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Recent studies on insect interactions on plants have revealed that herbivorous insects indirectly interact with each other through changes in plant traits following herbivory. However, less attention has been given to plant biomass relative to plant quality in relation to indirect interactions among herbivores. We explored the extent to which the larval food demand of two specialist butterflies (Sericinus montela and Atrophaneura alcinous) explains their interaction on a host plant, Aristolochia debilis. A laboratory experiment showed that plant mass consumption by A. alcinous larvae was 2.6 times greater than that by S. montela. We predicted that A. alcinous, which requires more food, is more vulnerable to food shortages than S. montela. In a cage experiment, an asymmetric interspecific interaction was detected between the two specialist butterflies; S. montela larval density significantly decreased the survival and prolonged the development time of A. alcinous, but A. alcinous density affected neither the survival nor the development time of S. montela. The prediction based on the food requirement was partly supported by the fact that increasing A. alcinous density likely caused a food shortage, which more negatively affected A. alcinous survival than S. montela survival. Conversely, increasing the density of S. montela did not reduce the remaining food quantity, suggesting that the negative effect of S. montela density on A. alcinous was unlikely to be due to food shortage. Although aristolochic acid I, a defensive chemical specific to Aristolochia plants, did not influence the food consumption or growth of either butterfly larva, unmeasured attributes of plant quality may have mediated an indirect interaction between the two butterflies. Consequently, our study suggests that not only the quality but also the quantity of plants should be considered to fully understand the characteristics, such as symmetry, of interspecific interactions among herbivorous insects on the same host plant.

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The variability in the genetic variance-covariance (G-matrix) in plant resistance and its role in the evolution of invasive plants have been long overlooked. We conducted an additional analysis of the data of a reciprocal transplant experiment with tall goldenrod, Solidago altissima, in multiple garden sites within its native range (USA) and introduced range (Japan). We explored the differences in G-matrix of resistance to two types of foliar herbivores: (a) a lace bug that is native to the USA and recently introduced to Japan, (b) and other herbivorous insects in response to plant origins and environments. A negative genetic covariance was found between plant resistances to lace bugs and other herbivorous insects, in all combinations of garden locations and plant origins except for US plants planted in US gardens. The G-matrix of the resistance indices did not differ between US and Japanese plants either in US or Japanese gardens, while it differed between US and Japanese gardens in both US and Japanese plants. Our results suggested that the G-matrix of the plant resistance may have changed in response to novel environmental differences including herbivore communities and/or other biotic and abiotic factors in the introduced range. This may have revealed a hidden trade-off between resistances, masked by the environmental factors in the origin range. These results suggest that the stability of the genetic covariance during invasion, and the environmentally triggered variability in the G-matrices of plant resistance may help to protect the plant against multiple herbivore species without changing its genetic architecture and that this may lead to a rapid adaptation of resistance in exotic plants. Local environments of the plant also have a critical effect on plant resistance and should be considered in order to understand trait evolution in exotic plants.

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Species often interact indirectly with each other via their traits. There is increasing appreciation of trait-mediated indirect effects linking multiple interactions. Flowers interact with both pollinators and floral herbivores, and the flower-pollinator interaction may be modified by indirect effects of floral herbivores (i.e., florivores) on flower traits such as flower size attracting pollinators. To explore whether flower size affects the flower-pollinator interaction, we used Eurya japonica flowers. We examined whether artificial florivory decreased fruit and seed production, and also whether flower size affected florivory and the number of floral visitors. The petal removal treatment (i.e., artificial florivory) showed approximately 50% reduction in both fruit and seed set in natural pollination but not in artificial pollination. Furthermore, flower size increased the number of floral visitors, although it did not affect the frequency of florivory. Our results demonstrate that petal removal indirectly decreased 75% of female reproductive output via decreased flower visits by pollinators and that flower size mediated indirect interactions between florivory and floral visitors.

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Despite our understanding of chemical defenses and their consequences for plant performance and herbivores, we know little about whether defensive chemicals in plant tissues, such as alkaloids, and their spatial variation within a population play unappreciated and critical roles in plant-herbivore interactions. Neighboring plants can decrease or increase attractiveness of a plant to herbivores, an example of a neighborhood effect. Chemical defensive traits may contribute to neighborhood effects in plant-herbivore interactions. We examined the effects of nicotine in leaves (a non-emitted defense chemical) on plant-herbivore interactions in a spatial context, using two varieties of Nicotiana tabacum with different nicotine levels. A common garden experiment demonstrated that visits by grasshoppers decreased with increasing density of neighboring plants with a greater nicotine level. In contrast, visits of leaf caterpillars were not affected by neighbors, irrespective of nicotine levels. Thus, our results clearly highlighted that the neighborhood effect caused by the nicotine in leaves depended on the insect identity, and it was mediated by plant-herbivore interactions, rather than plant-plant interactions. This study demonstrates that understanding of effects of plant defensive traits on plant-herbivore interactions requires careful consideration of the spatial distribution of plant defenses, and provides support for the importance of spatial context to accurately capture the ecological and evolutionary consequences of plant-herbivore interactions.

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Geographic variation in the traits of a species is shaped by variation in abiotic conditions, biotic interactions, and evolutionary history of its interactions with other species. We studied the geographic variation in the density of the lace bug, Corythucha marmorata, and the resistance of tall goldenrod Solidago altissima to the lace bug herbivory in their native range in the United States and invaded range in Japan. We conducted field surveys and reciprocal transplant experiments to examine what abiotic and biotic factors influence variation in lace bug density, and what ecological and evolutionary factors predict the resistance of the host plant between and within the native and invaded ranges. Lace bug density was higher throughout the invaded range than in the native range, higher in populations with warmer climates, and negatively affected by foliage damage by other insects in both ranges. The higher lace bug density in warmer climates was explained by the shorter developmental time of the lace bugs at higher temperatures. The resistance of S. altissima to lace bugs was higher in populations with lace bugs compared to populations without lace bugs in both native and invaded ranges, indicating that the evolutionary history of the interaction with the lace bugs was responsible for the variation in S. altissima resistance in both ranges. The present study revealed that abiotic and biotic factors, including temperature and other herbivorous insects, can drive the geographic variation in lace bug density, which in turn selects for variation in plant resistance in both in the native and invaded ranges. We conclude that the novel combination of factors such as higher temperature and lower number of other herbivorous insects is responsible for the higher lace bug density in the invaded range than in the native range.

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The interplay between evolution and ecological communities is critical for the integration of different levels of biological organization. Recent work has begun to unveil the importance of plant phenotypic plasticity and plant-herbivore (co)evolution to link plant evolution and associated insect communities. Specifically, herbivore-induced plant traits (i.e., plastic phenotypes) have significant effects on the structure and diversity of herbivore communities, which can in turn promote the evolution of not only the focal plant but also insect community members. Here, I will provide a conceptual framework on the eco-evolutionary dynamics of plant-herbivore communities to understand how biological organizations are integrated in plant-insect interactions. Research on eco-evolutionary dynamics of plant-herbivore communities will undoubtedly enrich understanding of a wide range of plant-insect interactions.

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Understanding the origins and diversity of invasive species can reveal introduction and invasion pathways, and inform an effective management of invasive species. Tall goldenrod, Solidago altissima, is a herbaceous perennial plant native to North America and it has become a widespread invasive weed in East Asian countries. We used microsatellite and chloroplast DNA markers to obtain information on neutral processes and on genetic diversity in native and invaded populations of S. altissima and to infer how it invaded and spread in Japan. We found that introduced (n = 12) and native (n = 20) populations had similar levels of genetic diversity at nuclear SSR loci. Genetic structure analysis indicated that at least two independent colonization events gave rise to current S. altissima populations in Japan. The majority (68%) of the Japanese S. altissima were genetically similar and likely shared a common origin from a single or a small number of populations from the southern USA populations, while the populations in Hokkaido were suggested to arise from a different source. Our results suggest that multiple and mass introductions have contributed to the persistence and rapid adaptation of S. altissima promoting its widespread establishment throughout Japan.

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Several recent studies have emphasised that community composition alters species trait evolution. Here, we demonstrate that differences in composition of local herbivore communities lead to divergent trait evolution of the leaf beetle Plagiodera versicolora through plant-mediated indirect interactions. Our field surveys, genetic analyses and community-manipulation experiments show that herbivore community composition determines the degree of herbivore-induced regrowth of willows (Salicaceae), which in turn, promotes the divergent evolution of feeding preference in the leaf beetle from exclusive preference for new leaves to a lack of preference among leaf-age types. Regrowth intensity depends both on the differential response of willows to different herbivore species and the integration of those herbivore species in the community. Because herbivore-induced regrowth involves phenological changes in new leaf production, leaf beetle populations develop divergent feeding preferences according to local regrowth intensity. Therefore, herbivore community composition shapes the selection regime for leaf beetle evolution through trait-mediated indirect interactions.

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PREMISE OF THE STUDY: Microsatellite markers were developed in the perennial herb Solidago altissima from populations within its introduced range in Japan to assess its population structure and to facilitate tracking of invasion expansion. • METHODS AND RESULTS: Using 454 pyrosequencing, 16 microsatellite primer sets were developed for S. altissima. The primer sets were tested on 70 individuals sampled from three populations in Japan. The primers amplified di- and trinucleotide repeats with five to 25 alleles per locus, and the expected heterozygosity ranged from 0.46 to 0.92. • CONCLUSIONS: These results indicate the utility of primers in S. altissima for future research on a wide range of applications, including tracking of invasion dynamics and investigating population genetics of the species.

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It is critical to incorporate the process of population dynamics into community genetics studies to identify the mechanisms of the linkage between host plant genetics and associated communities. We studied the effects of plant genotypic diversity of tall goldenrod Solidago altissima on the population dynamics of the aphid Uroleucon nigrotuberculatum. We found genotypic variation in plant resistance to the aphid in our experiments. To determine the impact of plant genotypic diversity on aphid population dynamics, we compared aphid densities under conditions of three treatments: single-genotype plots, mixed-genotype plots and mixed-genotype-with-cages plots. In the latter treatment plants were individually caged to prevent natural enemy attack and aphid movement among plants. The synergistic effects of genotypes on population size were demonstrated by the greater aphid population size in the mixed-genotype treatment than expected from additive effects alone. Two non-exclusive hypotheses are proposed to explain this pattern. First, there is a source-sink relationship among plant genotypes: aphids move from plant genotypes where their reproduction is high to genotypes where their reproduction is low. Second, natural enemy mortality is reduced in mixed plots in a matrix of diverse plant genotypes.

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Trophic cascades are often a potent force in ecological communities, but abiotic and biotic heterogeneity can diffuse their influence. For example, inducible defenses in many species create variation in prey edibility, and size-structured interactions, such as cannibalism, can shift predator diets away from heterospecific prey. Although both factors diffuse cascade strength by adding heterogeneity to trophic interactions, the consequences of their interactioh remain poorly understood. We show that inducible defenses in tadpole prey greatly intensify cannibalism in predatory larval salamanders. The likelihood of cannibalism was also strongly influenced by asymmetries in salamander size that appear to be most important in the presence of defended prey. Hence, variation in prey edibility and the size structure of the predator may synergistically affect predator-prey population dynamics by reducing prey mortality and increasing predator mortality via cannibalism. We also suggest that the indirect effects of prey defenses may shape the evolution of predator traits that determine diet breadth and how trophic dynamics unfold in natural systems.

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In mutualism under spatial structure, asynchrony between the dispersions of the interacting species can be a key determinant of their dynamics. We focused on the plant-mycorrhizal fungi system to theoretically analyze the colonization process by calculating the probability of colony establishment under environmental fluctuation. This can be considered a joint process of two sub-processes before and after the association between the host plant and the mycorrhizal fungi in a novel habitat. When colony growth undergoes environmental fluctuation, the dynamics of colony size can be considered a combination of the two stochastic sub-processes that mediated the association event between the plant and the fungi. Therefore, properties of whole system are influenced by five parameters, means and variances of colony growth rates of two sub-systems, and a rate of association of plant and fungi. For the successful establishment of a colony, the second sub-process must start before the first sub-process finishes (i.e., extinction), which we refer to as "stochastic tunneling." Our analysis of the establishment probability of a plant colony based on this concept revealed that (1) the mean colony growth rates of the host alone and the symbiotic association affect establishment probability in different ways, (2) the variance of colony growth rate of the symbiotic association reduces the establishment probability, although the variance of growth rate of the host alone facilitates the establishment probability when the mean growth rate of the host alone is negative, and (3) a trade-off between the mean colony growth rates of the host alone and the symbiotic association could result in the evolution of either a symbiotic or parasitic relationship, based on a host decision. The model we present is widely applicable to the colonization processes of both positive and negative species relationships, where the interacting species disperse independently.

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Much attention has been paid to ecology and evolution of damage-induced plant responses. Recently, it has been emphasized that phenotypic plasticity, such as induced plant responses, has the potential to lead to evolutionary changes of interacting partners. Here, we report that induced plant regrowth promotes a locally adaptive feeding preference of a leaf beetle, Plagiodera versicolora. We found that there was among-population variation in the strength of the feeding preference of the leaf beetle for leaf-age types of conspecific host plants. The strength of the preference was positively correlated to leaf production of host plants across populations, and the intensity of induced regrowth was likely to have been responsible for geographic variation in new leaf production. Within one population, we detected a significant additive genetic variance and heritability in the preference for consuming new vs. old leaves. Moreover, the strength of preference was significantly related to egg production depending on the leaf-age types. Thus, allopatric populations can evolutionarily develop different adaptive preference, according to locally distinct patterns of induced host regrowth.

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1. Herbivory often changes resource quality for herbivorous insects through induced plant responses depending on the intensity of damage. We hypothesized that the willow's response following herbivory affects the entire arthropod community structure on the tree due to changes in plant quality. To examine this hypothesis, we investigated arthropod communities on three willow species, Salix gilgiana, Salix eriocarpa and Salix serissaefolia. 2. In our common garden, cuttings were established in 2003 and experimental treatments were applied in 2005. Damage by a boring caterpillar of a swift moth (Endoclita excrescens) and artificial cutting of 25% stems were applied as partial herbivory within individual trees, and 100% cutting of stems was applied to represent severe herbivory to whole individual trees. These treatments stimulated lateral shoot production depending on damage intensity, resulting in full compensation for biomass loss. 3. Positive relationships were detected between within-tree variation in foliar nitrogen content and overall abundance/species richness of herbivores. Moth boring and 25% cutting increased herbivore abundance and species richness relative to controls. However, we found no significant differences in herbivore abundance and species richness between 100% cut and control trees. Community composition of herbivore species was significantly different between the following three groups: (i) bored and 25% cut; (ii) 100% cut; and (iii) control trees. Changes in community structure of herbivores were likely due to changes in plant quality depending on the intensity of damage. 4. Although total predator abundance and species richness were not significantly different among treatments, community composition of predator species was significantly different among treatments. 5. These results indicate that herbivore-induced willow responses can largely determine the entire arthropod community structure of multitrophic levels due to changes in plant quality. We suggest that heterogeneous resource conditions induced by herbivory within and among plant individuals increase the species diversity of arthropods.

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Insect herbivory can negatively or positively affect plant performance. We examined how a stem gall midge Rabdophaga rigidae affects the survival, growth, and bud production of current year shoots of the willow Salix eriocarpa. In mid-May, the gall midge initiates stem galls on the apical regions of shoots. The following spring, galled shoots had thicker basal diameters and more lateral shoots than ungalled shoots. Although galled shoots were on average 1.6 times longer than ungalled shoots, there were no significant differences in shoot length or in the numbers of reproductive, vegetative, and dormant buds per shoot. However, the subsequent survival of galled shoots was significantly higher than that of ungalled shoots, probably because of the thicker basal diameter. This increased shoot survival resulted in approximately two times greater reproductive, vegetative, and dormant bud production on galled shoots compared with ungalled shoots in the following spring. These results suggest that the willow regrowth induced by galling can lead to an increase in bud production through increased shoot survival.

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We experimentally examined the effects on other herbivorous insects of leaf shelters constructed by lepidopteran larvae on a willow, Salix miyabeana. Several insect species occupied the vacant leaf shelters. Our experiment using artificial leaf shelters showed that the number of aphids increased with the number of artificial leaf shelters on a shoot, as did the numbers of three ant species ( Camponotus japonicus, Lasius hayashi, and Myrmica jessensis) that entered leaf shelters to collect aphid honeydew. To determine the ant-mediated effect of leaf shelters on herbivorous insects that do not use leaf shelters, we transferred newly hatched larvae of a common leaf beetle, Plagiodera versicolora, to the leaves of shoots with and without artificial leaf shelters. One day after the transfer, larval survival rate was significantly lower on shoots with shelters than on those without shelters, and shoots with shelters had significantly more ants than did shoots without shelters. Our field experiments demonstrated clearly that shelter-making lepidopteran larvae increased the abundance of both aphids and ants and decreased the survival rate of leaf beetle larvae, probably because the larvae were removed by ants that were attracted to the leaf shelters by the aphid colonies.

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We investigated the relationship between oviposition preference and offspring performance in a herbivorous lady beetle Epilachna pustulosa on two co-occurring plant species, thistle Cirsium kamtschaticum and blue cohosh Caulophyllum robustum, in 1994 and 1995. The relative importance of bottom-up effects by host plants and top-down effects by natural enemies on offspring performance were determined using field and laboratory experiments. In both years, egg density on blue cohosh was significantly higher than on thistle. A laboratory experiment demonstrated that larval survival from hatching to adult emergence was significantly higher, and developmental period shorter when larvae were reared on blue cohosh compared to thistle. The positive preference-performance linkage varied between years in the field. Top-down effects had a different impact on larval survival on the two host plant species. Arthropod predators, a lady beetle Harmonia axyridis and an earwig Forficula mikado, considerably depressed immature survival on thistle, while they were negligible on blue cohosh. Although the lack of effective predation increased larval survival on blue cohosh, it led to defoliation due to increased larval feeding late in the season. Because of severe intraspecific competition, old larvae had significantly lower survival on blue cohosh than on thistle. In 1994, as larval survival decreased due to defoliation on blue cohosh, the overall survival rate was significantly higher on thistle than on blue cohosh. This survival pattern was opposite to that found in the laboratory experiment. In contrast, in 1995, the increase in predatory lady beetles on thistle caused greater larval mortality. Thus, the overall survival was significantly lower on thistle than on blue cohosh, although severe intraspecific competition occurred on blue cohosh as it had in 1994. Consequently, the offspring performance on the two host plants is largely determined by the relative importance of arthropod predation determining larval survival on thistle and host plant defoliation reducing late larval survival on blue cohosh. These results indicate the important role of spatial and temporal variability of natural enemies on the preference-performance linkage of herbivorous insects.

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A reproductive tradeoff between current egg production and subsequent survival in a lady beetle, Epilachna niponica, a specialist herbivore on a thistle, Cirsium kagamontanum, was investigated at the two study sites, A and F. Survival of reproductive females decreased consistently from early May until mid-June, but apparently increased thereafter. In contrast, males showed a consistent decrease in survival throughout the reproductive season, without any sign of recovery. Dissection of ovaries of sampled females revealed that egg resorption increased late in the reproductive season, coincident with increased female survival. Reproductive females stopped oviposition immediately after a large flood in 1979 at site F. Two weeks after the habitat perturbation, females resumed oviposition in response to a flush of new leaves on damaged plants. Female survival sharply increased during the nonoviposition period, and declined when egg-laying resumed. Approximately 40% of long-lived reproductive females at site F survived up to the following reproductive season in the next year. Also, some of these long-lived females were observed ovipositing in the following reproductive season. The long-lived reproductive females which had previously invested in reproduction survived equally well as newly emerged females which had not reproduced in summer. These results suggest that there is a reproductive tradeoff between current egg production and subsequent survival. Egg resorption may be an adaptive ovipositional response to habitat perturbation such as flooding, which considerably reduces offspring fitness due to absolute shortage of food. Also, increased female survival accompanied by egg resorption enhances the likelihood of the future oviposition in the second reproductive season, thereby increasing a female's lifetime reproductive success.