Gall Density Research Articles

Gall‐forming and free‐feeding herbivory along vertical gradients in a lowland tropical rainforest: the importance of leaf sclerophylly

In contrast to most insect guilds, gall‐forming insects are thought to reach highest diversity on sclerophyllous vegetation, such as Neotropical savannas and Mediterranean vegetation types. The water and nutrient stress endured by meristems of canopy trees in tall wet tropical rainforests may cause leaf sclerophylly. Hence, the upper canopies of such ecosystems may represent a suitable habitat for gall‐forming insects. At the San Lorenzo Protected Area, Panama, we estimated free‐feeding herbivory and gall densities within five sites in 2003 and 2004, by surveying leaves in vertical and horizontal transects. In each sample, we recorded leaf density (mature and young foliage), free‐feeding herbivore damage and number of galls, including the presence of live larvae, parasitoids or fungi. We surveyed 43 994 leaves, including 231 plants and 73 tree and liana species. We collected 5014 galls from 17 host‐plant species, including 32 gall species of which 59% were restricted to the canopy (overall infestation rates: 2.4% in 2003, 5.5% in 2004). In 2003, 16% of the galls were occupied by live larvae, against 5% in 2004. About 17–20% of leaves surveyed suffered from free‐feeding herbivory. Leaf sclerophylly increased significantly with sampling height, while free‐feeding herbivory decreased inversely. Conversely, the number of live galls collected in the canopy was 13–16 times higher than in the understorey, a pattern consistent across sites and years. Hence, the probability of gall survivorship increased with increasing leaf sclerophylly as death by fungi, parasitoids or accidental chewing were greater in the understorey. Increasing harsh ecophysiological conditions towards the upper canopy appear favourable to galls‐forming population maintenance, in support of the hypothesis of harsh environment. Hence, gall diversity and abundance in the upper canopy of tall tropical forests are perhaps among the highest in the world.

Variable responses of natural enemies to Salix triandra phenotypes with different secondary chemistry

Plant phenotypes often differ in their resistance to natural enemies, but the mechanism for this has seldom been identified. The aim of this study was to determine if the spatial patterns of phenotype use of a highly specialized insect herbivore (the galling sawfly Pontania triandrae) in a natural willow population can be related to phenotypic variation in plant secondary chemistry. Furthermore, we tested if traits that confer resistance to one type of natural enemy, i.e. the galling sawfly, also confer resistance to others, in our case a leaf beetle Gonioctena linnaeana and the rust fungus Melampsora amygdalinae. We identified 18 phenotypes with high and 18 phenotypes with low gall density in our field population and determined gall densities, the degree of leaf damage and rust infection on each phenotype and collected leaves for chemical analyses. The concentration of phenolics was higher in phenotypes with high density of galls suggesting that this galling sawfly may use phenolics as oviposition cues. Rust infection showed the opposite pattern, with lower levels on clones with high concentration of phenolics, while leaf damage by G. linnaeana did not differ between clone types. This indicates that these important natural enemies may assert divergent selection on willow phenotypes and that this might provide a mechanism for maintaining phenotypic variation within willow populations.

Eurytoma robusta (Hymenoptera: Eurytomidae), a local key factor in the population dynamics of Urophora cardui (Diptera: Tephritidae): a comparative analysis

Eurytoma robusta Mayr (Chalcidoidea) exploits host galls either as a primary or secondary parasitoid, an entomophytophagous inquiline or occasionally even as a predator. We present data on its ecology and impact on gall densities and population trends of the gall fly Urophora cardui (L.) on Cirsium arvense (L.) Scop. Habitat preference, host gall selection, clutch size, and high incidence of superparasitism causing empty gall cells show that E. robusta, a generalist with a broad host spectrum, is relatively poorly adapted to parasitising U. cardui. The influence of E. robusta on U. cardui in the Belfort-Sundgau region (1970-2004), in the Upper Rhine Valley (1973-2004) and in north-eastern Bavaria (1977-2004), differed considerably. In the forests of the Upper Rhine Valley and the Belfort-Sundgau region, where U. cardui has relatively stable source-sink populations, E. robusta is present but not the dominant mortality factor of the gall fly. In most areas of north-eastern Bavaria U. cardui occurs in fragmented populations and short lived non-equilibrium metapopulations. In these systems E. robusta became more abundant over the last five years, which resulted in a high incidence of superparasitism, an increase in the number of empty gall cells and reduced gall quality. The greatly increased degree of parasitism and an excess of empty cells resulted recently in the collapse of most local populations of U. cardui in the study area south of Bayreuth (north-eastern Bavaria). Together with earlier records the data presented here suggest that in north-eastern Bavaria E. robusta cause fluctuations in the abundance of U. cardui, which have a periodicity of 5-7 years. A remarkable feature of the oligophagous E. robusta is its high fidelity to formerly abundant U. cardui populations, which, with declining host densities, leads to overexploitation, resulting in a high incidence of superparasitism and high larval mortality. The possible influence of the habitat structure on the effect of E. robusta on the population dynamics of U. cardui is discussed. Our data plus that of other authors suggest that, with regard to U. cardui, E. robusta can develop a temporary local host specialisation.

GALL FORMATION ON THE DIFFERENT FORMS OF SKELETON WEED BY CYSTIPHORA SCHMIDTI RÜBSAAMEN (DIPTERA: CECIDOMYIIDAE)

Cystiphora schmidti produced similar numbers and densities of galls on the 3 Australian forms of its host weed Chondrilla juncea. The total number of galls increased with available leaf area. Density of galls was similar on all leaf areas offered. Low temperature significantly reduced the number and density of galls on all forms of the weed.

Biology, revised taxonomy and impact on host plants ofOphelimus maskelli, an invasive gall inducer onEucalyptus spp. in the Mediterranean Area

Ophelimus maskelli (Ashmead) (Hymenoptera: Eulophidae), an invasive species in the Mediterranean area that occurs naturally onEucalyptus camaldulensis at Wagga Wagga in New South Wales, was studied in Israel on the same host plant. The most distinctive characteristic ofO. maskelli as compared with other congeners is the presence of only a single seta on the submarginal vein.O. maskelli shows a clear tendency to oviposit in developed, immature leaves, and it prefers to oviposit on an area of the leaf blade near the petiole. The female lays an average of 109 eggs when maintained on water alone. The egglaying distribution tended towards aggregation, and the same tendency has also been observed within and among trees. The wasp prefers to attack the lower canopy. The gall diameter ranged from 1.2 to 0.9 mm, and gall density from 11.5 to 36.0 galls cm−2, respectively. The typical color of the gall appeared as soon as third instar larva developed. Galls on shaded leaves remained green or greenish-yellow, whereas those that were exposed to the sun were reddish.O. maskelli produces three generations per year in Israel. High flight occurs among the spring generations, starting in early March. The wasps were attracted to green sticky plates much more than to the other tested colors. Heavy galling byO. maskelli results in premature shedding of the leaves, soon after the emergence of the wasps. Among the 84 eucalyptus species tested, the following 14 species were found to be suitable hosts:E. botryoides, E. bridgesiana, E. camaldulensis, E. cinerea, E. globulus, E. gunii, E. nicholii, E. pulverulenta, E. robusta, E. rudis, E. saligna, E. tereticornis andE. viminalis. This study focused on morphological and development characters that can clearly distinguishO. maskelli fromOphelimus eucalypti Gahan, and on examining the effect of crossing between eucalyptus species on the development possibilities of the wasps. Also considered were the effect of the food on adult survival, the competition between the eulophid gall makerLeptocybe invasa Fisher & LaSalle andO. maskelli, the heavy damage inflicted on eucalyptus, and the health problems and nuisance to people caused by the wasp near heavily infested trees during the mass emergence of adults.

BOTTOM-UP AND TOP-DOWN EFFECTS ON INSECT HERBIVORES DO NOT VARY AMONG SITES OF DIFFERENT SALINITY

It has been suggested, but rarely tested, that the relative strength of top-down and bottom-up factors in communities varies along an environmental stress gradient. We compared the strength of bottom-up and top-down effects on the densities of insect herbivores along a range of sites of different salinities in west-central Florida. We used a 2 x 2 factorial design with plots divided into four treatments: (1) bottom-up manipulation, where fertilizer was applied to increase plant quality; (2) top-down manipulation, where sticky traps were used to reduce the effects of natural enemies (parasitoids); (3) bottom-up and top-down manipulation, where fertilizer was applied and sticky traps were used; and (4) control plots. These plots were established along a range of salinities among seven different sites containing the salt marsh plant Borrichia frutescens. In each plot, we determined the parasitism levels and abundances of the sap sucker Pissonotus quadripustulatus, the gall maker Asphondylia borrichiae, and the lepidopteran stem borer Argyresthia spp. Gall density, Pissonotus density, and stem borer density were significantly higher in lower salinity sites, suggesting a strong effect of environmental stress. There was a significant increase of galls and Pissonotus and a marginally significant increase of bored stems on fertilized plots but not on trapped plots. There was a significant interaction of site and fertilizer on gall parasitism. There were no interactions of either treatment with salinity on herbivore densities. The general lack of interaction between salinity level and other treatments on herbivore densities contrasts with our previous result where treatment effects did vary with salinity level on a large experimentally generated salinity gradient at one site. Thus, the results of the present paper suggest that, while environmental stress can modify top-down and bottom-up effects on herbivores at single sites, variation in site-to-site factors, possibly including clonal identity of plant, affects herbivore densities so much as to swamp out any observable interaction between environmental stress and top-down or bottom-up factors.

Distribution and frequency of galls induced by Anisodiplosis waltheriae Maia (Diptera: Cecidomyiidae) on the invasive plant Waltheria indica L. (Sterculiaceae)

The frequency of galls induced by Anisodiplosis waltheriae Maia, a recently described species, on Waltheria indica L. was studied. W indica is an invasive weed in regeneration areas of Atlantic Forest in southeastern Brazil. Plants were collected in May 2004 and above-ground biomass, main stem length, number of leaves, number of galls per leaf and leaf area of each individual were recorded. Nearly 90% of all plants and 25% of all leaves were attacked by the gall midge, with an average of 0.67 galls/leaf. Leaf area had a weak effect on gall abundance while the number of leaves had no effect on gall abundance. Only 31% of the variation in gall abundance was explained by plant biomass. Natural enemies killed one third of the sampled galls. Predation accounted for 22.9% of gall mortality, unknown factors killed 7.6%, microhymenopteran parasitoids killed 2.5% and fungi only 1%. Mortality factors were not influenced by leaf area or gall density.

Processes Driving Ontogenetic Succession of Galls in a Canopy Tree1

ABSTRACTHerbivores can be associated with distinct ontogenetic stages of their host in a nonseasonal, directional, and continuous pattern of colonization and extinction of species populations called ontogenetic succession, but the processes behind this pattern are still largely unknown. We used plants of Cryptocarya aschersoniana Mez (Lauraceae) belonging to different ontogenetic stages, to examine how the density of different gall‐inducing insects varies along the ontogeny of the host, and how gall density is influenced by mechanisms associated with host quality (plant height, plant shape, leaf area, specific leaf area, and hypersensitivity), and by mechanisms associated with their natural enemies (parasitoids, pathogens, and predators). In a remnant of Araucaria Forest, located in the São Francisco de Paula National Forest (Brazil), gall density (ind./100 g of leaf ) was obtained for 42 plants of C. aschersoniana divided into three height classes. Two galling species were recorded, showing quite distinct density patterns among height classes of C. aschersoniana. While Hymenoptera gall density decreased almost 50 times from small plants to canopy trees, Hemiptera gall density increased almost 10 times. Path analyses showed that Hymenoptera density was higher in smaller plants, independent of other host traits, while Hemiptera density was higher in plants exhibiting smaller leaves. Natural enemies were not detected in the Hemiptera population, and mortality rates due to predators, parasitoids, and pathogens did not affect Hymenoptera density. Processes associated with plant quality play the main role in generating the observed ontogenetic succession pattern.

Spatial density-independent parasitism of the inquiline, Diomorus aiolomorphi (Hymenoptera: Torymidae), on the bamboo gall maker, Aiolomorphus rhopaloides (Hymenoptera: Eurytomidae)

Diomorus aiolomorphi Kamijo (Hymenoptera: Torymidae) is an inquiline on bamboo galls formed by the bamboo gall maker Aiolomorphus rhopaloides Walker (Hymenoptera: Eurytomidae). I examined (1) the spatial and vertical distribution of galls and galls from which D. aiolomorphi emerged, and (2) the spatial density-dependent parasitism of D. aiolomorphi in a stand of Phyllostachys heterocycla. The percentages of galls from which D. aiolomorphi emerged tended to be higher than those from which A. rhopaloides emerged and ranged from 21.5% to 45.6%. Bamboo galls and galls from which D. aiolomorphi emerged were distributed contagiously on bamboo culms and branches. The attack of D. aiolomorphi was not dependent on gall density on culms and branches. D. aiolomorphi is synovigenic; females have no eggs in their ovaries when they emerge but develop eggs continuously as adults age. Thus, D. aiolomorphi females cannot lay eggs on freshly initiated galls aggregatively and successively, suggesting that D. aiolomorphi attacks regardless of gall density.

Patch Size and Patch Quality of Gall-inducing Aphids in a Mosaic Landscape in Israel

For weak flying insects feeding on two different host plants during their life cycle, such as gall-inducing aphids, patch and matrix characteristics may play a critical role in patch occupancy and population size in occupied patches. The aims of the present study were to define the basic patch size of Baizongia pistaciae (L) (Aphididae, Fordini), an aphid inducing galls on Pistacia palaestina Boiss (Anacardiaceae) using a genetic approach, and to estimate the impact of landscape structure and patch quality on patch occupancy and gall density on occupied trees of this aphid and four other closely related species. Using 42 genetic markers detected by RAPD-PCR in 117 clones of the galling aphid Baizongia pistaciae, we calculated Wright's F statistics and estimated the number of winged migrants between demes. We found that host trees at least 150 m apart supported genetically differentiated demes of B. pistaciae, and formed distinct patches. Since the annual cycle of this aphid involves alternation between two different hosts, P. palaestina trees and Poaceae roots, patch – the smallest area that sustains a deme – is a relatively small area that must be composed of at least a single P. palaestina tree and nearby secondary hosts. To assess the impact of landscape structure and patch quality on patch occupancy and gall abundance in occupied patches, two field surveys of P. palaestina trees in natural Mediterranean maquis were performed. Among the five species of gall-inducing aphids found, B. pistaciae was the most abundant of those surveyed. Host trees were occupied more often in the ecotone, the transition zone between Mediterranean closed maquis and open bata, than in the maquis. Mature and old trees were more often occupied than young ones, and shrubs more often than tree-like plants. There was no difference in the proportion of occupied trees between isolated host trees or those growing in groups. Species richness showed similar trends. We also found no significant differences in gall abundance in occupied trees among tree quality categories, except that trees growing in the ecotone tended to carry more galls than those growing in the maquis. In conclusion, the best patch of gall-inducing aphids seems to be a small area, composed of an old shrub of P. palaestina standing in an open landscape with nearby secondary hosts, grass roots, available for colonization by winged migrants.

Influence of synchronization between adult emergence and host plant phenology on the population density of Pseudasphondylia neolitseae (Diptera: Cecidomyiidae) inducing leaf galls on Neolitsea sericea (Lauraceae)

AbstractSynchronization between the appearance of herbivorous insects and their host‐plant phenology is a critical event, especially for short‐lived insects such as gall midges. We studied a natural population of Pseudasphondylia neolitseae (Diptera: Cecidomyiidae) that induces leaf galls on Neolitsea sericea (Lauraceae) to evaluate the effect of synchronization on gall density in the subsequent generation. To do so, we combined quantitative data on host resources with time lag between emergence and host‐available seasons. The gamma distribution model was applied to the emergence curve of P. neolitseae and the normal distribution model to the daily changes in the number of host buds suitable for oviposition; the latter model was transformed into an “available‐resource curve” based on the mean number of host buds required for a single female to realize her eggs. By superimposing the emergence curve on the available‐resource curve and calculating overlapped area, the degree of synchronization was evaluated more accurately than previous studies, which had treated only the time lag. The number of females that synchronized with host buds affected gall density in the next generation.

The biology of Dasineura dielsi Rübsaamen (Diptera: Cecidomyiidae) in relation to the biological control of Acacia cyclops (Mimosaceae) in South Africa

Abstract Acacia cyclops is an invasive Australian tree in South Africa and a target for biological control using seed‐reducing agents. In southern Australia, two gall‐forming Cecidomyiidae, Dasineura dielsi (Small Fluted Galler) and Asphondylia sp., develop on the flowers and seeds of A. cyclops, respectively. The larvae of D. dielsi form woody fluted galls on the ovaries of flowers and prevent the development of fruit. Immature Asphondylia sp. develop in the loculi of green fruit and destroy developing seeds. Dasineura dielsi was selected as a biological control candidate for A. cyclops in South Africa and was approved for official release after host specificity evaluation and consideration of potential conflicts of interest. Dasineura dielsi naturalised in South Africa in 2001 and after 3 years dispersed up to 450 km from a single population at Stellenbosch, Western Cape. At sites where D. dielsi has been present longest, high gall densities occur on A. cyclops during the peak flower season in summer. Four hymenopterans, ?Synopeas sp., Mesopolobus sp., Torymus sp. and an unidentified Platygastridae, were reared from D. dielsi galls and are suspected parasitoids of the cecidomyiid, with incidence levels less than 10%. Monitoring is required to evaluate trends in the population status of D. dielsi, its parasitoids and seed production of A. cyclops. Importantly, field monitoring should determine the extent and nature of possible competitive interactions between D. dielsi and an introduced seed‐feeding weevil, Melanterius servulus.

Long‐term population dynamics of a sawfly show strong bottom‐up effects

Summary 1 Population dynamics of the arroyo willow stem-galling sawfly Euura lasiolepis Smith (Hymenoptera: Tenthredinidae) on its host plant, Salix lasiolepis Bentham (Salicaceae), was studied for 22 years in the vicinity of Flagstaff, Arizona, USA. 2 Comparatively low levels of population change in drier and wetter sites ranged over two orders of magnitude. 3 Time-series analysis revealed best fit models of population change that included total winter precipitation from October to May and/or cumulative Pearson Drought Severity Index (PDSI) for the same period, plus a 1-year delay in precipitation or PDSI in some cases. 4 For populations on willow clones in drier sites, the model accounted for 73% of sawfly population change, and in wetter sites, with high mean populations, the model predicted 67% of the variation in gall density. 5 The mechanistic link between predictors of water availability to host plants and sawfly populations was better growth of willow after high winter precipitation, producing more and longer shoots. Also, willow clones with younger ramets, with more vigorous growth, were more favourable to sawflies than clones with older ramets. 6 Strong female preference for longer shoots resulted in more eggs laid, and this was coupled with higher larval survival on longer shoots. The major component of loss in a cohort was aborted ovipositions, with significant and negative influence on survival through the immature stages. Survival in one generation was correlated with gall density in the following generation. 7 Dynamics were entirely driven through resource supply. 8 Combining the evolutionary Phylogenetic Constraints Hypothesis with detailed mechanistic studies reveals an explanation of population dynamics which is more broadly applicable than purely ecological studies.

Spatial Distribution of Galls Caused by Aculus tetanothrix (Acari: Eriophyoidea) on Arctic Willows

The distribution of galls caused by Aculus tetanothrix (Acari: Eriophyoidea) on three Salix species was studied. The factors influencing this distribution were analysed, i.e. willow species, study area and shoot length. Spatial pattern of gall distribution within the shoot was also examined. The study was conducted in Russia, Kola Peninsula. Densities of galls caused by A. tetanothrix differed significantly among willow species. Considerably higher gall density was recorded in the White Sea coast than in the Khibiny Mountains. This may be explained by the influence of a milder maritime climate that favors mite occurrence compared to a harsh and variable mountain climate that limits mite abundance. There was no relationship between the gall density and the shoot length. The highest density of galls was recorded on the inner offshoots; within the offshoot, there was a maximum density on the fifth leaf. This pattern was repeatable for all shoots studied, independent of the study area, willow species and length of shoots, suggesting the optimal conditions for A. tetanothrix exist on leaves in the middle part of a shoot. This distribution pattern may be an effect of the trade-off between the costs and benefits resulting from leaf quality and mite movement along the shoot. This hypothesis, however, needs to be tested experimentally.

Scale-dependent mechanisms in the population dynamics of an insect herbivore

A multiscale approach has lead to significant advances in the understanding of species population dynamics. The scale-dependent nature of population processes has been particularly clearly illustrated for insect herbivores. However, one of the most well-studied insect herbivores, the galling sawfly Euura lasiolepis, has to date been examined almost exclusively at fine spatial scales. The preference-performance, plant vigour and larval survival hypotheses are well supported by this species. Here, we test these hypotheses at a spatial scale larger than that previously considered, i.e. across a landscape in northern Arizona represented by an altitudinal gradient encompassing a series of drainages. We also develop a qualitative model for understanding the population dynamics of E. lasiolepis based on patterns of survival and mortality found in this study and previous ones. Gall density was highly variable across the altitudinal gradient, not explained by host plant variables, and thus a poor surrogate for population abundance. These findings for the first time fail to support the plant vigour and preference hierarchy hypotheses for E. lasiolepis. Dispersal limitation most likely explains the lack of support for these hypotheses at this scale. By contrast, sawfly survival, gall abortion, parasitism and larval mortality were well explained by host plant quality variables and altitude. The larval survival hypothesis was well supported and is thus comparatively scale-invariant. A qualitative model developed here highlighted the importance of both willow water status and disturbance in determining host plant quality, as well as an apparent trade off between shoot length and plant moisture status in determining vital rates across the altitudinal gradient. This study thus demonstrated for the first time the scale-dependent nature of mechanisms underlying the population dynamics E. lasiolepis, and identified the interaction between parasitism and altitude as a novel mechanism underlying spatial patterns in the survival and mortality patterns of this species.

Slow response to strong disturbance in an insect population with a temporal refuge

The extent to which perturbations are modulated and delayed in population systems is a question of considerable theoretical and practical interest. Organisms with a temporal refuge, for example insects with prolonged diapause, are likely to have strongly buffered populations with long response lags, but little is known concerning how disturbances are actually mediated in natural populations. We show experimentally that the gall midge Contarinia vincetoxici has a prolonged diapause with a “bank” in the soil where diapausing larvae can reside for at least six winters. This bank acts as a strong buffer against disturbance. We stopped recruitment to the larval bank during eight years by removing all new galls, but no significant effects on gall density were found. Response lags may thus be very long even in populations of apparently shortlived animals. Population fluctuations during the last decade were evidently largely determined by events taking place prior to this period.

Time tells: long‐term patterns in the population dynamics of the yew gall midge,Taxomyia taxi(Cecidomyiidae), over 35 years

Abstract. 1. Three populations of the yew gall midge,Taxomyia taxi, and its two chalcid parasitoids,Mesopolobus diffinisandTorymus nigritarsus, from Kingley Vale, West Sussex, U.K., were monitored each year between 1966 and 2002.Taxomyia taxi's life cycles are either hemivoltine or univoltine and the parasitoids' are univoltine (T. nigritarsus) or multivoltine (M. diffinis).2. Growth of the host trees, yewTaxus baccata, each year between 1950 and 1997 was assessed by width of annual rings in cores taken in 1998. Temperature and rainfall data were provided by the weather station in Southampton, representative of the climate of central southern England.3. Time‐series analyses were conducted to explore the effects of climate, tree growth, and parasitoids on eachT. taxipopulation and on the three populations combined. Four time‐delayed models were developed: for hemivoltine and univoltineT. taxiand for each of the parasitoids.4. HemivoltineT. taxipopulations showed troughs in density in the periods 1972–1975 and 1991–1992, which were more pronounced in even than odd years; these were closely tracked byM. diffinisand followed, after a delay, byT. nigritarsus. Time‐series models showed that, forT. taxi, its current density depended on the density of its hemivoltine parents moderated by the negative effects ofT. nigritarsuson these parents. In reciprocal fashion,T. nigritarsusdensity was influenced by that of its hemivoltine host as well as by its own previous density. Host and parasitoid appear to participate in a coupled interaction that regulates the populations of both via delayed density dependence, resulting in cycles of at least 14 years.Mesopolobus diffinishad no effect on the dynamics of hemivoltineT. taxi, most of which escaped its parasitism. Although univoltineT. taxican give rise to hemivoltine offspring, there was no evidence that they affected hemivoltine dynamics.5. The density of univoltineT. taxiwas usually much lower than hemivoltine density and varied irregularly over time. It was not attacked byT. nigritarsusbut parasitism byM. diffiniswas always high. Despite this, its density depended only on the density of its parents, especially the hemivoltine parents. Parasitism had no effect on the dynamics of univoltineT. taxi. Conversely,M. diffinisdensity depended solely on the densities of both univoltine and hemivoltine hosts.6. Tree growth appeared to have little effect on the dynamics of the host or parasitoids and there was no evidence that gall density had any effect on growth of the tree. The primary effect of climate appeared to be in maintaining the cyclic nature of theT. taxi–T. nigritarsusinteraction. If stochastic weather variables are removed from the models, the oscillations are of equivalent periodicity but dampen to equilibrium.

Ant mutualists alter the composition and attack rate of the parasitoid community for the gall waspDisholcaspis eldoradensis(Cynipidae)

Abstract. 1. The strength or density dependence of pairwise species interactions can depend on the presence or absence of other species, especially potential mutualists.2. The gall waspDisholcaspis eldoradensisinduces plant galls that secrete a sweet honeydew from their top surfaces while the wasp larvae are active. These galls are actively tended by Argentine ants, which collect the honeydew and drive off parasitoids attempting to attack the gall wasp.3. When ants were excluded, the total rate of parasitism by seven species of parasitoids increased by 36%, and the rate of gall‐wasp emergence decreased by 54%.4. The total percentage parasitism was affected by gall density when ants were excluded but not when ants were unmanipulated, suggesting a change in parasitoid functional responses due to ant tending.5. In addition, excluding ants significantly altered the proportions of different parasitoid species that emerged from galls; one parasitoid species increased from 1% to 34%, and another decreased from 46% to 19%.6. The invasive Argentine ants studied are capable of maintaining the mutualism with the gall wasps that evolved in the presence of different ant species and also act as a selective filter for the local community of generalist parasitoids trying to attack this gall species.

Quality or quantity: the direct and indirect effects of host plants on herbivores and their natural enemies

Resource quality (plant nitrogen) and resource quantity (plant density) have often been argued to be among the most important factors influencing herbivore densities. A difficulty inherent in the studies that manipulate resource quality, by changing nutrient levels, is that resource quantity can be influenced simultaneously, i.e. fertilized plants grow more. In this study we disentangled the potentially confounding effects of plant quality and quantity on herbivore trophic dynamics by separately manipulating nutrients and plant density, while simultaneously reducing pressure from natural enemies (parasitoids) in a fully factorial design. Plant quality of the sea oxeye daisy, Borrichia frutescens, a common coastal species in Florida, was manipulated by adding nitrogen fertilizer to increase and sugar to decrease available nitrogen. Plant density was manipulated by pulling by hand 25 or 50% of Borrichia stems on each plot. Because our main focal herbivore was a gall making fly, Asphondylia borrichiae, which attacks only the apical meristems of plants, manipulating plant nitrogen levels was a convenient and reliable way to change plant quality without impacting quantity because fertilizer and sugar altered plant nitrogen content but not plant density. Our other focal herbivore was a sap-sucker, Pissonotus quadripustulatus, which taps the main veins of leaves. Parasitism of both herbivores was reduced via yellow sticky traps that caught hymenopteran parasitoids. Plant quality significantly affected the per stem density of both herbivores, with fertilization increasing, and sugar decreasing the densities of the two species but stem density manipulations had no significant effects. Parasitoid removal significantly increased the densities of both herbivores. Top-down manipulations resulted in a trophic cascade, as the density of Borrichia stems decreased significantly on parasitoid removal plots. This is because reduced parasitism increases gall density and galls can kill plant stems. In this system, plant quality and natural enemies impact per stem herbivore population densities but plant density does not.

Spatial abundance structures in an assemblage of gall‐forming sawflies

SummaryExamination of the fine‐scale internal structure of species geographical ranges, and interspecific variation therein across landscapes, contributes to a more comprehensive understanding of the structure of geographical ranges. Two components of this internal structure that require further examination are the occurrence, extent and position of spatial autocorrelation, and relationships between the spatial abundance structures of closely related, ecologically similar species.Here we compare the abundance structures of an assemblage of gall‐forming sawflies (Tenthredinidae) across a landscape. We identify the relative roles of spatial and non‐spatial factors in explaining their abundance structures and test the hypothesis that sawfly density is explained by host plant quality, as has been demonstrated repeatedly at finer scales. We use these results to distinguish between alternative sets of mechanisms that may be operating at the landscape scale.Species densities were mainly multimodal across the landscape and significantly spatially structured, with patch, periodic and trend components. The abundance structures thus mimic those found generally for species across the full extent of their geographical ranges.Many abundance structure characteristics were unique to species, with differences in their correlogram profiles, distances over which densities were positively autocorrelated, and the absence of significant spatial structure in one species.In contrast to previous, fine‐scale studies, host plant quality explained little of the variation in sawfly gall density across the landscape, whereas unexplained spatial structure contributed between 30% and 50%. Based on knowledge of the biology of these species and the absence of competitive interactions, species dispersal characteristics and the Moran effect are suggested as probable alternative hypotheses at this scale.Therefore, a spatial approach has identified the hierarchical nature of mechanisms underlying the population dynamics of this sawfly assemblage for the first time. Furthermore, it has highlighted the importance of spatial processes in explaining the densities of species at the landscape scale, as well as the individualistic nature of their abundance structures.