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A diverse array of organisms induce plants to form galls. This phenomenon is one of the most striking instances of convergent evolution, yet the underlying mechanism is only well understood in gall-inducing microbes. To determine if gall induction by insects is associated with bacterial symbiosis, we used 16S rRNA gene sequencing to examine the microbiome of a variety of gall-inducing and non-gall-inducing insects. Overall, we did not find any bacterial signature of gall induction among the insect species we surveyed. There were no specific bacterial taxa that were consistently associated with gall induction. Microbiome diversity differed significantly among species of host insects, but not between gall- and non-gall-inducing insect species. Bacterial community composition also differed strongly among insect species, but not in a systematic way between gall-inducing and non-gall-inducing species. Furthermore, two gall-inducing species harbored highly variable microbiomes with relatively few bacterial sequences, characteristics that suggest a lack of abundant bacterial symbionts. Together, these findings argue that gall induction is not consistently mediated by a bacterial symbiont or bacterial community and may be symbiont-independent, at least in some insect species. While symbionts may still contribute to gall induction in specific instances, we suggest that the convergent evolution of gall induction is more typically driven by endogenous mechanisms, with potential contributions from horizontal gene transfer.

The interaction between the arroyo willow, Salix lasiolepis Bentham, and its specialist herbivore, the arroyo willow stem-galling sawfly, Euura lasiolepis Smith (Hymenoptera: Tenthredinidae), was studied for 32 yr in Flagstaff, AZ, emphasizing a mechanistic understanding of insect population dynamics. Long-term weather records were evaluated to provide a climatic context for this study. Previously, predictive models of sawfly dynamics were developed from estimates of sawfly gall density made between 1981 and 2002; one model each for drier and wetter sites. Predictor variables in these models included winter precipitation and the Palmer Drought Severity Index, which impact the willow growth, with strong bottom-up effects on sawflies. We now evaluate original model predictions of sawfly population dynamics using new data (from 2003-2012). Additionally, willow resources were evaluated in 1986 and in 2012, using as criteria clone area, shoot density, and shoot length. The dry site model accounted for 40% of gall population density variation between 2003 and 2012 (69% over the 32 yr), providing strong support for the bottom-up, mechanistic hypothesis that water supply to willow hosts impacts sawfly populations. The current drying trend stressed willow clones: in drier sites, willow resources declined and gall density decreased by 98%. The wet site model accounted for 23% of variation in gall population density between 2003 and 2012 (48% over 30 yr), consistent with less water limitation. Nonetheless, gall populations were reduced by 72%.

A guild of six parasitic insects (Hymenoptera) which attack cocoons of the same sawfly populations was composed of four indigenous and two introduced species. The indigenous parasitoids included the ichneumonids, Pleolophus indistinctus (Prov.), Mastrus aciculatus (Prov.), Gelis urbanus (Brues) and Endasys subclavatus (Say), and the introduced species were the ichneumonid, Pleolophus basizonus (Grav.), and the eulophid, Dahlbominus fuscipennis (Zett.). Their distributions were sampled on five variable requirements, or resource sets. Litter moisture content and seasonal activity varied within plots, and host density, host species, and plant community varied between lots. Although the distributions of the indigenous species overlapped, in relative terms each species occupied one position in the niche space that was poorly exploited by all other guild members. Such an enclave permitted each of the first three species to develop a zone of dominance over the other parasitoids. The introduced species had no recognizable enclave. The most abundant guild member, P. basizononus, was dominant in the sites most favorable to litter-searching parasitoids. It had the broadest niche over the range of litter moisture content and length of seasonal activity, and it interacted more with all other species than any other guild member. It was a better competitor than the next most broadly adapted species, P. indistinctus. Competitive superiority is proposed as the driving force behind abundance, leading to dominance in favorable sites. The density-dependent interaction between individuals, which results in dispersal, appears to be responsible for the occupation of a broader niche.

Records of 232 moth species spanning 26 years (total catch of ca. 230,000 specimens), obtained by continuous light-trapping in Kevo, northernmost subarctic Finland, were used to examine the hypothesis that life-history traits and taxonomic position contribute to both relative abundance and temporal variability of Lepidoptera. Species with detritophagous or moss-feeding larvae, species hibernating in the larval stage, and species pupating during the first half of the growing season were over-represented among 42 species classified as abundant during the entire sampling period. The coefficients of variation in annual catches of species hibernating as eggs averaged 1.7 times higher than those of species hibernating as larvae or pupae. Time-series analysis demonstrated that periodicity in fluctuations of annual catches is generally independent of life-history traits and taxonomic affinities of the species. Moreover, closely related species with similar life-history traits often show different population dynamics, undermining the phylogenetic constraints hypothesis. Species with the shortest (1 year) time lag in the action of negative feedback processes on population growth exhibit the largest magnitude of fluctuations. Our analyses revealed that only a few consistent patterns in the population dynamics of herbivorous moths can be deduced from life-history characteristics of the species. Moreover, the diversity of population behaviour in one moth assemblage challenges any conventional wisdom suggesting predictable patterns. Our results raise several questions about perceptions and paradigms in insect population dynamics and stress the need for research on detritivorous insect population dynamics, as well as the need for more assemblage-wide studies using common trapping methods to provide comparative data on related and unrelated species with different life-history traits.

Summary The adaptive radiation of four taxa of gall-inducing insects is compared: sawflies (Hymenoptera: Tenthredinidae), oak gall wasps (Hymenoptera: Cynipidae), aphids (Homoptera: Aphididae), and gall midges (Diptera: Cecidomyiidae). The sawflies, with only three genera but many species per genus, colonized many adaptive zones. Many sibling species have been recently recognized. Such characteristics suggest a dynamic evolutionary state of radiation. Innovations in the substrates used for oviposition by sawflies, probably involving ovipositional accidents, have enabled evolution from free-feeding ancestors into diverse gall types and galling sites – from leaf edge to leaf lamina, leaf midrib, petiole, bud, and stem. Host shifting among species of willows (Salix) has been extensive and opportunistic. The oak gall wasps, like the sawflies, colonized many adaptive zones, in terms of galling sites on the plant and diverse gall types. The gall wasps include large genera, and use a geographically widespread host-plant genus rich in species, with largely opportunistic colonization of new hosts. The gall-inducing aphids on Pistacia in Israel show radiation into many adaptive zones, with different gall types and galling sites on the host plant, even though the radiation is much less extensive than in sawflies and gall wasps. A major limiting factor in this smaller radiation appears to be the low richness of Pistacia species and the host specificity of the galling aphids to this plant genus. The gall midges are similar to the sawflies and gall wasps with many large genera. Their radiation is much greater than in the related fungus-feeding midges, indicating the importance of a rich host flora for insect radiation. The study of adaptive radiation in gall-inducing insect taxa offers many opportunities for comparative studies in evolutionary biology.

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.

Planning conservation of insect herbivores requires knowing what needs to be conserved and developing a set of predictor variables that aid management. We conducted a state-wide survey to examine the species richness of gall wasps (Hymenoptera: Cynipidae) on six oak species dominant in the threatened scrub-oak vegetation in peninsular Florida. Eighty-eight cynipid species were recorded; 23 were new species to Florida (a 35% increase), including 17 species new to science and 6 species newly recorded in the state. Th ec ynipid species represented 68% of cynipids of Florida, on only 24% of oak species sampled. This fauna represents a hotspot of richness, justifying conservation initiatives in scrub-oak habitat and throughout the state. We derived predictor variables from general ecological concepts: (1) the theory of island biogeography that insect species richness increases as host plant geographic area increases and as local abundance increases, (2) the plant-architecture hypothesis that insect species richness increases with increased plant size, and (3) phytochemical patterns in leaves, including nutrients and digestibility reducers predicting suitability for insect herbivores. Concepts 1 and 2, developed for large scales and species numbers, were tested at smaller scales relevant to much conservation research and management. A stepwise multiple regression including all predictor variables accounted for 99% of the variance in cynipid species richness with three variables: foliar hemicellulose concentration (81%), host geographic area (16%), and tree height (2%). The trends were negative, however, and opposite to those predicted by concepts 1 and 2. Ecological theory was not applicable to discovery of predictors of cynipid species richness on six oak species. Thus, we promote caution in applying ecological theory to a narrow set of species without specific testing of how patterns conform to theoretical predictions.

A group of six unusual sawfly species, which do not conform to the phylogenetic constraints hypothesis as it has been applied to sawflies, was exam- ined in natural populations. All species were in the genus Pontania (Hymenoptera: Tenthredinidae), which induce galls on leaves of willow species (Salicaceae). An understanding of these non-conformist species was important as a test of the validity of the general hypothesis. 2. The six species of sawfly, Pontania mandshurica, P. cf. arcticornis, P. aestiva, P. arcticornis, P. pacifica, and P. nr. pacifica, showed no oviposition preference for long, vigorous shoots, in contrast to 37 documented tenthredinid species that have demonstrated such a preference. Rather, the non-conformist species attacked the shortest shoot length classes more frequently and larval establishment in galls was successful. 3. The evident escape from the phylogenetic constraint, which commonly limits sawfly attack to the most vigorous shoots in a willow population, resulted from low apparent heterogeneity of the resources exploited by these Pontania species. At the time of female oviposition, shoots and leaves were too uniform to allow discrimination by females among shoot length classes, resulting in random, or near random attack of shoots. 4. The unusual relative uniformity of resources to which sawflies were exposed resulted from several characteristics. (1) Females emerged early relative to shoot growth phenology, making discrimination among shoot length and vigour difficult or impossible. (2) Low heterogeneity in leaf length resulted in resource similarity independent of shoot length. (3) Abscission of leaves occurred after emergence of larvae from leaf galls so that differential abscission of leaves in relation to shoot length became irrelevant. (4) In some cases, low variance in shoot lengths was evident in old ramets lacking long, vigorous shoots. Probably as a result of low resource heterogeneity, larvae survived well across all shoot length classes, revealing no ovipositional preference and larval performance linkage related to the exploita- tion of the longest shoot length classes in a population of willows, as in the conformist species. Therefore, larval survival did not provide positive feedback on female preferential behaviour for long shoots, as in the conformist species studied.

Oviposition preference was examined in relation to offspring performance in a bud gall-forming sawfly, Euura sp., which attacks the Scouler or fire willow, Salix scouleriana (Barratt). The only known locality for this sawfly in Arizona is in the inner basin of the San Francisco Peaks, at 3,100–3,450 m above sea level. A random sample of shoots was taken in October of 1995, 1996, 1997, and 1998 to record shoot length, number of buds per shoot, number of galls per shoot, percent mortality, and overall percent survival for each generation. Oviposition preference was quantified for the first 3 yr, and larval performance was measured over all 4 yr. We found a strong oviposition preference for longer shoots. The longest shoot-length classes contained the smallest proportion of buds, and yet they were attacked more frequently than the short shoot-length classes. Therefore, the rarest shoot-length classes were the most preferred. Larval establishment and survival in galls were used to evaluate offspring performance. No significant correlation existed between larval establishment and shoot length, thus negating the prediction of a preference-performance linkage. Overall survivorship was high in 1995 (77%), 1997 (80%), and 1998 (82%), but drastically dropped in 1996 (16%), possibly because of a severe drought during that year. We discuss two alternative hypotheses to possibly explain these results: (1) all modules (long and short shoots) could provide sufficient nutrients for larvae to survive; and (2) selective pressures acting on female oviposition behavior are independent of larval performance.

Summary 1 Examination 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. 2 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. 3 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. 4 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. 5 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. 6 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.

The distribution and ecology of insects in arctic regions are poorly known. The aim of this study was to determine the distribution of galling sawflies in the Canadian arctic and their oviposition preference. The Swedish Tundra Northwest 1999 expedition visited 17 sites in the Canadian arctic. We determined the occurrence of galling sawflies at all the sites and studied the oviposition preference of two leaf-galling sawflies, Eupontania arctica and Pontania nivalis, on Salix reticulata and S. glauca, respectively. Galling sawflies were abundant at only one site, the mainland site at Ivvavik National Park. Only a few galls in total were found at the remaining sites, suggesting that galling sawflies are rare in the higher arctic, and potential explanations for this pattern are discussed. Shoots with leaf galls were longer than shoots without galls on both S. reticulata and S. glauca. These differences could not be explained by a higher number of leaves on longer shoots. This suggests that long shoots are preferred by sawflies because of faster development and better survival of larvae on long shoots.

1 Predictions from the Phylogenetic Constraints Hypothesis were tested for the first time in an eruptive forest Lepidopteran species, the western spruce budworm. 2 In previous work, we established that western spruce budworm females exhibit oviposition preferences with regard to tree age, tree vigour and host species. However, there was no evidence to support a link between oviposition preference and larval performance, which supports the Phylogenetic Constraints Hypothesis. 3 Our preference data led us to test whether female budworms use oviposition strategies to select the sites where they lay their egg masses. Our experiments were designed to make direct comparisons between latent and eruptive insect herbivores with respect to two oviposition behaviours: egg retention and avoidance of conspecifics. This type of research has not previously been conducted on any eruptive forest Lepidopteran. 4 Female budworms retained eggs instead of laying them on less preferred hosts in two of three experiments, but the percentage of eggs they retained was significantly less compared to latent insect herbivores. 5 In addition, female budworms actively avoided oviposition in areas with the highest density of conspecific egg masses, but they laid egg masses in all the other locations provided. This contrasts with the pattern seen in latent insect herbivores, which consistently avoid laying their eggs near any sites already used by conspecifics. 6 Our research indicates that there are extreme differences between latent and eruptive insect herbivores with respect to egg retention and avoidance of conspecifics, thus supporting the Phylogenetic Constraints Hypothesis.

Studies on insects have provided a wellspring for major advances in ecology and related fields over the past two centuries. From animal–plant interactions to zoogeography, the insects have inspired and motivated innovation in science.

Host-plant association data, gathered from field surveys conducted throughout Florida and from the literature, were used to identify the specificity of cynipid gall inducers to one or more of six Quercus species that occur at Archbold Biological Station, Lake Placid, Florida, USA, including the red oaks Q. laevis, Q. myrtifolia, and Q. inopina, and the white oaks Q. chapmanii, Q. geminata, and Q. minima. Quercus myrtifolia had the highest cynipid richness and diversity (37 cynipid species, Shannon H′ = 3.61, Simpson's D = 0.97), followed by Q. chapmanii, Q. laevis, Q. inopina, Q. geminata, and finally Q. minima (10 species, H′ = 2.30, D = 0.90). All cynipid species showed strong fidelity to a particular host plant or a restricted set of host plants. An ordination of gall-wasp host associations indicated that the cynipid communities of each oak species were distinct and specific to a given oak species. Leaf samples taken from each oak species were analyzed for condensed and hydrolyzable tannins, total phenolics, lignin, cellulose and hemicellulose, nitrogen, and carbon. All of these chemical traits, with the exception of carbon, differed by oak species, and the differences were strongly correlated with the axes of the cynipid--species ordination. These results suggest that gall-wasp occurrence is influenced by oak chemistry and imply that experimental studies of cynipid gall inducers that examine host-plant chemistry and female oviposition choice and larval performance will yield useful insights.

Terrestrial food webs based on living plants may well represent 75% of global terrestrial biodiver- sity. The majority of component species are specialists and a large proportion is parasitic as herbi- vores and carnivores, with consequences for high sensitivity to heterogeneity on a variety of scales. Relatively large primary producers support relatively small insect herbivores and carnivores, with plants providing both food and habitat, making resource-driven effects very strong. Complexity of resources provided by plants, with influences up the food web, is generated by at least seven major factors: (i) plants as food; (ii) plants as habitat; (iii) the physical traits of plants such as size, tough- ness and trichomes; (iv) traits of plants that require evolutionary responses by herbivores in terms of crypsis, phenological synchrony, life history and behavioral adaptations; (v) the constitutive chemicals in plants; (vi) the induced changes in plants caused by herbivory; and (vii) landscape and biogeographic variation in vegetation types and food web richness. No other trophic level has such a wide-ranging impact on other trophic levels. But such broad impact makes the term 'food web' overly narrow and inadequate. The term 'interaction web' is preferable, aiding recognition of the many kinds of interactions that pass up food webs from living plants. Any claim that top-down impact is stronger than bottom-up influences is necessarily couched in a narrow sense of biomass or numbers reduction.

Eleven species of native or naturalized grasses were found hosting spittlebug nymphs in the natural grassland areas around Brasília in the Fe-deral District of Brazil. A large number of nymphs were found on the species Axonopus marginatus (Trin.) Chase. Out of a total of 224 observed nymphs, 85.7% were found feeding on A. marginatus. Apparently the presence of the spittlebug nymphs in the cerrado vegetation, the Brazilian savanna, was associated with any form of disturbance, such as cattle grazing and partial deforestation. Nymphs were found only in disturbed areas of the "cerrado campo sujo" and in the cerrado sensu strictu. The densities of spittlebug nymphs in the cerrado were lower than the density found in the cultivated pasture. The nymphal density in the cerrado sensu strictu was approximately 105 times lower than the densities in Brachiaria ruziziensis Germain & Evrard area. In two sites of "campo sujo" form of cerrado the densities were, approximately, 2,273 and 1,212 times lower than the densities in B. ruziziensis areas. The distribution and density of the spittlebug in the natural vegetation were influenced by the presence of adjacent infested areas of cultivated pasture. This fact indicates that areas of Brachiaria are the source of infestation for other host plants. Onze espécies de gramíneas nativas ou naturalizadas foram encontradas hospedando ninfas da cigarrinha-das-pastagens em áreas de cerrado na região de Brasília, Distrito Federal. Grande número de ninfas foi encontrado na espécie Axonopus marginatus (Trin.) Chase. Do total de 224 ninfas observadas, 85,7% foram encontradas alimentando-se nessa espécie de gramínea. Aparentemente a presença da cigarrinha-das-pastagens nas gramíneas do cerrado estava associada com qualquer forma de perturbação, tais como pastejo pelo gado ou desmatamento parcial. Ninfas foram encontradas somente em áreas perturbadas de cerrado do tipo campo sujo e sensu strictu. As densidades de ninfas no cerrado foram muito mais baixas do que as densidades encontradas em pastagens cultivadas. O número de ninfas/m² no cerrado sensu strictu foi aproximadamente 105 vezes mais baixo do que as densidades encontradas nas áreas de Brachiaria ruziziensis Germain & Evrard . Em dois sítios de cerrado do tipo campo sujo as densidades foram, aproximadamente, 2.273 e 1.212 vezes mais baixas do que as densidades nas áreas de B. ruziziensis. A distribuição e as densidades populacionais da cigarrinha-das-pastagens em áreas de vegetação natural foram influenciadas pela presença de pastagens cultivadas infestadas por este inseto nas adjacências, sugerindo que áreas de Brachiaria são focos de infestação para outras hospedeiras.

This study addresses four hypotheses that may account for differences in the number of insect herbivore species among plant species. These hypotheses are based on the assumption that insect diversity is a function of the number, quantity, and distribution of plant resources used by herbivores. The study investigated predictions that herbivore species richness will increase as a function of increasing the following: (1) host plant distribution over the landscape (host plant geographical distribution hypothesis), (2) host plant density within a habitat (resource concentration hypothesis), (3) size of individual plants (plant size hypothesis), or (4) abundance of resources (resource abundance hypothesis). We tested predictions from these hypotheses by examining the species richness of insect herbivores on five sympatric species of fabaceous plants that varied in their local dispersion of individual plants and plant architecture. Among these five species, plant geographical distribution varied threefold, density varied 38-fold, plant size and food resources available to insect herbivores varied ≈100-fold. Plant geographical distribution, plant size, and the resource concentration hypotheses were not corroborated in this study. Resource abundance, measured as plant dry weight, accounted for the differences in number and abundance of insect species between host plant species. Leaf biomass accounted for 44.15% of the variation in number of insect herbivore species and 51.76% of the variation in abundance of insect herbivores using leaves as resources. Flower biomass accounted for 32.86% of the variation in the number of insect herbivore species using flowers. Therefore, plants that offered a greater quantity of resources, especially leaves and flowers, had greater species richness and abundance in insect herbivores.

We studied the movement and feeding behavior of nymphs of the grass feeding spittlebug Deois flavopicta Stal to evaluate the role of host plant species on the spatial distribution of the nymphs. The inter- and intraplant foraging patterns of the nymphs contributed to their patchy distributions. The young nymphs, despite their weak motility, moved from the hatching spot and chose feeding sites. They tend to move less when host plants were present. Plant quality near hatching sites influenced the movement pattern of newly hatched nymphs. Egg densities at the oviposition sites did not influence the movement of newly hatched nymphs. The distribution of nymphs in relation to differences in host species and shoot age of host plant was studied in the field and in multiple choice experiments. Both young and old nymphs preferred plants of the introduced host in the genus Brachiaria over the native grass Axonopus marginatus (Trinius) Chase. The nymphs fed gregariously on buds and young shoots as first and second instars and in small groups of third, fourth, and fifth instars on old shoots of different sizes. These patterns of attack may be related to differences in tissue hardness or amino acid content between young and old shoots, and between host species. Nymphal survivorship was higher on B. ruziziensis, the preferred host plant. In addition, survivorship of newly hatched nymphs was higher on young buds and shoots compared with old shoots. The possible consequences of nymphal feeding behavior on population dynamics are discussed.

Summary 1. Predictions from the Phylogenetic Constraints Hypothesis were tested for the first time in a tropical system using the pasture pest Deois flavopicta Stal, which oviposits into the ground and not into plant tissues. The prediction that there is no oviposition preference–larval performance linkage was supported. The absence of such a link provides an evolutionary basis for eruptive population dynamics. 2. The effects of host species and host plant quality on the preference of ovipositing females of D. flavopicta and performance of their offspring on the selected host plants were tested at the population level. 3. Female oviposition behaviour was affected by the presence of the host plant. Females of D. flavopicta showed a strong preference to oviposit close to host plants. The number of eggs was higher in pots containing Brachiaria ruziziensis (121.88 ± 13.70) than in pots containing only the wet oviposition substrate (5.2 ± 1.98) or dry oviposition substrate (0.067 ± 0.067). Ovipositing females did not, however, discriminate between plants of Brachiaria decumbens and Axonopus marginatus and did not show a strong oviposition preference in relation to differences in plant quality (protein and fibre content). They did show oviposition preference for plants under the high watering regime. The mean number of eggs collected from pots with non water-stressed plants was 60% higher than the mean number of eggs collected on pots with water-stressed plants. 4. Although females did not show ovipositional preference, spittlebug larval performance, measured as percentage survival and duration of nymphal period, was better on plants of high protein and low fibre content. These results indicate that there is not a linkage between female oviposition preference and subsequent nymphal performance in relation to differences in protein and fibre content in the host plants. There was, however, a limited linkage between oviposition preference and nymphal performance in relation to plant water content. Females showed preference for moist sites that have high survival of newly hatched nymphs. 5. Evidence indicates that for D. flavopicta, the influences of natality and female oviposition behaviour in response to plant quality are not the major factors driving population outbreaks, which is in accordance with the Phylogenetic Constraints Hypothesis.

We examined the effect of shoot size distribution within plants and different stage of host plant development on the pattern of attack of Andricus sp. on gambel oak, Quercus gambelli Nuttall. The gall-inducing wasp preferred large diameter shoots to small ones in 2 populations of host plant. Attack on larger shoots was significantly nonrandom and occurred even though these shoots were relatively scarce in the shoot population. Large shoots accounted for only 5.51 and 12.72% of the total shoots in an urban and forest site, respectively. The percentage of attack on shoots in the largest shoot diameter class was 10.53 and 48.74% in the urban and forest site, respectively. Gall density decreased with maturity of the host plant and was 3 times higher on juvenile plants compared with mature plants. The distribution of attack in relation to plant age was not related to changes in shoot size with the age of the plants. Gambel oak may become resistant to gall formation with increasing age. Survival was 33.72% higher on large shoots (83.72%) compared with small shoots (50.00%). Larval survival did not change with plant age among host plants. The pattern of attack in relation to shoot size suggests that Andricus sp. prefer to oviposit on large shoots in which larval performance was highest. Andricus sp. might show a flexible oviposition preference hierarchy for shoot lengths. Females attacked the longest shoots available rather than showing a fixed preference for specific shoot lengths.

Summary 1. Ants have evolved mutualistic relationships with a diverse array of plant and animal species. Usually, the predatory/aggressive behaviour of ants near food sources can limit herbivore damage. 2. Galls of Disholcaspis edura on Quercus turbinella produce a secretion that is harvested by three species of ants (Formica neorufibarbis, Liometopium apiculatum, and Monomorium cyaneum) in the chaparral vegetation of Arizona, U.S.A. The study reported here provides evidence of a mutualistic relationship between these species of ants and the gall-forming wasp Disholcaspis edura. 3. An ant exclusion experiment showed that when ants tended galls, the rate of parasitism by Platygaster sp. on Disholcaspis edura was nearly halved in comparison to a treatment in which ants were excluded. 4. In the presence of ants, galls with the largest diameter suffered a lower mortality rate due to parasitoid attack than when ants were excluded. Thus, ant presence reduced the selective pressure imposed by Platygaster sp. on the galls with larger diameter.

The ovipositional stimulation for the shoot galling sawfly, Euura lasiolepis, whose natural host is the North American willow, Salix lasiolepis, was studied in response to its original host, Finnish willows, poplar species, and individual phenolic glucosides and fractions. The major phenolic glucosides in the natural host were salicortin and tremulacin, which were also the major components in the Finnish S. purpurea and S. rosmarinifolia. The natural host, S. lasiolepis, was chosen over six Finnish willow species in multiple-choice tests. Multiple-choice tests with six Finnish willow species and one Finnish popular species showed that S. rosmarinifolia was significantly more attractive than any of the other species by at least 16-fold. No-choice testing with phenolic glucosides showed the tremulacin fraction to be the significant ovipositional stimulant, which elicited probes with the ovipositor at least 19 times more frequently than other glucosides. A tremulacin fraction at 1.5% elicited significantly more probes than a tremulacin fraction tested at 0.4%. In addition, tremulacin was the only glucoside to induce prolonged and repeated probing with the ovipositor, lasting a mean of 6.4 sec per bout per female. Only single probes lasting a fraction of a second were observed in response to other glucosides. This is the second study showing a direct effect of a phenolic glucoside as an ovipositional stimulant for a sawfly, and it also suggests the existence of a mechanism that enables females to respond positively in a graded way to shoot length heterogeneity in host plants and populations.

We investigated the relationships among oviposition preference and larval performance of Nematus oligospilus Foerster, a solitary, free-feeding sawfly, and the vigor (shoot length) of its host plant, arroyo willow, Salix lasiolepis Bentham. In the 3 populations studied, the number of eggs laid by N. oligospilus increased 20- to 170-fold from shoots ≤100 mm to shoots >800 mm, even though shoots >800 mm accounted for only 0.20–3.17% of the available shoots. The distributions of leaves and leaf area did not account for greater attack on longer shoots, nor did the size (target area) of leaves. N. oligospilus had strong and relatively inflexible oviposition preferences for rapidly growing shoots of S. lasiolepis . In multiple choice oviposition experiments, females laid 4.0–6.2 times more eggs on vigorous shoots than on medium and short shoots. In no choice oviposition tests, females laid 83.5% of their available eggs on vigorous shoots, but only 54.4% on medium shoots and 31.0% on short shoots. The performance of larvae corresponded to the oviposition preference of females because short shoots were poor quality resources for developing larvae. In the field and laboratory, larval mortality on vigorously growing shoots averaged 26% less at establishment and 33.5% less after establishment compared with mortality on short shoots. Development time was 2.3 d less on average for larvae reared on vigorous shoots compared to those reared on short shoots. The fecundity of females reared from larvae developing on vigorous shoots averaged 4.8 eggs more than that of females reared from larvae developing on short shoots, although this difference was only nearly statistically significant ( P = 0.06). Natural enemies (egg parasites) did not playa role in the correlation between larval survival and oviposition preference. A life table analysis suggests that the influence of variation in plant quality (vigor) of S. lasiolepis on natality and larval performance significantly influences the demography of N. oligospilus .

We evaluate a three-part hypothesis explaining why gall-inducing insect species richness is so high in scleromorphic vegetation: (1) persistence of low nutrient status scleromorphic leaves facilitates the galling habit in warm temperate latitudes; (2) favourable colonization sites for gallers result from reduced hygrothermal stress, high phenolics in the outer cortex of the gall, and reduced carnivore and fungal attack in the gall; and (3) in more mesic sites, mortality is high due to carnivore attack and invasion of galls by fungi. Over 280 samples of local species of galling herbivorous insects from fourteen countries on all continents except Antarctica revealed a strong pattern of highest richness in warm temperate latitudes, or their altitudinal equivalents. The peak of galling species richness on the latitudinal gradient from the equator into the Arctic was between 25 to 38° N or S. Galling species were particularly diverse in sclerophyllous vegetation, which commonly had greater than twelve species per local sample. In mesic, non-sclerophyllous vegetation types the number of galling species was lower with twelve or fewer species present. Many sites in sclerophyllous vegetation supported between thirteen and forty-six galling species locally, including campina islands in Amazonia, cerrado savanna in central Brazil, the Sonoran Desert in Arizona and Mexico, shrubland in Israel, fynbos in South Africa and coastal scleromorphic vegetation in Australia. At the same latitude, or its elevational equivalent, galling species richness was significantly higher in relatively xeric sites when compared to riparian or otherwise mesic habitats, even when scleromorphic vegetation dominated the mesic sites. The results were consistent with the hypothesis and extend to a more general level the patterns and predictions on the biogeography of gall-inducing insects.

SummaryDelayed density dependence, and the cycles in insect populations that it can generate, are often investigated using time-series analysis. Recently, several authors have raised concerns about the validity of using time-series analysis to detect density dependence. One particular concern is the suggestion that exogenous driving variables, such as cyclic weather patterns, can lead to the spurious detection of density dependence in natural populations. Using non-biological data (the electricity bills of one of the authors), we show how easy it is to be misled by the results of time-series analysis. We then present 16 years’ data on the gall-forming sawfly, Euura lasiolepis (Hymenoptera: Tenthredinidae), and show that cycles in weather, specifically winter precipitation, lead to the spurious detection of density dependence in time-series analysis. We conclude that time-series analysis cannot stand alone as a method for inferring the action of density dependence, and urge further investigation of the effects of apparent cycles in abiotic forces on insect populations.