We used European geometrid moths (>630 species) as a model group to investigate how life history traits linked to larval host plant use (i.e., diet breadth and host-plant growth form) and seasonal life cycle (i.e., voltinism, overwintering stage and caterpillar phenology) are related to adult body size in holometabolous insect herbivores. To do so, we applied phylogenetic comparative methods to account for shared evolutionary history among herbivore species. We further categorized larval diet breadth based on the phylogenetic structure of utilized host plant genera. Our results indicate that species associated with woody plants are, on average, larger than herb feeders and increase in size with increasing diet breadth. Obligatorily univoltine species are larger than multivoltine species, and attain larger sizes when their larvae occur exclusively in the early season. Furthermore, the adult body size is significantly smaller in species that overwinter in the pupal stage compared to those that overwinter as eggs or caterpillars. In summary, our results indicate that the ecological niche of holometabolous insect herbivores is strongly interrelated with body size at maturity.

Induced responses in plants, initiated by herbivory, create potential for trait-mediated indirect interactions among herbivores. Responses to an initial herbivore may change a number of plant traits that subsequently alter ecological processes with additional herbivores. Although common, indirect interactions between taxonomically distant herbivores, such as mammals and insects, are less studied than between taxonomically related species (i.e., insect-insect). In terms of mammal-insect interactions, effects on insect numbers (e.g., density) are relatively well studied, whereas effects on performance (e.g., fecundity) are rarely explored. Moreover, few studies have explored mammal-insect interactions on coniferous plants.The aim of this study was to investigate the effect of mammalian induced responses on insect performance. We specifically investigated the effect of moose (Alces alces) browsing on Scots pine (Pinus sylvestris) and subsequent effects on sawfly (Neodiprion sertifer) performance.Sawfly larvae were reared on browsed, clipped, and unbrowsed control pine trees in a controlled field experiment. Afterward, cocoon weight was measured. Needle C:N ratio and di-terpene content were measured in response to browsing.Sawfly performance was enhanced on trees browsed by moose. Cocoon weight (proxy for fecundity) was 9 and 13% higher on browsed and clipped trees compared to unbrowsed trees. Cocoon weight was weakly related to needle C:N ratio, and browsed trees had lower a C:N ratio compared to unbrowsed trees. Needle di-terpene content, known to affect sawfly performance, was neither affected by the browsing treatments nor did it correlate with sawfly weight.We conclude that mammalian herbivory can affect insect herbivore performance, with potential consequences for ecological communities and with particular importance for insect population dynamics. The measured plant variables could not fully explain the effect on sawfly performance providing a starting point for the consideration of additional plant responses induced by mammalian browsing affecting insect performance.

This study investigated the interactive effects of growth on drought stressed host plants and pathogen challenge with the baculovirus Autographa californica nucleopolyhedrovirus (AcMNPV) on survival and fitness-related traits using the Speckled Wood butterfly, Pararge aegeria (L.). Exposure to AcMNPV significantly reduced survival to pupation. For surviving larvae, sub-lethal infection significantly decreased daily mass acquisition rates and pupal mass. Growth on drought stressed plants increased daily mass acquisition rates resulting in heavier pupae, and increased resource allocation to adult reproduction. The interaction between host plant drought and viral exposure resulted in different resource allocation strategies, and thus different growth trajectories, between larvae. This in turn resulted in significantly different allometric relationships between larval mass (at inoculation) and both development time and investment in flight muscles. For larvae with relatively lighter masses there was a cost of resisting infection when growth occurred on drought stressed host plants, both within the larval stage (i.e. longer larval development times) and in the adult stage (i.e. lower investment in flight muscle mass). This multi-factor study highlights several potential mechanisms by which the complex interplay between low host plant nutritional quality due to drought, and pathogen exposure, may differentially influence the performance of P. aegeria individuals across multiple life stages.

Despite the increasing rate of urbanization, the consequences of this process on biotic interactions remain insufficiently studied. Our aims were to identify the general pattern of urbanization impact on background insect herbivory, to explore variations in this impact related to characteristics of both urban areas and insect-plant systems, and to uncover the factors governing urbanization impacts on insect herbivory. We compared the foliar damage inflicted on the most common trees by defoliating, leafmining and gall-forming insects in rural and urban habitats associated with 16 European cities. In two of these cities, we explored quality of birch foliage for herbivorous insects, mortality of leafmining insects due to predators and parasitoids and bird predation on artificial plasticine larvae. On average, the foliage losses to insects were 16.5% lower in urban than in rural habitats. The magnitude of the overall adverse effect of urbanization on herbivory was independent of the latitude of the locality and was similar in all 11 studied tree species, but increased with an increase in the size of the urban area: it was significant in large cities (city population 1-5 million) but not significant in medium-sized and small towns. Quality of birch foliage for herbivorous insects was slightly higher in urban habitats than in rural habitats. At the same time, leafminer mortality due to ants and birds and the bird attack intensity on dummy larvae were higher in large cities than in rural habitats, which at least partially explained the decline in insect herbivory observed in response to urbanization. Our findings underscore the importance of top-down forces in mediating impacts of urbanization on plant-feeding insects: factors favouring predators may override the positive effects of temperature elevation on insects and thus reduce plant damage.

Adults of the wood-boring beetlePhoracantha semipunctata F. showed variability in their attractiveness to five varieties ofEucalyptus when presented with an array of logs in a natural setting. Logs of two host varieties (E. camaldulensis Dehnhardt and the hybridE. trabutii) attracted two to three times more adult beetles than did logs of other host species (E. cladocalyx F.,E. grandis Hill ex Maiden andE. tereticornis Small). In the field, high oviposition rates byP. semipunctata adults resulted in severe competition among larvae. Larval survivorship was low in field logs ofE. trabutii and high inE. cladocalyx logs, although these hosts were the most and least attractive to the adult beetles, respectively. However, when logs were hand infested at low larval densities, survivorship ofP. semipunctata larvae was highest in logs of bothE. camaldulensis andE. trabutii. These findings suggest that adult beetles in the field were most attracted to those logs ofEucalyptus species that represented the highest quality hosts for their progeny under conditions of reduced larval competition.

This study provides an example of how variation in the quality of overwintering sites provided by the host plant of an insect seed predator can influence both the probability of overwintering survival and the size and composition of postwintering populations. Thus, the concept of host plant quality is extended to include variation in the suitability of the overwintering site of temperate region insects that overwinter within, or in habitats created by, their host plant. Adult Acanthoscelides alboscutellatus (Horn) (Coleoptera: Bruchidae) overwinter inside the fruit of Ludwigia alternifolia (L.) (Onagraceae). In early winter, however, fruits begin to dehisce, i.e., one or more of the fruit\'s four sides and/or top are shed. Variation in the onset and extent of dehiscence creates a range of overwintering habitats that vary in exposure to ambient conditions. In this study the frequency of possible overwintering sites in natural populations of L. alternifolia was determined by monitoring the phenology of fruit dehiscence from October through May in two populations for four years and for a third population for three years. Winter survivorship of adult A. alboscutellatus was assessed experimentally in eight environments representative of the conditions created by variation in dehiscence. These environments were produced by crossing four levels of exposure (degree of dehiscence) with two locations of the overwintering site, i.e., above or on the ground surface. The onset, phenology, and overall frequency of fruit dehiscence varied markedly among populations and years. Exposure, location, and their interaction had strong effects on survival and accounted for 80% of the observed variation in winter survival. Survivorship was higher on than above the ground, and in both locations decreased with increasing exposure. Thus, variation in fruit dehiscence among L. alternifolia populations will influence the size of postwintering A. alboscutellatus populations by dictating the quality of overwintering sites. Adult beetles that over-winter inside indehiscent fruit experience selection for small body size, associated with high mortality, when they attempt to exit the fruit at eclosion. As a consequence, the frequency of fruit dehiscence at eclosion coupled with the relative survival rates of adults within indehiscent fruit will determine the body size composition of postwintering populations and hence the response to selection for small body size in this species.

Phytophagous insects of estuarine salt marshes which live inside their host plants are not directly exposed to estuarine gradients. Host plant quality, however, may change along the estuary as a result of the direct effects of these gradients; as a consequence growth and development of endophagous insects may be influenced. The results of a study of the life cycle of Agapanthia villosoviridescens (Coleoptera, Cerambycidae), a stem-borer of the halophyte Aster tripolium, on three salt marshes along the Westerschelde estuary (the Netherlands) are in line with this hypothesis. It was shown that in upstream direction (1) mean larval weights were consistently higher during the entire period of larval development; (2) the percentage of late instars on a given sampling date generally was higher; (3) the percentage of larvae which underwent successful metamorphosis increased. Furthermore, (4) dry weight of the imagos was highest on the least saline marsh. The effects of estuarine gradients on the Aster host plants was indicated by differences in growth and chloride content between the populations of the three marshes. The non-overlapping geographic distribution of Agapanthia villosoviridescens and its host plant Aster tripolium on the Westerschelde salt marshes may be related to the effects of estuarine gradients on the suitability of the host plant.

Host plant quality for herbivores is modulated by different factors including symbiosis with soil organisms, such as arbuscular mycorrhiza (AM), as well as plant age. However, the role of the developmental stage of the AM in such plant-microbe-herbivore interactions has been neglected. To investigate the effects of AM stage and plant age on aphid performance, individuals of the generalist Myzus persicae were reared on leaves of non-mycorrhized (NM) or mycorrhized (AM) Plantago lanceolata plants at two time points, on young plants (with a minor established AM) and on older plants (with a well-established AM), respectively. Various performance traits were measured in the first aphid generation. Additionally, the body mass of the offspring was recorded to determine effects of previous infestation on the next generation. At the end of the herbivore experiments, plant carbon (C), nitrogen (N) and phosphorus (P) as well as leaf mass per area (LMA) were analyzed as measures of plant quality. Developmental performance traits of the aphids were either affected by AM and time point (nymph body mass at day 6) or by the interaction of both (relative growth rate). However, body mass at day 10 and reproductive performance traits were lower on older plants, independent of AM treatment. In line with these results, host plant quality changed little due to AM but strongly with age, with decreases in leaf N and P, but increases in C and LMA. Furthermore, nymphs gained a higher body mass when feeding on previously infested plants compared to their parents that started to feed on non-infested hosts, likely due to an aphid-induced modification in host traits. In summary, our results indicate that effects of both the developmental stage of AM and the plant age on aphid performance change throughout aphid development and between generations, attributable to shifts in plant quality.

Baculoviruses are food-borne microbial pathogens that are ingested by insects on contaminated foliage. Oxidation of plant-derived phenolics, activated by insect feeding, can directly interfere with infections in the gut. Since phenolic oxidation is an important component of plant resistance against insects, baculoviruses are suggested to be incompatible with plant defences. However, plants among and within species invest differently in a myriad of chemical and physical defences. Therefore, we hypothesized that among eight soybean genotypes, some genotypes would be able to maintain both high resistance against an insect pest and high efficacy of a baculovirus. Soybean constitutive (non-induced) and jasmonic acid (JA)-induced (anti-herbivore response) resistance was measured against the fall armyworm Spodoptera frugiperda (weight gain, leaf consumption and utilization). Indicators of phenolic oxidation were measured as foliar phenolic content and peroxidase activity. Levels of armyworm mortality inflicted by baculovirus (SfMNPV) did not vary among soybean genotypes when the virus was ingested with non-induced foliage. Ingestion of the virus on JA-induced foliage reduced armyworm mortality, relative to non-induced foliage, on some soybean genotypes. Baculovirus efficacy was lower when ingested with foliage that contained higher phenolic content and defensive properties that reduced armyworm weight gain and leaf utilization. However, soybean genotypes that defended the plant by reducing consumption rate and strongly deterred feeding upon JA-induction did not reduce baculovirus efficacy, indicating that these defences may be more compatible with baculoviruses to maximize plant protection. Differential compatibility of defence traits with the third trophic level highlights an important cost/trade-off associated with plant defence strategies.

Understanding the mechanisms by which abiotic drivers, such as climate and pollution, influence population dynamics of animals is important for our ability to predict the population trajectories of individual species under different global change scenarios. We monitored four leaf beetle species (Coleoptera: Chrysomelidae) feeding on willows (Salix spp.) in 13 sites along a pollution gradient in subarctic forests of north-western Russia from 1993 to 2014. During a subset of years, we also measured the impacts of natural enemies and host plant quality on the performance of one of these species, Chrysomela lapponica. Spring and fall temperatures increased by 2.5-3°C during the 21-year observation period, while emissions of sulfur dioxide and heavy metals from the nickel-copper smelter at Monchegorsk decreased fivefold. However, contrary to predictions of increasing herbivory with climate warming, and in spite of discovered increase in host plant quality with increase in temperatures, none of the beetle species became more abundant during the past 20years. No directional trends were observed in densities of either Phratora vitellinae or Plagiodera versicolora, whereas densities of both C. lapponica and Gonioctena pallida showed a simultaneous rapid 20-fold decline in the early 2000s, remaining at very low levels thereafter. Time series analysis and model selection indicated that these abrupt population declines were associated with decreases in aerial emissions from the smelter. Observed declines in the population densities of C. lapponica can be explained by increases in mortality from natural enemies due to the combined action of climate warming and declining pollution. This pattern suggests that at least in some tri-trophic systems, top-down factors override bottom-up effects and govern the impacts of environmental changes on insect herbivores.