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The organization of phytophagous guilds in Cardueae flower heads: Conclusions from null models
Abstract
Rich and diversified assemblages of larval stage phytophagous insects exploit the flower heads of the thistle tribe Cardueae. As these insects form communities within a shared resource unit with well-defined spatial boundaries, they offer a good opportunity to study the guild organization of communities of endophytic insects. We used random combinations of members of phytophagous taxa, which radiated on hosts of the subtribes Centaureinae and Carduinae, as species pools for null models to determine whether and to what extent the composition of the guilds in Cardueae flower heads follows predictable assembly rules. We tested two hypotheses and found that in both cases the null model can be rejected. Assembly rule 1: species assemblages in flower heads follow the rule of intra-generic isolation (i.e. each guild member belongs to a different genus) significantly more often than random combinations of Cardueae insects. We show that, where violations of this rule occur, they are almost exclusively due to the occurrence of a combination of those congeners of the genera Urophora, Larinus and Cera-jocera, whose larval activities differ to some extent temporally and/or spatially. Assembly rule 2: the composition of guilds tends to develop towards a maximum of intra-guild differentiation – the three complementary trophic types (e.g. gall and callus feeders, receptacle and ovary chewers, omnivores and intra-guild predators) co-occur in guilds significantly more often than in random combinations. Our results show that the organization of phytophagous guilds in Cardueae flower heads is mainly a result of invasions due to host shifts. We suggest that in addition to larval competition for space and food, constraints such as the availability of enemy-free space and/or 'free rendezvous arenas' have shaped the structure of the guilds investigated.
... Application of null models to analyse community orga - nisational processes is based on the use of presence / absence data ( Graves & Gotelli , 1993 ; Gotelli , 2000 ; Gotelli & McCabe , 2002 ; Feeley , 2003 ; Va´zquez & Aizen , 2003 ; Zwoïfer & Stadler , 2004 ) , which , however , might be less powerful in detecting competitive relations . This is para - doxical , because population interactions are influenced by abundance , and members of a community can be repre - sented by various frequencies that are not related to inter - actions ( e . ...
... Strengthening the above, Hawkins and Mills (1996) found great variability in guild composition of parasitoid communities reflecting historical events such as chance colonisation, hosts shifts, and specialisation. Application of null models to analyse community organisational processes is based on the use of presence/absence data (Graves & Gotelli, 1993; Gotelli, 2000; Gotelli & McCabe, 2002; Feeley, 2003; Va´zquezVa´zquez & Aizen, 2003; Zwoïfer & Stadler, 2004), which, however, might be less powerful in detecting competitive relations. This is paradoxical , because population interactions are influenced by abundance, and members of a community can be represented by various frequencies that are not related to interactions (e.g. ...
... Hawkins and MacMahon (1989) emphasise that generalist members may often destabilise guilds. Even in a case where competition was found important among phytophagous insect guild members, it was not the sole organising force (Zwoïfer & Stadler, 2004). One of the factors that produces variability in guild composition can be microhabitat requirements of members (Joern & Lawlor, 1981). ...
1. The variability of species combinations and membership assembly patterns in an insect pre-dispersal seed predator guild were studied at various spatial and temporal resolutions using presence/absence and abundance data in null models. The guild consists of seven beetle species (four bruchids and three curculionids) and one moth species that live on a perennial vetch, Vicia tenuifolia Roth (Leguminosae).
2. The seed predator guild varied considerably in the number of members and species combinations in space and time, and, contrary to expectations, there was no evidence of interspecific competition among guild members, supporting the view that other processes, including chance events, could contribute to guild formation.
3. It is concluded that, apart from the possibility of stochastic co-occurrence, it is the narrow host specialisation that constrains seed predator members to participate in the guild, and small differences in habitat preference can also lead to spatial and temporal variation.
... Females of L. sturnus were able to detect the "occupied" capitula and never laid eggs in them (Angermann, 1986, cited after Zwölfer, 1988). Zwölfer and Stadler (2004) noted that females of L. turbinatus also avoided the capitula infested with L. carlinae, but gave no reference to the corresponding field observations or experiments. ...
The following actions performed by females of several Larinus Dej. species during egg laying are described: search of an appropriate place on the plant, making the hole for the egg, oviposition proper, and sealing the hole. The hole preparation takes the longest time and the greatest effort. Only one individual usually completes development in one flower head. Females of Larinus vulpes Ol. prefer larger flower heads for oviposition and occasionally lay eggs into stems. The ability of females to distinguish the flower heads with already laid eggs is discussed. Species of Larinus may be divided into two groups with “early” and “late” oviposition. The evolution of egg laying in the genus Larinus is discussed.
... Females deposit eggs between the florets of the open flower heads by using their stout rostrum to make a small burrow between the single florets and depositing eggs vertically in the burrow (Zwölfer andBrandl 1989, Zwölfer et al. 1971). Larvae feed on seeds and tissue of the flower head (Zwölfer and Stadler 2004). In the Palearctic Region (Russia) (Yegorenkova et al. 2011), females oviposit into inflorescences of Cirsium arvense (L.) Scop. in mid-July. ...
Larinus turbinatus Gyllenhal, a Palearctic weevil that feeds on thistle species of the genera Cirsium and Carduus (Asteraceae: tribe Cardueae), is newly reported for North America. It was first collected by sweep net sampling of flowers of pasture thistle Cirsium pumilum (Nutt.) Spreng. at Fort Indiantown Gap, Lebanon County, Pennsylvania in late July 2011 and specimens were hand-collected from field thistle Cirsium discolor (Muhl. exWilld.) Spreng. at Gettysburg National Park, Adams County, Pennsylvania in late August 2012. This is the first published record of this Old World species in the Western Hemisphere. At present, five species of the genus Larinus are known as introduced into North America. These are L. turbinatus Gyllenhal, L. carlinae (Olivier) (known formerly as L. planus), L. curtus Hochhuth, L. obtusus Gyllenhal, and L. minutus Gyllenhal. Brief summaries of their hosts, distributions, biology and a key for their separation are provided.
... Previous studies have shown that the distribution of endophagous insects e such as gall-formers and leaf miners e is typically aggregated (Bultman and Faeth, 1986;Veech et al., 2003) and when tissues of better nutritional quality are selected, competition between co-occurring insects can arise (Cornelissen and Stiling, 2008). For sessile insects and free-feeding herbivores in general, most of the studies that indirectly addressed interspecific competition by analyzing species co-occurrence have used presenceabsence distributional data as a surrogate for competition (e.g., Stiling et al., 1999;Kagata and Ohgushi, 2001; other examples in Denno et al., 1995), although more recent studies advocate for the use of more refined statistical tests based upon random distributions such as null models (e.g., Gotelli and Graves, 1996;Ribas and Schoereder, 2002;Zwolfer and Stadler, 2004). ...
... Gall formers also have the potential to modify plant quality by physically and/or chemically modifying plant vasculature , architecture and nutritional quality (Johnson, Mayhew, Douglas, & Hartley, 2002), and some studies have demonstrated that galled leaves exhibit higher nutritional quality than neighboring non-galled leaves (e.g., Abrahamson & Weis, 1986, but see Hartley & Lawton, 1992). For sessile insects and free-feeding herbivores in general, most of the studies that indirectly addressed inter-specific competition by analyzing species co-occurrence have used presence–absence distributional data as a surrogate for competition (e.g., Kagata & Ohgushi, 2001; Stiling, Rossi, Catell, & Bowdish, 1999, and examples in Denno et al., 1995), although more recent studies advocate for the use of more refined statistical tests based on random distributions such as ARTICLE IN PRESS null models (e.g., Gotelli & Graves, 1996; Ribas & Schoereder, 2002; Zwoïfer & Stadler, 2004). The purpose of this study was to investigate how differences in plant phenology and nutritional quality influence the distribution and abundance of leaf miners on Quercus geminata (Fagaceae) and Quercus laevis, both between and within plants. ...
Leaf miners typically show non-random distributions both between and within plants. We tested the hypothesis that leaf miners on two oak species were clumped on individual host trees and individual branches and addressed whether clumping was influenced by aspects of plant quality and how clumping and/or interactions with other oak herbivores affected leaf-miner survivorship. Null models were used to test whether oak herbivores and different herbivore guilds co-occur at the plant scale. Twenty individual Quercus geminata plants and 20 Quercus laevis plants were followed over the season for the appearance of leaf miners and other herbivores, and foliar nitrogen, tannin concentration, leaf toughness and leaf water content were evaluated monthly for each individual tree. The survivorship of the most common leaf miners was evaluated by following the fate of marked mines in several combinations that involved intra- and inter-specific associations. We observed that all leaf miners studied were clumped at the plant and branch scale, and the abundance of most leaf-miner species was influenced by plant quality traits. Mines that occurred singly on leaves exhibited significantly higher survivorship than double and triple mines and leaves that contained a mine or a leaf gall and a mine and damage by chewers exhibited lowest survivorship. Although leaf miners were clumped at individual host trees, null model analyses indicated that oak herbivores do not co-occur significantly less than expected by chance and there was no evidence for biological mechanisms such as inter-specific competition determining community structure at the plant scale. Thus, despite co-occurrence resulting in reduced survivorship at the leaf scale, such competition was not strong enough to structure separation of these oak herbivore communities.
... Gall-formers also have the potential to modify plant quality by physically and/or chemically modifying plant vasculature, architecture and nutritional quality (Johnson, Mayhew, Douglas, & Hartley, 2002), and some studies have demonstrated that galled leaves exhibit higher nutritional quality than neighboring nongalled leaves (e.g., Abrahamson & Weis, 1986, but see Hartley & Lawton, 1992. For sessile insects and free-feeding herbivores in general, most of the studies that indirectly addressed interspecific competition by analyzing species co-occurrence have used presence-absence distributional data as a surrogate for competition (e.g., Stiling, Rossi, Catell, & Bowdish, 1999;Kagata & Ohgushi, 2001, andexamples in Denno et al., 1995), although more recent studies advocate for the use of more refined statistical tests based on random distributions such as null models (e.g., Gotelli & Graves, 1996;Ribas & Schoereder, 2002;Zwolfer & Stadler, 2004). ...
This study investigated effects of plant quality and natural enemies on the abundance and survivorship of several leaf miner species on Florida scrub oaks over several ecological scales. Three oak species (Quercus laevis, Q. geminata, and Q. myrtifolid) and four leafminer species (Acrocercops albinatella, Brachys tesselatus, Stilbosis quadripustulatus, and Cameraria sp. nova) were the main focus of five separate studies, addressing effects of bottom-up and top-down factors at regional, local, and individual scales. At the regional scale, it was observed that Cameraria sp. nova was aggregated into sites, and sites closer to each other exhibited similar densities of mines than sites farther apart. None of the bottom-up and top-down factors studied were spatially structured, but did influence the variation in Cameraria abundance over the range of the host plant Q. myrtifolia. At the local scale, all leaf miners studied were aggregated between and within plants, and variation in bottom-up factors among individual plants explained variation in abundance for some of the leaf miners studied. Intra-specific competition was identified as an important factor influencing mine survivorship, but inter-specific competition among leaf miners and gall-formers did not shape the community structure of oak herbivores. Experimental manipulation of bottom-up and top-down factors via fertilization and natural enemy removal showed that bottom-up effects were important determinants of leaf miner abundance, as fertilized plants supported 2 to 5-fold more herbivores than control plants. The removal of natural enemies, on the other hand, did not significantly impact the abundance and/or the survivorship of leaf miners and other guilds studied. At individual scales, it was demonstrated that two leaf miner species responded to random variations in leaf morphology, by increasing in abundance in individual host plants with more asymmetric leaves and/or higher levels of fluctuating asymmetry. These results offered support for the plant stress hypothesis and differences in host plant quality were again partially responsible for the results found.
New observations on biology, ecology, host plants and parasitoids of four Larinus species are presented. Centaurea phrygia L. is confirmed as a host plant of Larinus obtusus Gyllenhal, 1835. The east Polish populations of Larinus obtusus were confirmed to be regionally monophagous on Centaurea stoebe L. and those of Larinus pollinis (Laicharting, 1781) on Carlina vulgaris L. Carduus crispus L. is identified for the first time as a host plant of Larinus sturnus (Schaller, 1783). Cirsium oleraceum (L.) Scop. and C. vulgare (Savi) Ten. were confirmed as host plants of Larinus sturnus, and Cirsium heterophyllum (L.) Hill as a host plant of Larinus turbinatus Gyllenhal, 1835. Life strategies including type of oviposition and development of these weevils are described and photographs are provided of immature stages, adults, host plants, biotopes and parasitoids.
Assuming that differences or similarities in morphology among congeneric parasite species living in the same habitat are not a random pattern, several hypotheses explaining morphological differences were tested: (i) reproductive isolation, (ii) niche restriction resulting from competition, and (iii) niche specialization. Congeneric monogenean (platyhelminth) ectoparasites parasitizing the gills of one host species were used as an ecological model. Morphometric distances of the attachment organ and morphometric distances of the copulatory organ between species pairs were calculated, Levin’s niche size and Renkonen niche overlap indices were applied. Our results support the prediction that the function of niche segregation is to achieve reproductive isolation of related species in order to prevent hybridization (reinforcement of reproductive barriers). Parasite species living in the same niche differ greatly in the size of copulatory organ. Moreover, species coexistence is facilitated by an increase in morphometric distances of copulatory organ and niche centre distances. Our results also show that species living in overlapping niches have similar attachment organs, which supports the prediction that morphologically similar species have the same ecological requirements within one host and suggests small effects of interspecific competition for the evolution of morphological diversity of attachment organs. Specialist adaptations also seem to facilitate species coexistence and affect the niche distribution within host species. Parasite species that can colonize more than one host species, i.e. generalists, occupy more distant niches within host species than strictly host-specific parasites. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76, 125–135.
The extent to which insect communities are organized is poorly understood because few studies have examined both population-level processes and community-level patterns. Furthermore, our view of phytophagous insect populations is probably biased by the more frequent attention given to economic pests whose dynamics might be expected to be somewhat exceptional. Herein, we report on the population dynamics and community-level features in a diverse, native fauna of phytophagous insects associated with goldenrod (Solidago altissima), a native perennial plant that forms a dominant, long-persisting element in old-field succession. The data consist of censuses taken in six consecutive years at 16 old fields in the Finger Lakes Region of New York; five additional stands were followed for four consecutive years. Our analyses address two questions: (a) to what extent is the functional structure of the community (as reflected in such attributes to the herbivore load, guild spectrum, and dominance hierarchy) maintained by compensatory changes in the densities of the member species and (b) are certain traits associated with a species' tendency to fluctuate in density or to dominate the community? In general, populations of goldenrod insects fluctuated less than those of insects reported in the literature. Few were abundant; only 7 of the 138 species in the goldenrod fauna ever reached densities at which their biomass exceeded 0.1% of the leaf biomass. The functional structure of the community, as reflected in the total herbivore load and the proportions of that load contributed by various guilds, varied widely in space and time. A small subset of the fauna remained dominant throughout the study; these species, whose populations occasionally irrupted, retained their high rank in relative abundance even during those periods when their populations were waning. There was no evidence for compensatory changes in the densities of species within guilds, as would be expected if a relatively distinct set of limiting resources were available to insects with different feeding styles. Species were no more likely to exhibit negative correlations with their guild mates than they were with members of other guilds. There was also little evidence of sporadic or diffuse competition within the fauna as a whole; even during population outbreaks, the dominant species rarely engendered decreases in the biomass or diversity of the remaining species, and increases in the total herbivore load were seldom associated with losses of species from the community. Several species were positively associated with one another on the basis of similar habitat requirements or the use by one species of conditions created by the presence of another. The community was predictable only in the sense that dominant species remained dominant and rare species rare. The abundance and population variability of species were not correlated with such traits as body size, generation time, or host range. Taxon-related trends, on the other hand, stood out; coleopterans tended to be abundant whereas lepidopterans were rare, and hemipterans tended to fluctuate widely whereas dipterans remained relatively constant. Furthermore, species that were clumped in space (onto stems within fields) tended to be abundant and variable. The "boom and bust" dynamics of these aggregating species may be related to the ability of colonies to survive the inroads of generalist predators. The low degree of regional synchrony in the performance of populations at different sites suggests that large-scale, weather-driven fluctuations are uncommon. Our analysis of this well-developed, native insect fauna reveals a community that has a somewhat predictable structure that reflects the idiosyncracies of the component species. These species do not fluctuate in an integrated community-structuring manner, but as an outcome of each species performing at its "individualistic" level, the community displays a characteristic dominance hierarchy. The members of this diverse fauna rarely achieve densities at which they compete with one another. Our results are consistent with Hairston, Smith, and Slobodkin's hypothesis (1960) which predicts that terrestrial herbivores rarely deplete plant resources to levels at which there is widespread interspecific competition.
Biotypes within the Insecta are most commonly entomophagous or phytophagous parasites or parasitoids distinguished by survival and development on a particular host or by host preferences for feeding, oviposition, or both. Other insect biotypes differ in diurnal or seasonal activity patterns, size, shape, colour, insecticide resistance, migration and dispersal tendencies, pheromone differences or disease vector capacities. This paper aims to integrate the current concepts used by evolutionary biologists to describe various patterns and levels of differentiation in insects with the views of the applied biologist. A classification system of insect biotypes, based on the mechanisms underlying their differentiation, is given. The significance of biotypes in adaptation, speciation and pest management is also reviewed.-from Authors
The successful colonization of novel host-plant species by herbivorous insects may be facilitated by a reduction in natural-enemy attack on insect populations associated with the novel (derived) host plant. This is particularly true if natural enemies use host-plant or habitat cues in searching for their herbivore prey. In order to test whether the acquisition of enemy-free space could have influenced the host shift in the goldenrod ball gallmaker, Eurosta solidaginis, we estimated levels of natural-enemy attack in 25 host-race populations associated with Solidago altissima and S. gigantea (Compositae) spanning the zone of sympatry between S. altissima and S. gigantea host races in New England. Mortality due to attack by the parasitoid wasp Eurytoma obtusiventris was significantly higher for the ancestral than for the derived host race (30.5% versus 0.4%) across the transect, which is consistent with the enemy escape hypothesis. Contrary to this hypothesis, mordellid beetles caused significantly higher mortality on the derived than ancestral host race (17.1% versus 2.6%). Mortality by a second parasitoid wasp and birds showed no significant differences between the two host races. Overall, the derived host race had significantly higher survivorship across the transect (36.6% versus 20.8%). An analysis of survivorship and parasitoid mortality levels from sympatric sites in this study and previous studies showed a highly significant correlation between the levels of Eurytoma obtusiventris attack and the survivorship advantage of the derived host race. Observations of this parasitoid's searching behavior confirmed that it preferentially searches the ancestral host for fly larvae. Current patterns of host-race mortality and naturalenemy behavior and abundance are consistent with the facilitation of the host shift by escape from a specialist parasitoid.
We define 'enemy free space' as ways of living that reduce or eliminate a species' vulnerability to one or more species of natural enemies. Many aspects of species' niches, in ecological and evolutionary time have apparently been moulded by interactions with natural enemies for enemy free space. We review a large number of examples. Yet many ecologists continue to think and write as though classical resource based competition for food or space is the primary determinant of species' niches. Often it is not. The recognition that the struggle for enemy free space is an important component of many species' ecologies may have important consequences for studies of community convergence, limits to species packing, and the ratio of predator species to prey species in natural communities. © 1984.
Deals with 31 species of braconids (Braconinae, Rogadinae, Cheloninae, Microgasterinae, Agathidinae, Macrocentrinae, Helconinae and Alysiinae) reared from hosts in flower heads, roots or on leaves of 9 genera of Cynaroideae host plants. The 16 members of the ectoparasitic subfamily Braconinae are "niche-specific' idiobionts, whereas the 15 species of the remaining subfamilies are all endoparasitic and represent "taxon-specific' koinobionts. -from Authors
Symphagy, as used in this chapter, is the collective sharing of flower heads of a single host-plant species of Asteraceae by more than one species from one or more genera of nonfrugivorous fruit flies (Diptera: Tephritidae) (Goeden 1987, 1989, 1992, 1993, 1994). The manner in which this symphagy is achieved, e.g., whether by spatial or temporal partitionings of the resource that a host-species’ flower heads collectively represent (Zwölfer 1987, 1988; Zwölfer and Arnold-Rinehart 1993), is beyond the scope of this chapter and best examined at the individual tephritid and host plant species level (cf. Zwölfer 1965, 1982, 1987, 1988, 1990; Goeden and Ricker 1986, 1987; Headrick and Goeden 1990; Goeden and Headrick 1991, 1992; Goeden et al. 1994). This chapter describes and analyzes 14 years of data on symphagy among 15 genera of florivorous Tephritidae and 12 tribes of Asteraceae in southern California. This is the first comprehensive analysis of symphagy by a multigeneric grouping of geographically defined, nonfrugivorous, Nearctic fruit flies, and was inspired in part by Helmut Zwölfer’s comprehensive, long-term field and laboratory studies of the biology, ecology, and evolution of Tephritidae, and other Palearctic insect associates of asteraceous thistles and knapweeds (Zwölfer 1965, 1982, 1988, 1990). I dedicate this analysis of original data as a token of my admiration, respect, and friendship for Helmut on the occasion of his recent retirement from the Faculty of the University of Bayreuth.
An analysis of ecosystems can follow two different Unes of research by concentrating its efforts either on the functions or on the structures of the system. In the first case the analysis will deal with the interaction between the components of the system and their environment and it will attempt to predict the behavior of the system under defined conditions. The second approach, which is the domain of community ecology as defined by Diamond and Case (1986), gives emphasis to an explanation of community structure and to predictions concerning structural parameters of the system. Whereas the analysis of the functions of an ecosystem investigates proximate factors and processes, the analysis of community structure has also to consider ultimate factors and processes which have adapted the components and integrated them into systems. Thus, community ecology asks whether ecosystems are organized in a predictable way and which processes have shaped them. In this contribution predictability of species richness, a particular structural aspect of ecological systems, will be investigated.
Investigating entire natural communities is a formidable task because one must take a high diversity of species into account. To focus on a more manageable unit, ecologists usually restrict their attention to some portion of the larger system, such as the “plant” or “bird” community. Such “taxon”-defined communities, however, are rather unwieldy, and they may contain a heterogeneous mix of interactions. Thus, most beetle communities contain predators, herbivores, and scavengers but ignore the bugs, caterpillars, flies, and gastropods that share many of the beetles' resources. The guild concept, which groups species according to the manner in which they exploit a common resource, provides a manageable, functional unit for studying patterns of adaptation and the organization of natural communities. The existence of species with mixed requirements and different evolutionary histories, however, makes it difficult to determine standard procedures for defining guilds. The indefinite boundaries that surround most guilds are a fundamental outcome of evolution, which is also at play in all of our attempts to group species into communities, trophic levels, and other ecological categories. Discovering how to draw valid conclusions from entities with vague boundaries is a special challenge to ecologists and provides us with a quest of broad significance.
The species of the tephritid genus Urophora differ greatly in the structure and function of the galls, which they induce on their Cardueae host plants. As this suggests the existence of trade-offs, we studied the benefits and costs of Urophora galls of different complexity using U. quadrifasciata, an oligophagous species with a primitive achene gall, and U. jaceana, a monophagous species with a complex multilocular ovariole-receptacle gall. Both Urophora species share Centaurea jacea as host plant and may co-exist in the same host populations without any sign of interspecific competition. Therefore, C. jacea was used in our tests as the standard host plant. We examined life-history data of the two species and tested the hypothesis that they respond in different ways to differences in host quality. Oviposition behaviour and larval development were studied on unfertilized and well-fertilized test plants grown under standardized nutrient regimes. The females of the specialist tephritid species, U jaceana, had a much higher egg load, higher oviposition decisiveness, reduced time costs per deposited egg and they were able to select and adjust clutch size to the quality of the host plant. This oviposition behaviour maximized reproductive fitness, as even high larval densities in galls on well-fertilized hosts had no negative effect on larval weight. In addition, larvae in well-fertilized buds developed more biomass and produced females with a higher egg potential. Females of the generalist species, U. quadrifasciata, did not respond to differences in host quality. They could use flower heads during a broader temporal window, but had a lower reproductive reward per handling time and the larvae extracted much less energy and nutrients from the host. However, their development was distinctly faster, making a second generation per annum possible. Moreover, U. quadrifasciata has a much broader host range and is better adapted for long-distance dispersal, which has made it a very successful pioneer species in North America. The comparison of the two Urophora species shows that the evolution of a refined oviposition behaviour and the formation of a complex gall, which allows the import of nutrients via a newly formed vascular system, greatly increases the energy and nutrient rewards, as long as stable host populations can be exploited. On the other hand, the advanced gall type of Urophora involves higher costs of developmental time, univoltine generations and a high dependence on a specific host plant.
Outlines features of the evolutionary history of insect-Cardueae associations and compares guild structures in connection with current theory on insect-plant systems. Sections are on: thistles as host plants of insects; insects as biogeographical tracers of the Cynaroideae; evolutionary aspects (stressing the importance of preadapted species pools, infraspecific evolution of thistle insects, and adaptations of phytophagous insects to thistle hosts); and guild structure in thistle insects (insects capable of an early aggregated attack being able to achieve high levels of resource exploitation). -P.J.Jarvis
(1) The distribution of two gall flies, Urophora affinis and U. quadrifasciata, among flower heads of diffuse and spotted knapweed was determined during a population explosion following their introduction to Canada as biological control agents. (2) The distribution of Urophora quadrifaseiata was more clumped than that of U. affinis and galls of both flies had more clumped distributions in seed heads of spotted knapweed, which has larger heads and more synchronous bud development than diffuse knapweed. Similar patterns are observed in European populations. (3) On diffuse knapweed Urophora quadrifasciata galls were more randomly distributed in peak populations and U. affinis galls have been more contagiously distributed since the post-introduction peak. (4) Apparently the equilibrium density of flies is not maintained through adjustment of ovipositional behaviour, but is more likely mediated through the response of the plant to fly attack, and by environmental factors which influence the phenology of bud development and fly emergence. (5) The introduction of these two closely related flies has led to greater seed destruction than would have been achieved by either species alone.
Metzneria paucipunctella Zeller is a seed head moth introduced for biological control of spotted knapweed, Centaurea maculosa Lamarck, in the Pacific Northwest. A cage study was conducted in Montana in 1987 and 1988 to assess the feeding strategy of the M. paucipunctella larva and its impact on two seed head flies, Urophora spp., also introduced on spotted knapweed. Each moth larva destroyed an average of 8.13 knapweed seeds. Most of the seeds were destroyed prior to seed dispersal in early September but the remaining seeds were attacked in the spring of the following year. Each larva tied an average of 7.0 seeds with silk webbing which allowed for feeding long after normal seed dispersal. Seed survival in cages containing both the moth and the two fly species (4.71 seeds per head) was significantly less than in fly-only cages (9.75 seeds per head). The moth larvae caused extensive mortality to the two Urophora species, especially during the spring months. Moth-caused mortality to U. affinis larvae increased from 19% during June through October to 67% in May of the following year. Mortality to U. quadrifasciata also occurred but did not increase significantly in the spring. Data from 10 field sites showed very little association between M. paucipunctella and the two Urophora species which suggests that the unnatural conditions associated with the cages did not increase the incidence of joint moth and fly attack to individual seed heads. The moth is a valuable biocontrol agent as it is contributing significantly to the overall consumer pressure on spotted knapweed seed heads.
Intrinsic and extrinsic factors responsible for niche restriction in parasites and particularly in Monogenea of fish are discussed. Interspecific competition may result in competitive exclusion of one or several species, or it may lead to a change in the microhabitat of one or all co-occurring species (interactive site segregation), but there is no evidence that such effects lead to evolutionary changes and avoidance of competition, i.e., to selective site segregation. That extrinsic factors, particularly competition, are not of such great evolutionary significance among parasites as is usually assumed, is indicated by the following observations. Most effects of competitive exclusion may also be caused by intraspecific crowding. Interspecific effects may also be positive, enhancing the chances of species co-occurring. Host-mediated effects are usually more harmful to the species evoking them than to competing species. Parasite species with coinciding or overlapping microhabitats often show no interactions...
Phytophagous insects representing six orders, 26 families, 49 genera, and 59 species are reported as composing the insect fauna of Cirsium californicum Gray (Asteraceae) in southern California. Similarly, phytophagous insects representing six orders, 20 families, 29 genera, and 31 species are reported from C. proteanum J. T. Howell. Most insects feeding on these thistles are polyphagous, ectophagous, sap- and foliage-feeding species. Ten (17%) of 58 species assessed for host specificity on C. californicum are stenophagous (i.e., their host plants restricted at least to Cynareae); 7 (23%) of 31 species on C. proteanum are stenophagous. The six oligophagous species (i.e., only attacking Carduinae) on C. californicum and four oligophagous species on C. proteanum include the flower head-infesting weevil Rhinocyllus conicus (Froelich), introduced and established for the biological control of alien, asteraceous thistles of European origin. Species packing within flower heads of both thistles averaged approximately two (range, 0-4) native insect species. Species packing in or on shoots of C. californicum averaged 10.5 ± 0.9 ( x ¯ ± SEM) species; for C. proteanum, 8.4 ± 0.8 species.
Phytophagous insects representing seven orders, 19 families, 25 genera, and 30 species compose the insect fauna on Cirsium mohavense (Green) Petrak (Asteraceae) in southern California. Similarly, phytophagous insects representing six orders, 17 families, 18 genera, and 23 species from Cirsium neomexicanum Gray, and insects representing six orders, 11 families, 12 genera, and 12 species from Cirsium nidulum (Jones) Petrak, are reported. Most insects feeding on these thistles are polyphagous, ectophagous, sap- and foliage-feeding species. Few or no insect species or individuals were reared from within the flower heads, stems, crowns, and roots of these desert thistles. Like other native Cirsium thistles surveyed in southern California and discussed here, the food niches represented by this natural assemblage of host plants are not saturated with phytophagous insect species. Significance of these underused resources and apparently “empty food niches for biological control of weedy thistles in North America is discussed. Some members of the insect faunas on Cirsium thistles have markedly broader feeding niches than their counterparts in Europe, compensating for this host-plant underuse. Taxonomic compositions and niche characteristics of phytophagous insect faunas on native thistles in Europe and southern California are compared and discussed in evolutionary terms.
The presence of a host refuge stabilises the Nicholson-Bailey model.
The seed-head gall flies Urophora affinis and U. quadrifasciata create powerful metabolic sinks in both Centaurea maculosa and C. diffusa that extends beyond the attacked heads to other parts of the plant growing at the time. Thus, seed production is affected in attacked as well as unattacked heads and the weight of individual larvae in a head is reduced little by crowding. It is suggested that the sink effect of the galls also restricts the Urophora larval population to the number that can be adequately nourished by the plant: the diversion of the plant's resources to the galls restricts the development of additional flower buds which abort and the newly hatched larvae in them perish. Seed reduction, as a result of the flies on the main C. diffusa stand studied, is estimated to be from around 25,000 seeds/m2 to 1,500 seeds/m2.
Wirkungen von Urophora affinis und U. quadrifasciata auf Centaurea maculosa und C. diffusa
Die Distelblüten-Gallfliegen U. affinis und U. quadrifasciata bewirken eine erhebliche Schwächung bei den Disteln C. maculosa und C. diffusa, die außer den befallenen Blütenkopfen auch andere Teile der Pflanze betrifft. So wird die Samenproduktion in befallenen wie unbefallenen Blütenköpfen einer Pflanze beeinflußt. Das Gewicht der einzelnen Larve hängt nur wenig von der Larvendichte ab. Es wird angenommen, daß der Schwächungseffekt durch die Gallen auch die Larven-Population der Gallfliege vermindert bis auf eine Zahl, die von der Pflanze ernährt werden kann. Die Samenproduktion ging bei den untersuchten Pflanzen von C. diffusa als Folge des Gallfliegen-Befalls von 25 000 Samen/m2 auf 1500 Samen/m2 zurück.
The ecological meaning of size relationships within guilds is still a matter of debate. We analyzed the niches and size relationships in Coleoptera associated with Cardueae host plants. Species were grouped into guilds using distributional data, host records and feeding strategies: a) The species of the genus Larinus are inhabitants of flower heads within the Cynaroideae. Two types of Larinus species were distinguished: one type attacks immature flower heads, the other exploits the floer heads in a more advanced stage. The females of the first group have elongated rostres adapted to piercing through the bracts of closed flower heads, the other group possesses blunt, short rostres. For an oligophagous group of four Larinus species we are able to show that the distribution of average female rostre length is non-random. b) In southern France four stem boring species of the genera Agapanthia and Lixus coexist within the same hosts. The frequency distributions of body length from these species are clearly overdispersed. c) Coexisting species of the folivorous genus Cassida show no differences in body sizes. We conclude that morphometric differences within the investigated guild of endophytic species (Larinus and Agapanthia/Lixus) evolved in response to size of the used plant structures and the size of potentially competing species, a pattern not evident in ectophytic species. We suggest that these differences are part of a general pattern as the evolution of herbivorous guild may strongly depend on the way how the host resource is exploited (endophagy vs ectophagy). So current differences in statements on the organization of herbivore communities could perhaps be reconciled.
Intra- and interspecific resource competition are potentially important factors affecting host plant use by phytophagous insects. In particular, escape from competitors could mediate a successful host shift by compensating for decreased feeding performance on a new plant. Here, we examine the question of host plant-dependent competition for apple (Malus pumila)- and hawthorn (Crataegus mollis)-infesting larvae of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae) at a field site near Grant, Michigan, USA. Interspecific competition from tortricid (Cydia pomonella, Grapholita prunivora, and Grapholita packardi) and agonoxenid (subfamily Blastodacninae) caterpillars and a curculionid weevil (Conotrachelus crataegi) was much stronger for R. pomonella larvae infesting the ancestral host hawthorn than the derived host apple. Egg to pupal survivorship was estimated as 52.8% for fly larvae infesting hawthorn fruit without caterpillars and weevils compared to only 27.3% for larvae in harthorns with interspecific insects. Survivorship was essentially the same between fly larvae infesting apples in the presence (44.8%) or absence (42.6%) of interspecific insects. Intraspecific competition among maggots was also stronger in hawthorns than apples. The order or time that a larva exited a hawthorn fruit was a significant determinant of its pupal mass, with earlier emerging larvae being heavier than later emerging larvae. This was not the case for larvae in apples, as the order or time that a larva exited an apple fruit had relatively little influence on its pupal mass. Our findings suggest that decreased performance related to host plant chemistry/nutrition may restrict host range expansion and race formation in R. pomonella to those plants where biotic/ecological factors (i.e. escape from competitors and parasitoids) adequately balance the survivorship equation. This balance permits stable fly populations to persist on novel plants, setting the stage for the evolution of host specialization under certain mitigating conditions (e.g. when mating is host specific and host-associated fitness trade-offs exist).
Sympatric speciation has become increasingly accepted in the past decade, as a result of new models substantiating its plausibility and new evidence that the conditions specified by the models are met in many natural populations. Retrospective phylogenetic and population genetic signatures of sympatric speciation have also been derived, and these are beginning to be tested. This new work has helped increase the acceptance of sympatric speciation as a plausible process, although it remains difficult to show conclusively that specific pairs of taxa have speciated through sympatric processes alone. It might be time for a re-evaluation of the geographical classification of speciation modes in favor of one based primarily on evolutionary mechanisms
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Note pour servir a l’histoire des insectes qui vivent dans le chardon penché
- C C Goureau
Goureau, C.C. 1845. Note pour servir a l’histoire des insectes qui vivent dans le chardon penché. Ann. Soc. Ent. France, 14: 75–102
Patterns in the parasitoid complex of the genus Isocolus Förster (Hym. Cynipidae)
- T Baumann
- S Vidal
Baumann, T. and Vidal, S. 1992. Patterns in the parasitoid complex of the genus Isocolus Förster (Hym. Cynipidae). Zool. Anzeiger, 229: 1–12
Insetti del Carduus nutans L. III. Euribia solstitialis L. (Diptera Trypetidae)
- E Mellini
Mellini, E. 1952. Insetti del Carduus nutans L. III. Euribia solstitialis L. (Diptera Trypetidae). Boll. Entom. Bologna, 19: 96–119
Evolution of Gall Forming Insects – Gall Midges UK: The British Library
- B M Mamaev
Mamaev, B.M. 1975. Evolution of Gall Forming Insects – Gall Midges. Boston Spa, UK: The British Library, Lending Division
Braconids associated with insects inhabiting thistles Insect herbivore communities colonising the flower-heads of Berkheya in South Africa and Carduoideae in Europe and California. Doctoral dissertation
- M Capek
- H Zwölfer
Capek, M. and Zwölfer, H. 1990. Braconids associated with insects inhabiting thistles. Acta Entomol. Bohemoslavica, 87: 262–277 Clark, M.M. 1989. Insect herbivore communities colonising the flower-heads of Berkheya in South Africa and Carduoideae in Europe and California. Doctoral dissertation, University of Cape Town, Department of Zoology
