasitoid eats the gall tissue, individuals developing in small galls have little food and mature with shorter ovipositors than those which develop in large galls. The simulation showed that the population mean parasitoid size is set by mean gall size. Since small galls are more frequently parasitized, there is a selection pressure on the gallmaker to induce larger galls. But, an additional simulation showed that since parasitoid ovipositor length depends on gall size, an evolutionary increase in gall size will also result in a non-evolutionary increase in parasitoid body size and ovipositor length over several generations.     

Partitioning of herbivore hosts across time and food plants promotes diversification in the Megastigmus dorsalis oak gall parasitoid complex

Communities of insect herbivores and their natural enemies are rich and ecologically crucial components of terrestrial biodiversity. Understanding the processes that promote their origin and maintenance is thus of considerable interest. One major proposed mechanism is ecological speciation through host‐associated differentiation (HAD), the divergence of a polyphagous species first into ecological host races and eventually into more specialized daughter species. The rich chalcid parasitoid communities attacking cynipid oak gall wasp hosts are structured by multiple host traits, including food plant taxon, host gall phenology, and gall structure. Here, we ask whether the same traits structure genetic diversity within supposedly generalist parasitoid morphospecies. We use mitochondrial DNA sequences and microsatellite genotypes to quantify HAD for Megastigmus (Bootanomyia) dorsalis, a complex of two apparently generalist cryptic parasitoid species attacking oak galls. Ancient Balkan refugial populations showed phenological separation between the cryptic species, one primarily attacking spring galls, and the other mainly attacking autumn galls. The spring species also contained host races specializing on galls developing on different host‐plant lineages (sections Cerris vs. Quercus) within the oak genus Quercus. These results indicate more significant host‐associated structuring within oak gall parasitoid communities than previously thought and support ecological theory predicting the evolution of specialist lineages within generalist parasitoids. In contrast, UK populations of the autumn cryptic species associated with both native and recently invading oak gall wasps showed no evidence of population differentiation, implying rapid recruitment of native parasitoid populations onto invading hosts, and hence potential for natural biological control. This is of significance given recent rapid range expansion of the economically damaging chestnut gall wasp, Dryocosmus kuriphilus, in Europe.     

GENETIC DIFFERENTIATION AMONG HOST-ASSOCIATED POPULATIONS OF THE GALLMAKER EUROSTA SOLIDAGINIS (DIPTERA: TEPHRITIDAE)

Eurosta solidaginis, a gallmaking tephritid (Diptera), infests at least two species of Solidago in the eastern United States. We used horizontal starch gel electrophoresis of enzymes to examine whether populations on the two principal host plant species, Solidago altissima and S. gigantea, have diverged genetically. At the d -β-hydroxybutyrate dehydrogenase (HBDH) locus, the predominant allele in all nine populations from S. altissima (HBDH^1.00) was absent or uncommon in all but 1 of the 12 populations from S. gigantea. At the phosphoglucomutase locus (PGM), the most common allele in all populations was PGM^1.00, but the frequency of this allele was, with one exception, higher in populations from S. gigantea than in populations from S. altissima. Genetic heterozygosity was usually greater in populations from S. altissima (H? = 0.028) than in populations from S. gigantea (H? = 0.009). A phylogenetic tree derived from a genetic distance matrix clustered gallmaker populations from the same host plant together.     

Factors determining the structure and diversity of parasitoid complexes in tephritid fruit flies

Summary The relative importance of phylogenetic affinity of hosts versus their ecological characteristics in determining the composition of their parasitoid complexes was examined using the parasitoid complexes of six species of frugivorous fruit flies from Central Europe. The hosts were four Rhagoletis and two other trypetine species, ranging in their relatedness from host races to members of different genera. They also differed in ecological characteristics, utilizing host plants of three different families, and developing either as pulp- or seedfeeders inside the host fruit. These features made it feasible to test the following pair of hypotheses. The ecological hypothesis predicts that ecological traits such as host-plant and fruit fly phenologies and host-fruit texture should be more important for the composition of parasitoid complexes than the taxonomic relatedness of the fly species. Assuming that ecological relationships do not parallel phylogenetic ones, the alternative phylogenetic hypothesis predicts the opposite. In fruit and soil samples, taken between 1983 and 1989, three guilds of parasitoids comprising 20 species were found: guild 1 — koinobiotic larval parasitoids (e.g. Opius spp., which attack the host larvae but develop inside the host puparia); guild 2 — idiobiotic larval parasitoids (e.g. Pteromalus spp., which consume the host larvae at once); and guild 3 — idiobiotic puparium parasitoids (e.g. Phygadeuon spp.). Although some results support the phylogenetic hypothesis, the majority of results support the ecological hypothesis.     

Host gall size predicts host quality for the parasitoidEurytoma gigantea (Hymenoptera: eurytomidae), but can the parasitoid tell?

Simulation models presented here show that gall size of Eurosta solidaginisFitch (Diptera: Tephritidae) is a reliable predictor of its quality as a host to the parasitoid Eurytoma giganteaWalsh (Hymenoptera; Eurytomidae). The nutritional value of a gall to a parasitoid increases with diameter, but so does the likelihood that ovipositing parasitoids will fail to penetrate to the gall's central chamber. Despite the large differences in gall quality with size, this parasitoid seems incapable of distinguishing large, impenetrable galls from smaller, more suitable ones (Weis, A. E., et al., Ecol. Entomol. 10:341–348, 1985). This paper shows that Eurytomais capable of quick rejection of nonhost galls of similar size and shape to suitably sized host galls. Several lines of reasoning are explored to understand the seemingly maladaptive lack of sizediscrimination ability on host galls.     

Factors affecting refuge from parasitoid attack in a cynipid wasp, Aphelonyx glanduliferae

We examined seasonal patterns of gall morphology, growth, and survivorship of the agamic generation of a cynipid wasp, Aphelonyx glanduliferae, and discussed its mortality factors, especially from the point of view of refuge from parasitoid attack. Although the initiation period varied greatly among individual galls, the larvae of A. glanduliferae grew rapidly and reached their maximum size within 3 weeks before pupating in late September to early October. This growth period corresponded to the period when the gall walls became thinner. Parasitoid attack, which was the principal factor in the mortality of A. glanduliferae in the tree crown, was concentrated around the pupation period of the cynipid. Gall walls were significantly thinner in galls attacked by parasitoids than in those still containing a living cynipid. Therefore, the period available to parasitoids seems to be limited by both gall wall thickness and cynipid size. Thus, the growth pattern of A. glanduliferae larvae can have significance in that it narrows the window of vulnerability to parasitoids to a particular period. Although delaying gall initiation will also shorten the exposure period to parasitoid attacks, it was likely to increase the risk of death from gall abortion caused by seasonal degradation in the quality of host plant tissues. Although many cynipids were killed by disease in the galls that fell to the ground, the falling of mature galls to the ground may be another way to a parasitoid-free space. It is thus suggested that a trade-off among life history traits against multiple factors operates in the refuge of A. glanduliferae from parasitoid attack. Received: May 15, 2001 / Accepted: February 1, 2002     

The role of geographic setting on the diversification process among Tephritis conura (Tephritidae) host races

We address the controversy over the processes causing divergence during speciation. Host races of the fruit fly Tephritis conura attack the thistles Cirsium oleraceum and Cirsium heterophyllum. By studying the genetic divergence of T. conura in areas where host plants are sympatric, parapatric and allopatric, we assessed the contribution of geography in driving host-race divergence. We also evaluated the relative importance of genetic drift and selection in the diversification process, by analysis of the geographic distribution of genetic variation. Host races were significantly diverged at five out of 13 polymorphic allozyme loci. Variance at two loci, Hex and Pep D, was almost exclusively attributable to host-plant affiliation in all geographic settings. However, Hex was significantly more differentiated between host races in sympatry/parapatry than in allopatry. This result might be explained by selection against hybridisation or against incorrect host choice in contact areas. Linkage disequilibrium tests suggest the latter: gene flow in contact areas may occur from males of the host-race C. heterophyllum to females of the host-race C. oleraceum, whereas incorrect oviposition events were never observed. The distinctive patterns of genetic differentiation at the two highly differentiated loci implicate the action of selection (acting directly or on linked loci) rather than genetic drift. Despite their restricted interactions in sympatry, we conclude that host races are stable and that the major diversification process took place before species arrived in today's geographical settings.     

Behavioural modification of a social spider by a parasitoid wasp

1. Manipulation of host behaviour by parasitoids has long captured the imagination of ecologists. Parasitoid wasps in the Polysphincta group of genera develop as external parasitoids of spiders. 2. In the present study, the previously undescribed interaction between a Zatypota sp. wasp (Ichneumonidae) and a social spider Anelosimus eximius (Theridiidae) is described. The larva of this Zatypota wasp is found to induce its host to disperse from their communal web and build an entirely enclosed web consisting of densely spun silk. 3. The wasp is observed to target primarily immature A. eximius individuals, with 37.5–44% of nests in a given area being parasitised. Of those nests, approximately 1.3–2.0% of individuals are hosts to the parasitoid larvae. Larger spider colonies had a significantly higher probability of harbouring parasitoids. 4. This interaction results in unusual behaviours for A. eximius induced by the parasitoid: (i) leaving the protection of the social nest and (ii) building a unique, altered web that it would not otherwise build. It is suggested that the wasp may be tapping into ancestral dispersal behaviours in its host and that a social species provides this wasp an evolutionary advantage by allowing a stable host source.     

The parasitoid wasp Eruga unilabiana Pádua & Sobczak,sp. nov. (Hymenoptera: Ichneumonidae) induces behavioral modification in its spider host

Some polysphinctine parasitoid wasps can alter the web building behavior of their host spiders. In this paper, we describe and illustrate a new species Eruga unilabiana sp. nov. and report for the first time, to the best of our knowledge, the interaction between this parasitic wasp and the linyphiid spider Dubiaranea sp. We investigated the wasp's host selection, development, and manipulation of host behavior. We found that most of the parasitized spiders were intermediate‐sized adult females that probably provide sufficient resources for parasitoid larvae and are less vulnerable for parasitoid females than larger host individuals at attack. The cocoon web of Dubiaranea sp. consists of a complex three‐dimensional tangle structure with several non‐stick radial lines that converge at the cocoon. In addition, E. unilabiana individuals construct their cocoons horizontally, which differ from cocoons of the majority of polysphinctine wasps. This study provides important information and discussion to further understand the evolution of parasitoid wasp–spider interactions.     

Behavioural,ecological and genetic evidence confirm the occurrence of host‐associated differentiation in goldenrod gall‐midges

Host‐associated differentiation (HAD) is considered a step towards ecological speciation and an important mechanism promoting diversification in phytophagous insects. Although the number of documented cases of HAD is increasing, these still represent only a small fraction of species and feeding guilds among phytophagous insects, and most reports are based on a single type of evidence. Here we employ a comprehensive approach to present behavioural, morphological, ecological and genetic evidence for the occurrence of HAD in the gall midge Dasineura folliculi (Diptera: Cecidomyiidae) on two sympatric species of goldenrods (Solidago rugosa and S. gigantea). Controlled experiments revealed assortative mating and strong oviposition fidelity for the natal‐host species. Analysis of mitochondrial DNA showed an amount of genetic divergence between the two host‐associated populations compatible with cryptic species rather than host races. Lower levels of within‐host genetic divergence, gall development and natural‐enemy attack in the S. gigantea population suggest this is the derived host.