Summary Intraspecific variation in dietary breadth can influence important ecological and evolutionary processes, yet the mechanisms generating this variation are usually unknown. Maternally transmitted bacterial symbionts frequently infect insect herbivores, and many have been shown to mediate key ecological interactions. For polyphagous herbivores, infection with particular symbionts is often strongly correlated with feeding on particular plant species, suggesting that facultative symbionts might directly determine herbivore food plant specificity. However, previous tests of this hypothesis have returned inconsistent results, providing little empirical support for a causal relationship between facultative symbiont infection and dietary breadth. Here, we investigate whether heritable facultative symbionts mediate dietary breadth in the polyphagous aphid, Aphis craccivora. We first determined that asexual clones of the aphid differ dramatically in performance across two leguminous food plants, locust and alfalfa, and could be considered biotypes with distinct ecological characteristics. The heritable symbiont Arsenophonus is strongly associated with locust‐origin aphids. We created experimental lines that share aphid genotypes but differed with respect to Arsenophonus infection status, and compared performance across three food plant species. Naturally Arsenophonus‐infected clones performed 2–4× better on locust and up to 75% worse on two alternate plant species than uninfected controls, clearly demonstrating that Arsenophonus promotes specialization on locust. In both laboratory and field experiments, uninfected locust‐ and alfalfa‐origin clones exhibited similar and modest performance on locust, indicating that the ‘locust‐associated biotype’ would not exist without Arsenophonus. We also hypothesized that moving Arsenophonus, via transinfection, to an alfalfa‐origin lineage would improve performance on locust and serve to expand dietary breadth. Indeed, transinfection doubled aphid performance on locust and halved aphid performance on alfalfa. However, because this aphid lineage naturally performs better on alfalfa, the transinfected symbiont functionally equalized aphid performance between locust and alfalfa, making the alfalfa biotype more generalized. Thus, the same symbiont can either reduce or expand dietary breadth, depending on host genotype. Our results unequivocally demonstrate that symbiont gain or loss can instantaneously and substantially change the topology of food plant use in a polyphagous insect, modifying diet in ways that potentially influence the insect's ecological niche, evolutionary trajectory and pest status.

Dinoponera Roger 1861 has been revised several times. However, species limits remain questionable due to limited collection and undescribed males. We re-evaluate the species boundaries based on workers and known males. We describe the new species Dinoponera hispida from Tucuruí, Pará, Brazil and Dinoponera snellingi from Campo Grande, Mato Grosso do Sul, Brazil and describe the male of Dinoponera longipes Emery 1901. Additionally, we report numerous range extensions with updated distribution maps and provide keys in English, Spanish and Portuguese for workers and known males of Dinoponera. ResumenEl género Dinoponera Roger 1861 ha sido revisado varias veces. Sin embargo, la distribución geográfica de las especies de este género todavía es cuestionable debido a colecciones limitadas, y a que en general los machos continúan sin descripción. Reevaluamos los límites geográficos de las especies utilizando caracteres merísticos y morfométricos basados en obreras y machos conocidos. Describimos las nuevas especies Dinoponera hispida de Tucuruí, Pará, Brasil, y Dinoponera snellingi de Campo Grande, Mato Grosso do Sul, Brasil, y describimos el macho de Dinoponera longipes Emery 1901. Además, reportamos numerosas extensiones nuevas de las distribuciones geográficas con sus respectivos mapas actualizados, y proporcionamos claves de identificación en inglés, español, y portugués para las obreras y los machos conocidos del género Dinoponera. ReseARCH ARtiClePaul A. Lenhart et al. / Journal of Hymenoptera Research 31: 119-164 (2013) 120 Resumo O género Dinoponera Roger1861, é tem revisados varios vezes. No entanto, a clarificação das especies ainda segue questionável devido a um coleção limitado, e a que em geral os representantes do sexo masculino continuam sem descrição. Reavaliamos os limitas de espécies utilizado caráteres merísticos e morfométricos, baseado em operários e machos conhecidos. Descrevemos novas espécies Dinoponera hispida de Tucuruí, Para, Brasil e Dinoponera snellingi de Campo Grande, Mato Grosso do Sul, Brasil, e descrevemos o macho de Dinoponera longipes Emery 1901. Además, reportamos distribuições geográficas com seus respetivos mapas atualizados, e proporcionamos chaves de identificação em inglês, espanhol, e português para operários e machos conhecidos do género Dinoponera.

Global climate change is altering precipitation patterns. Th e eff ect of water stress on plant -herbivore interactions is poorly understood even though this is a primary ecological interaction that will be altered by climate change. Th is is especially true for grasslands where water is often limiting. In this study we manipulated water inputs in open grassland plots (1 m 2 ) during a severe drought and assessed plant and insect herbivore responses. Th ere were two watering treatments: ambient and supplemented. Supplemented plots received water weekly in amounts that mimicked average seasonal rainfall. For plants, we were interested in how water input aff ected protein and digestible carbohydrate content; previous studies predicted water stress would increase the concentration of these two nutrients. Grasshoppers are the dominant insect herbivores in grasslands and we assessed their responses to water inputs by measuring abundance and diversity. Previous studies suggested grasshoppers would prefer water-stressed plots. Protein and carbohydrate content in bulk grass and forb samples, plus plant biomass and diversity, were measured monthly (May -August). Immediately prior to harvesting plant tissue, we counted and identifi ed individual grasshoppers in each plot. Grass biomass was reduced with water stress, but macronutrient content and species diversity were unaff ected. After three months water-stressed forbs were less protein biased, and diverse, relative to watered forbs; forb biomass was indistinguishable between treatments. Grasshopper abundance and diversity were lower in water-stressed plots as the season progressed. However, grasshopper-feeding biology mattered: densities of mixed-feeders and grass-feeders, but not forb-specialists, decreased over time in water-stressed plots, but not in water supplemented plots. Our results demonstrate the importance of focusing on plant and insect herbivore functional groups and provide valuable new data that can be incorporated into models to explore the eff ects of global climate change in greater detail.

Elemental data are commonly used to infer plant quality as a resource to herbivores. However, the ubiquity of carbon in biomolecules, the presence of nitrogen-containing plant defensive compounds, and variation in species-specific correlations between nitrogen and plant protein content all limit the accuracy of these inferences. Additionally, research focused on plant and/or herbivore physiology require a level of accuracy that is not achieved using generalized correlations. The methods presented here offer researchers a clear and rapid protocol for directly measuring plant soluble proteins and digestible carbohydrates, the two plant macronutrients most closely tied to animal physiological performance. The protocols combine well characterized colorimetric assays with optimized plant-specific digestion steps to provide precise and reproducible results. Our analyses of different sweet corn tissues show that these assays have the sensitivity to detect variation in plant soluble protein and digestible carbohydrate content across multiple spatial scales. These include between-plant differences across growing regions and plant species or varieties, as well as within-plant differences in tissue type and even positional differences within the same tissue. Combining soluble protein and digestible carbohydrate content with elemental data also has the potential to provide new opportunities in plant biology to connect plant mineral nutrition with plant physiological processes. These analyses also help generate the soluble protein and digestible carbohydrate data needed to study nutritional ecology, plant-herbivore interactions and food-web dynamics, which will in turn enhance physiology and ecological research.

The structure of a genome can be described at its simplest by the number of chromosomes and the sex chromosome system it contains. Despite over a century of study, the evolution of genome structure on this scale remains recalcitrant to broad generalizations that can be applied across clades. To address this issue, we have assembled a dataset of 823 karyotypes from the insect group Polyneoptera. This group contains orders with a range of variations in chromosome number, and offer the opportunity to explore the possible causes of these differences. We have analysed these data using both phylogenetic and taxonomic approaches. Our analysis allows us to assess the importance of rates of evolution, phylogenetic history, sex chromosome systems, parthenogenesis and genome size on variation in chromosome number within clades. We find that fusions play a key role in the origin of new sex chromosomes, and that orders exhibit striking differences in rates of fusions, fissions and polyploidy. Our results suggest that the difficulty in finding consistent rules that govern evolution at this scale may be due to the presence of many interacting forces that can lead to variation among groups.

Summary Herbivores often defend themselves from predation by transmitting toxic plant‐produced chemicals to their enemies. Polyphagous herbivores sometimes exhibit differential toxicity when found on various host plant species, which is generally assumed to reflect variation in plant chemistry. Here, however, we provide evidence that host‐associated herbivore lineages can intrinsically differ in their toxic properties. Lineages of Aphis craccivora originating from black locust (Robinia pseudoacacia) are unsuitable food for the ladybeetle Harmonia axyridis, resulting in death of both larvae and adults, whereas aphid lineages originating from alfalfa (Medicago sativa) support larval development and adult reproduction. We show that locust‐origin aphids remain toxic and alfalfa‐origin aphids remain non‐toxic when reared on any of three legume plants (fava, alfalfa or locust). Furthermore, toxicity is not a function of the facultative bacterial symbiont Arsenophonus, which is naturally present in locust‐origin aphid lineages and facilitates aphid use of locust. Experimentally cured locust‐origin lineages remain toxic, and an experimentally transinfected alfalfa‐origin lineage remains non‐toxic to H. axyridis. Instead, Arsenophonus plays an indirect role in the distribution of toxic aphid lineages by facilitating aphid use of locust. It is the parthenogenetic coinheritance of Arsenophonus and the toxic trait that observationally correlates locust‐feeding with toxicity in A. craccivora, rather than host plant chemistry per se. Our results clearly demonstrate that aphid lineages intrinsically vary in their toxic properties in a way that neither plant chemistry nor bacterial symbionts can explain. A more inclusive paradigm is needed for understanding variation in herbivore defence against predators.

1. The value of protective mutualisms provided by some facultative endosymbionts has been well demonstrated in the laboratory, yet only recently has their effectiveness in the field been studied. ‘Candidatus Hamiltonella defensa’ is known to defend aphids from parasitoid wasps in laboratory trials. However, the efficacy of this defence varies among parasitoids, suggesting that protection will vary spatially and temporally depending on parasitoid community composition.2. This demonstrated specificity and a dearth of studies on Hamiltonella in the field prompted the authors to quantify parasitism rates of Hamiltonella‐infected and ‐uninfected Aphis craccivora Koch aphid colonies in a manipulative field study.3. It was found that A. craccivora in central Kentucky alfalfa were parasitised by Lysiphlebus testaceipes (Cresson) and Aphelinus sp. Surprisingly, Hamiltonella infection did not lower successful parasitism by the naturally occurring parasitoid wasps. Whether Hamiltonella was effective against L. testaceipes was subsequently tested in a controlled laboratory assay, and no effect on parasitism rate was found.4. This study emphasises the fact that defensive symbionts sometimes provide no tangible defensive benefits under field conditions, depending on parasitoid community composition. It is hypothesised that the protective mutualism may be beneficial in geographically localised areas. When the symbiosis is effective against a local parasitoid community, aphid clones may experience eruptive population growth and rapidly disperse across a large area, allowing spread to habitats with different parasitoid communities where the mutualism is an ineffective defence.

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