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Plants protect themselves against pest attack utilizing both direct and indirect modes of defense. The direct mode of defense includes morphological, biochemical, and molecular barriers that affect feeding, growth, and survival of herbivores whereas the indirect mode of defense includes release of a blend of volatiles that attract natural enemies of the pests. Both of these strategies adopted by plants are reinforced if the plants are supplied with one of the most abundant metalloids, silicon (Si). Plants absorb Si as silicic acid (Si(OH)4) and accumulate it as phytoliths, which strengthens their physical defense. This deposition of Si in plant tissue is up-regulated upon pest attack. Further, Si deposited in the apoplast, suppresses pest effector molecules. Additionally, Si up-regulates the expression of defense-related genes and proteins and their activity and enhances the accumulation of secondary metabolites, boosting induced molecular and biochemical defenses. Moreover, Si plays a crucial role in phytohormone-mediated direct and indirect defense mechanisms. It is also involved in the reduction of harmful effects of oxidative stress resulting from herbivory by accelerating the scavenging process. Despite increasing evidence of its multiple roles in defense against pests, the practical implications of Si for crop protection have received less attention. Here, we highlight recent developments in Si-mediated improved plant resistance against pests and its significance for future use in crop improvement.

Drought stress reduces annual global wheat yields by 20%. Silicon (Si) fertilisation has been proposed to improve plant drought stress tolerance. However, it is currently unknown if and how Si affects different wheat landraces, especially with respect to their innate Si accumulation properties. In this study, significant and consistent differences in Si accumulation between landraces were identified, allowing for the classification of high Si accumulators and low Si accumulators. Landraces from the two accumulation groups were then used to investigate the effect of Si during osmotic and drought stress. Si was found to improve growth marginally in high Si accumulators during osmotic stress. However, no significant effect of Si on growth during drought stress was found. It was further found that osmotic stress decreased Si accumulation for all landraces whereas drought increased it. Overall, these results suggest that the beneficial effect of Si commonly reported in similar studies is not universal and that the application of Si fertiliser as a solution to agricultural drought stress requires detailed understanding of genotype-specific responses to Si.

Grasses accumulate large concentrations of silicon (Si) which alleviates a range of stresses including defence against herbivores. Likewise, grasses symbiotically associate with foliar Epichloë-fungal endophytes which provide herbivore defence, mainly via the production of alkaloids. Some Epichloë-endophytes increase foliar Si concentrations, particularly in tall fescue (Festuca arundinacea) but also in perennial ryegrass (Lolium perenne); it is unknown whether this impacts herbivores. Likewise, while Si is primarily a physical defence against herbivores, it can also affect defensive secondary metabolites; Si supply might therefore also affect alkaloids produced by Epichloë-endophytes, however, this remains untested. We grew tall fescue and perennial ryegrass in a factorial combination with or without Si supplementation, in the absence or presence of a chewing herbivore; Helicoverpa armigera. Grasses were associated with four different Epichloë-endophyte strains (tall fescue: AR584; perennial ryegrass: AR37, AR1, or wild type) or as Epichloë-free controls. Specifically, we assessed how Si supply and Epichloë-endophyte presence impacts plant growth and chemistry, and how their interaction with herbivory affects foliar Si concentrations and alkaloid production. Subsequently, their effects on H. armigera relative growth rates (RGR) were evaluated. In Fescue, the AR584-endophyte increased constitutive (herbivore-free) and induced (herbivore-inoculated) silicon concentrations when Si was supplied. In perennial ryegrass, AR37-endophyte increased constitutive and induced silicon concentration, meanwhile, AR1-endophyte increased constitutive levels only. Si supply and herbivory did not affect alkaloids produced by AR584- or AR1/Wt-endophyte in tall fescue and perennial ryegrass, respectively. However, Si suppressed herbivore-induced production of alkaloids in the AR37-endophyte perennial ryegrass association. Si was a more effective defence in tall fescue than perennial ryegrass, significantly reducing H. armigera RGR. Our results suggest that Si reduced herbivore performance to such an extent in tall fescue that it was operating at maximum effect and endophyte-mediated increases in Si concentration made no further difference. Si had a more modest impact on herbivores in perennial ryegrass, potentially linked to silicon decreasing herbivore feeding and thus, suppressing herbivore-induced alkaloids. We provide novel evidence that increased Si concentrations in some cases interact with endophyte-produced chemical defences, which could ultimately impact plant resistance to herbivores.

Plants deploy an arsenal of chemical and physical defenses against arthropod herbivores, but it may be most cost efficient to produce these only when attacked. Herbivory activates complex signaling pathways involving several phytohormones, including jasmonic acid (JA), which regulate production of defensive compounds. The Poaceae also have the capacity to take up large amounts of silicon (Si), which accumulates in plant tissues. Si accumulation has antiherbivore properties, but it is poorly understood how Si defenses relate to defense hormone signaling. Here we show that Si enrichment causes the model grass Brachypodium distachyon to show lower levels of JA induction when attacked by chewing herbivores. Triggering this hormone even at lower concentrations, however, prompts Si uptake and physical defenses (e.g., leaf hairs), which negatively impact chewing herbivores. Removal of leaf hairs restored performance. Crucially, activation of such Si-based defense is herbivore-specific and occurred only in response to chewing and not fluid-feeding (aphid) herbivores. This aligned with our meta-analysis of 88 studies that showed Si defenses were more effective against chewing herbivores than fluid feeders. Our results suggest integration between herbivore defenses in a model Si-accumulating plant, which potentially allows it to avoid unnecessary activation of other costly defenses.

Global climate change may increase invasions of exotic plant species by directly promoting the success of invasive/exotic species or by reducing the competitive abilities of native species. Changes in plant chemistry, leading to altered susceptibility to stress, could mediate these effects. Grasses are hyper-accumulators of silicon, which play a crucial function in the alleviation of diverse biotic and abiotic stresses. It is unknown how predicted increases in atmospheric carbon dioxide (CO2 ) and air temperature affect silicon accumulation in grasses, especially in relation to primary and secondary metabolites. We tested how elevated CO2 (eCO2 ) (+240 ppm) and temperature (eT) (+4°C) affected chemical composition (silicon, phenolics, carbon and nitrogen) and plant growth in eight grass species, either native or exotic to Australia. eCO2 increased phenolic concentrations by 11%, but caused silicon accumulation to decline by 12%. Moreover, declines in silicon occurred mainly in native species (-19%), but remained largely unchanged in exotic species. Conversely, eT increased silicon accumulation in native species (+19%) but decreased silicon accumulation in exotic species (-10%). Silicon and phenolic concentrations were negatively correlated with each other, potentially reflecting a defensive trade-off. Moreover, both defences were negatively correlated with plant mass, compatible with a growth-defence trade-off. Grasses responded in a species-specific manner, suggesting that the relative susceptibility of different species may differ under future climates compared to current species rankings of resource quality. For example, the native Microlaena stipoides was less well defended under eCO2 in terms of both phenolics and silicon, and thus could suffer greater vulnerability to herbivores. To our knowledge, this is the first demonstration of the impacts of eCO2 and eT on silicon accumulation in grasses. We speculate that the greater plasticity in silicon uptake shown by Australian native grasses may be partly a consequence of evolving in a low nutrient and seasonally arid environment.

Arbuscular mycorrhizal fungal (AMF) colonisation of plant roots is one of the most ancient and widespread interactions in ecology, yet the systemic consequences for plant secondary chemistry remain unclear. We performed the first metabolomic investigation into the impact of AMF colonisation by Rhizophagus irregularis on the chemical defences, spanning above- and below-ground tissues, in its host-plant ragwort (Senecio jacobaea). We used a non-targeted metabolomics approach to profile, and where possible identify, compounds induced by AMF colonisation in both roots and shoots. Metabolomics analyses revealed that 33 compounds were significantly increased in the root tissue of AMF colonised plants, including seven blumenols, plant-derived compounds known to be associated with AMF colonisation. One of these was a novel structure conjugated with a malonyl-sugar and uronic acid moiety, hitherto an unreported combination. Such structural modifications of blumenols could be significant for their previously reported functional roles associated with the establishment and maintenance of AM colonisation. Pyrrolizidine alkaloids (PAs), key anti-herbivore defence compounds in ragwort, dominated the metabolomic profiles of root and shoot extracts. Analyses of the metabolomic profiles revealed an increase in four PAs in roots (but not shoots) of AMF colonised plants, with the potential to protect colonised plants from below-ground organisms. Electronic supplementary material The online version of this article (10.1007/s10886-017-0921-1) contains supplementary material, which is available to authorized users.

Background Glucosinolates, anionic sulfur rich secondary metabolites, have been extensively studied because of their occurrence in the agriculturally important brassicaceae and their impact on human and animal health. There is also increasing interest in the biofumigant properties of toxic glucosinolate hydrolysis products as a method to control agricultural pests. Evaluating biofumigation potential requires rapid and accurate quantification of glucosinolates, but current commonly used methods of extraction prior to analysis involve a number of time consuming and hazardous steps; this study aimed to develop an improved method for glucosinolate extraction. Results Three methods previously used to extract glucosinolates from brassicaceae tissues, namely extraction in cold methanol, extraction in boiling methanol, and extraction in boiling water were compared across tissue type (root, stem leaf) and four brassicaceae species (B. juncea, S. alba, R. sativus, and E. sativa). Cold methanol extraction was shown to perform as well or better than all other tested methods for extraction of glucosinolates with the exception of glucoraphasatin in R. sativus shoots. It was also demonstrated that lyophilisation methods, routinely used during extraction to allow tissue disruption, can reduce final glucosinolate concentrations and that extracting from frozen wet tissue samples in cold 80% methanol is more effective. Conclusions We present a simplified method for extracting glucosinolates from plant tissues which does not require the use of a freeze drier or boiling methanol, and is therefore less hazardous, and more time and cost effective. The presented method has been shown to have comparable or improved glucosinolate extraction efficiency relative to the commonly used ISO method for major glucosinolates in the Brassicaceae species studied: sinigrin and gluconasturtiin in B. juncea; sinalbin, glucotropaeolin, and gluconasturtiin in S. alba; glucoraphenin and glucoraphasatin in R. sativus; and glucosatavin, glucoerucin and glucoraphanin in E. sativa. Electronic supplementary material The online version of this article (doi:10.1186/s13007-017-0164-8) contains supplementary material, which is available to authorized users.

The distinctive ecology of root herbivores, the complexity and diversity of root-microbe interactions, and the physical nature of the soil matrix mean that plant responses to root herbivory extrapolate poorly from our understanding of responses to aboveground herbivores. For example, root attack induces different changes in phytohormones to those in damaged leaves, including a lower but more potent burst of jasmonates in several plant species. Root secondary metabolite responses also differ markedly, although patterns between roots and shoots are harder to discern. Root defences must therefore be investigated in their own ecophysiological and evolutionary context, specifically one which incorporates root microbial symbionts and antagonists, if we are to better understand the battle between plants and their hidden herbivores.

Research investigating interactions between aboveground (AG) and below- ground (BG) herbivores has been central to characterizing AG-BG linkages in terrestrial ecosystems, with many of these interactions forming the basis of complex food webs spanning the two subsystems. Despite the growing literature on the effects of AG and BG herbivores on each other, underlying patterns have been difficult to identify due to a high degree of context dependency. In this study, we present the first quantitative meta-analysis of AG and BG herbivore interactions. Previous global predictions, specifically that BG herbivores normally promoted AG herbivore performance and AG herbivores normally reduced BG herbivore performance, were not supported. Instead, the meta-analysis identified four factors that determined the outcome of AG-BG interactions. (1) Sequence of herbivore arrival on host plants was important, with BG herbivores promoting AG herbivore performance only when introduced to the plant simultaneously, whereas AG herbivores had negative effects on BG herbivores only when introduced first. (2) AG herbivores negatively affected BG herbivore survival but tended to increase population growth rates. (3) AG herbivores negatively affected BG herbivore performance on annual plants, but not on perennials, and these effects were observed more consistently in laboratory than field studies. (4) The type of herbivore was also important, with BG insect herbivores belonging to the order Diptera (i.e., true flies) having the strongest negative effects on AG herbivores. Coleoptera (i.e., beetles) species were the most widely investigated BG herbivores and had positive impacts on AG Homoptera (e.g., aphids), but negative effects on AG Hymenoptera (e.g., sawflies). The strongest negative outcomes for BG herbivores were seen when the AG herbivore was a Coleoptera species. We found no evidence for publication bias in AG-BG herbivore interaction literature and conclude that several biological and experimental factors are important for predicting the outcome of AG- BG herbivore interactions. The sequence of herbivore arrival on the host plant was among the most influential.

Silica defences in grasses have recently been suggested to be a potential driver of vole population dynamics. However, the ability of grasses to induce silica in response to herbivory has not been tested in northern ecosystems where small rodents are important herbivores. We conducted a large-scale field experiment in subarctic tundra using three river catchments differing in herbivore densities, and examined the effects of small rodent and/or reindeer exclusion on leaf silica levels in five grass species (Avenella flexuosa, Anthoxanthum nipponicum, Calamagrostis phragmitoides, Deschampsia cespitosa and Phleum alpinum). We also conducted a greenhouse experiment using three of these species (A. flexuosa, A. nipponicum and D. cespitosa) and Festuca ovina to determine whether intraspecific genotypic variation affects baseline silica concentrations and the capacity to induce silica in response to simulated grazing. Baseline leaf silica concentrations and silica induction varied with plant species in both experiments, with catchment in the field experiment and with genotype in the greenhouse experiment. These findings show that the allocation to silica defences in grasses is highly variable, and suggest that the combined effects of grazing pressure, plant species and intraspecific genotypic differences are likely to determine the circumstances under which silica induction may be an optimal defence strategy. A better understanding of the interplay between grazing and other factors influencing silica induction is necessary to interpret the role of silica in plant–herbivore interactions.

Many insect herbivores and plant pathogens influence each other via plant-mediated mechanisms. Although there is speculation that these interactions may be important in structuring terrestrial food webs, few studies have empirically demonstrated the mechanisms by which pathogens manipulate the behaviour of their insect vectors. We investigated how infection of red raspberry, Rubus idaeus L. (Rosaceae), with two viral pathogens, black raspberry necrosis virus (BRNV) and raspberry leaf mottle virus (RLMV), affected the behaviour of their vector, the large raspberry aphid, Amphorophora idaei Borner (Hemiptera: Aphididae: Macrosiphini). As semi-persistently transmitted viruses, comparatively little is known about how such viruses affect vector biology. We also examined the effect of infection on plant volatile emissions and amino acid content, which could drive changes in aphid behaviour and performance. Virus-infected plants were initially more attractive to the aphid and the insects remained on infected plants for 30 min, but were found equally on uninfected plants 12 h after inoculation. Twenty-seven volatile compounds were identified. Two green leaf volatiles were emitted at higher concentrations by infected plants: (Z)-3-hexenyl acetate and 2-hexenal. In dose-response assays, (Z)-3-hexenyl acetate was attractive to the aphid at concentrations of 50 ng ml−1. When reared on infected plants, aphids took more than 3 days longer to reach adulthood compared with those on uninfected plants, although the number of offspring remained the same. Soluble amino acid (essential and non-essential) concentrations in raspberry leaves increased more than two-fold with virus infection. Amino acid composition was dominated by glutamate, accounting for 64 and 77% of the total in uninfected and infected leaves, respectively. Excessive glutamate may have underpinned the negative effects of viral infection on aphid performance. These results demonstrate the capacity of viruses to alter their host plant to manipulate vector behaviour, which may have evolved to be consistent with the transmission requirements of the virus.

The vine weevil Otiorhynchus sulcatus is a major pest of horticultural crops worldwide, with root-feeding larvae causing most damage. Adult oviposition aboveground may therefore influence levels of damage as the larvae are relatively immobile after oviposition. 2 The present study investigated feeding and oviposition behaviour on red raspberry Rubus idaeus using intact plants, ensuring that choices reflected the realistic differences in cultivar appearance and chemical composition. Previous studies investigating vine weevil feeding and oviposition on other crops have used excised plant material, which may inadvertently influence behaviour. 3 Adult weevils significantly preferred to feed on particular cultivars in the choice experiment (e.g. Tulameen), although they consumed significantly more foliage (0.22-1.03 cm 2 /day) on different raspberry cultivars (e.g. Glen Moy, Glen Rosa and a wild accession) in no-choice situations. 4 In choice experiments, weevils tended to avoid laying eggs on some cul- tivars (e.g. Glen Moy and the wild accession). The number of eggs laid (1.91-4.32 eggs per day) did not, however, differ significantly between the cul- tivars in a no-choice situation. Foliar nitrogen and magnesium concentrations were positively, although weakly, correlated with the total number of eggs laid. 5 The present study highlights the importance of considering both choice and no- choice tests when assessing crop susceptibility to attack because weevils may avoid feeding on certain cultivars (e.g. Glen Moy) when given a choice, although this would cause significant damage to such cultivars if they were grown in monoculture (i.e. when there is no alternative).

1. A substantial amount of research on host-plant selection by insect herbivores is focused around the preference–performance hypothesis (PPH). To date, the majority of studies have primarily considered insects with aboveground life cycles, overlooking insect herbivores that have both aboveground and belowground life stages, for which the PPH could be equally applicable. 2. This study investigated the factors influencing the performance of the root-feeding vine weevil (Otiorhynchus sulcatus) larvae and whether this was linked to the oviposition behaviour of the maternal adult living aboveground. 3. Maternal insects feeding aboveground reduced root biomass by 34% and increased root carbon by 4%. Larvae feeding on plants subjected to aboveground herbivory had reduced mass. Irrespective of the presence of maternal herbivory, larval mass was positively correlated with root biomass. 4. Larval mass was also reduced by conspecific larvae, previously feeding on roots (19% reduction). However, the mechanism underpinning this effect remains unclear, as in contrast to maternal herbivory aboveground, prior larval feeding did not significantly affect root biomass or root carbon concentrations. 5. Maternal insects did not distinguish between plants infested with larvae and those that were free of larvae, in terms of their egg-laying behaviour. Conversely, maternal insects tended to lay eggs on plants with smaller root systems, a behaviour that is likely to negatively affect offspring performance. 6. The PPH is not supported by our findings for the polyphagous vine weevil feeding on the host plant raspberry (Rubus idaeus), and in fact our results suggest that there is the potential for strong parent–offspring conflict in this system.

1. Individual plants are often simultaneously consumed by many different organisms and mediate important indirect interactions between their consumers, even when their consumers are phylogenetically distant and feed on different parts of the plant. 2. We examined the effects on two xylem feeders of sharing a plant host in greenhouse pot trials and outdoor mesocosms. The spittle bug Neophilaenus lineatus, which feeds on grass shoots, and a hemiparasitic angiosperm, Rhinanthus minor, which attaches to the roots, were added separately or together on grass host plants growing in pots or in model grassland swards. 3. We found strong asymmetric competition between the hemiparasite and the insect. In greenhouse experiments, sharing a grass host with R. minor significantly increased mortality of N. lineatus, but R. minor performance was unaffected. 4. Mortality of N. lineatus was also increased on swards containing R. minor in model communities grown in the field. R. minor was more sensitive to sharing a host with N. lineatus in these communities, and experienced a modest (non-significant) reduction in flower production. 5. The hemiparasite was the stronger competitor for xylem resources, possibly because of its ability to extract resources from the plant roots, and because of its significant ability to appropriate the xylem stream via its strong transpirational pull. 6. We believe these are the first experiments to demonstrate competition between insect herbivores and hemiparasitic plants mediated via a shared host plant. The consequences for the invertebrate population were potentially highly significant and we propose that future research should further explore the role of this asymmetric relationship in structuring invertebrate communities.

1. Euedaphic collembola alter their soil distribution in response to above-ground aphid herbivory of Poa annua L. Graminae, a host grass. 2. Two mechanisms potentially underpin this effect. Carbon-rich aphid honeydew falling onto the soil surface may affect mycophagous collembola; alternatively aphid-induced changes in root biomass may be necessary to produce changes in collembola abundance. 3. When compared to a plant-only control, aphid herbivory increased the number of collembola in the top 5 cm of soil, reduced both foliar and root biomass, and increased shoot/root ratio. Honeydew addition had no effect on collembola numbers or any recorded host-plant parameter. 4. Honeydew deposition is not responsible for the increased numbers of collembola found in the upper soil after aphid herbivory; aphid-induced reductions in root biomass may be the most important factor explaining knock-on effects of aphid herbivory on soil fauna.

1. Silica in the leaves of grasses can act as a defence against both vertebrate and invertebrate herbivores. The mechanisms by which silica affects herbivore performance are not well characterized. Here we expose an insect herbivore Spodoptera exempta to high-silica diets and test two mechanisms by which silica has been proposed to act as a defence. First, that silica reduces the digestibility of leaves and second, that silica causes wear to insect mandibles, both of which could potentially impact on herbivore performance. 2. Silica reduced the efficiency with which S. exempta converted ingested food to body mass and the amount of nitrogen absorbed from their food, leading to reduced insect growth rates. The measure of how efficiently herbivores utilize digested food (ECD) was unaffected by silica. 3. These effects occurred even with short-term exposure to silica-rich diets, but they also increased markedly with the duration of exposure and affected late instars more than early instar larvae. This appears to be due to the progressive impacts of silica with longer exposure times and suggests that herbivores cannot adapt to silica defences, nor do they develop a tolerance for silica with age. 4. Exposure to silica-rich diets caused increased mandible wear in S. exempta. This effect was extremely rapid, occurring within a single instar, further reducing feeding efficiency and growth rates. These effects on insect growth and feeding efficiency are nonreversible, persisting after the herbivore has switched diets. Up to a third of this residual impact can be explained by the degree of mandible wear caused by previous silica-rich diets. 5. The impacts of silica on S. exempta larvae were progressive with exposure time and could not be compensated for, even by switching to a different diet. Thus, herbivores cannot easily adapt to physical defences such as silica, suggesting this defence will have major implications for herbivore fitness.

Seedlings of five species of dipterocarp trees were planted in experimental plots in rain forest gaps in Sabah, Malaysia, and the rates of herbivory on their mature leaves recorded over 6 mo. A novel method was used to estimate the feeding pressure exerted by the local insect herbivore community, derived from the relative abundances of the dominant generalist herbivores and their feeding preferences. Characteristics of the leaves related to their defense and nutritional value were measured--phenolic content, laminar fracture toughness, laminar thickness, and nitrogen content. Three main groups of herbivorous insects were present--coleopteran and lepidopteran herbivores, which were sampled by hand from the seedlings, and orthopteran herbivores, which were sampled by sweep netting. The feeding preferences of the main coleopteran and orthopteran herbivores were determined using laboratory feeding trials. Combining variables in a Principal Components Analysis, a clear separation was found between the five seedling species along the first extracted component. This correlated closely with herbivory rates between species. The first extracted component comprised a negative influence of phenolic content and positive effects of nitrogen content, laminar fracture toughness, abundances of coleopteran and lepidopteran herbivores, and estimated feeding pressure of the coleopteran community. Further studies are required to determine the potential applications of the latter measure of estimated herbivore community impact.

Seedlings of five species in the Dipterocarpaceae were grown in experimental plots in Sabah, Malaysia. These were sited both in gaps and understorey and on alluvial and sandstone soils. Half of all seedlings were provided with a complete fertilizer. Herbivore damage levels were recorded on over 25 000 individual leaves in four surveys over the course of 2 y. Rates of herbivory were lower on mature leaves (0.07–0.8% leaf area mo−1 among species) than new leaves (2.1–4.4% leaf area mo−1). There were no overall effects of light conditions, soil type, fertilizer treatment or time on rates of herbivory. The only consistent source of variation was that between species, with the three alluvial-specialist species suffering higher rates of damage than the two sandstone-specialists. Mature leaves of alluvial species received greater damage in sandstone soils, whereas sandstone species were damaged at equivalent rates on both soil types. New leaves were more damaged on their native soil type. Published herbivory rates vary in the timescales and methods of measurement. Nevertheless, the few comparable studies confirm that herbivory rates on seedlings in tropical rain forests are remarkably constant over time and across experimental treatments.

1. Silica, deposited as opaline phytoliths in the leaves of grasses, constitutes 2-5% of dry leaf mass, yet its function remains unclear. It has been proposed that silica may act as an antiherbivore defence by increasing the abrasiveness and reducing the digestibility of grass leaves, although there is little direct experimental evidence to support this. 2. We investigated the effects of manipulated silica levels on the abrasiveness of the leaves of five grass species. We also examined the effects of silica levels on the feeding preferences, growth performance and digestion efficiency of two folivorous insects and one phloem-feeding insect. 3. Silica addition resulted in increases to leaf abrasiveness in four of the five grass species studied. Silica addition also deterred feeding by both folivores and reduced their growth rates and digestion efficiency. 4. These effects resulted in lower pupal mass of the lepidopteron larvae Spodoptera exempta and compensatory feeding by the orthopteran, Schistocerca gregaria. In contrast, silica had no effects on the feeding preference or the population growth of the phloem feeder, Sitobion avenae. 5. Our results demonstrate that silica is an effective defence against folivorous insects, both as a feeding deterrent, possibly mediated by increased abrasiveness, and as a digestibility reducer. The effects of silica on pupal mass and development time may impact on herbivore fitness and exposure to natural enemies. 6. These results are the first demonstration of a direct effect of silica on the abrasiveness of grasses and the adverse impact of silica on herbivore preference and performance.

Gender-related differences in growth and concentration of secondary metabolites have been documented in dioecious plants. Males usually grow faster than females, whilst females allocate more resources to reproduction and chemical defences than males, hence their growth is reduced. This hypothesis was tested on prickly juniper ( Juniperus oxycedrus macrocarpa ), a Mediterranean evergreen tree. The results showed that males grew faster than females but also had higher concentrations of both phenolics and terpenoids. However, neither phenolic nor terpenoid concentrations were correlated with growth in either sex, nor with a “reproductive effort” index in female trees. No browsing occurred on any of the trees sampled. Contrary to predictions, this study suggests that allocation of resources to reproduction in plants may reduce the resources available to both growth and secondary compounds. This might be particularly pronounced in plants with “expensive” fruits, such as prickly juniper, characterised by relatively large, long-lived fruits.

The induction of carbon-based secondary metabolites in leaves following damage has been proposed to be a result of a shift in the carbon:nutrient balance, when growth is limited by nutrients in relation to carbon. Here we test this hypothesis using seedlings of a tropical tree, Shorea leprosula (Dipterocarpaceae). In the short term, we analysed the phenolic content of leaves 7 d after damage on seedlings grown under differing light and nutrient treatments. In the long term, we examined the effect of nutrients, over 12 mo, on leaf phenolic concentration and seedling growth. In both the long and short term, levels of phenolics increased in damaged leaves under low nutrient treatments. No changes in leaf phenolics were detected under high nutrient regimes, or in the short term under low light. In addition, it was found that defoliation of seedlings in high-nutrient environments led to greater rates of leaf production than in undamaged seedlings, suggesting compensation.

1. The spatial and temporal abundance of the aphid Euceraphis betulae was investigated in relation to heterogeneity in host plant (Betula pendula) vigour and pathogenic stress. The performance of aphids feeding on vigorous and stressed foliage was also examined. 2. The plant stress and plant vigour hypotheses have been suggested as opposing ways in which foliage quality influences herbivore abundance. In many plants, however, vigorous growing foliage co-exists with stressed or damaged foliage. 3. There was a negative correlation between branch growth (vigour) and branch stress (leaf chlorosis), with the most vigorous branches displaying little or no stress, and the most stressed branches achieving poor growth. There was a similar negative correlation between vigour and stress at the level of individual trees, which themselves represented a continuum in quality. 4. At the beginning of the season, E. betulae were intermittently more abundant on vigorous branches than on branches destined to become stressed, but aphids became significantly more abundant on stressed branches later in the season, when symptoms of stress became apparent. Similar patterns of aphid abundance were seen on vigorous and stressed trees in the following year. 5. Euceraphis betulae performance was generally enhanced when feeding on naturally stressed B. pendula leaves, but there was some evidence for elevated potential reproduction when feeding on vigorous leaves too. 6. Overall, plant stress probably influences E. betulae distribution more than plant vigour, but the temporal and spatial variability in plant quality suggests that plant vigour could play a role in aphid distribution early in the season.

The role of indirect interactions in structuring communities is becoming increasingly recognised. Plant fungi can bring about changes in plant chemistry which may affect insect herbivores that share the same plant, and hence the two may interact indirectly. This study investigated the indirect effects of a fungal pathogen ( Marssonina betulae) of silver birch ( Betula pendula) on an aphid ( Euceraphis betulae), and the processes underpinning the interaction. There was a strong positive association between natural populations of the aphid and leaves bearing high fungal infection. In choice tests, significantly more aphids settled on leaves inoculated with the fungus than on asymptomatic leaves. Individual aphids reared on inoculated leaves were heavier, possessed longer hind tibiae and displayed enhanced embryo development compared with aphids reared on asymptomatic leaves; population growth rate was also positively correlated with fungal infection when groups of aphids were reared on inoculated branches. Changes in leaf chemistry were associated with fungal infection with inoculated leaves containing higher concentrations of free-amino acids. This may reflect a plant-initiated response to fungal attack in which free amino acids from the degradation of mesophyll cells are translocated out of infected leaves via the phloem. These changes in plant chemistry are similar to those occurring during leaf senescence, and are proposed as the mechanistic basis for the positive interaction between the fungus and aphid.

Summary 1 We conducted an experiment to investigate the effect of organic matter distribution on below-ground competition between the dwarf shrub Calluna vulgaris (L.) Hull and the coarse upland grass Nardus stricta (L.). 2 The two species were grown alone, or in competition in pots where the substrate was either a sand layer overlain by a peat layer (layered), or the same volume of peat evenly mixed with sand throughout the pot (mixed). 3 Root length and allocation in the top, middle and bottom of each pot was measured, along with plant biomass. 4 Calluna allocated the greatest proportion of root length to the top of the pot, irrespective of the peat distribution. Nardus allocated a large proportion of root length to the bottom of the pot. Root allocation by Nardus to the top and middle of the pot was positively related to the distribution of peat. 5 Nardus grown alone attained greater shoot mass in the layered than in the mixed substrate, but this was not the case when Calluna was present. Calluna was the superior competitor in both layered and mixed-substrate pots, but more so in the latter. 6 This was attributed to (i) the ability of Calluna to exclude Nardus roots from the upper organic material in the layered substrate, and (ii) the greater plasticity of root allocation of Nardus, enabling it to avoid competition with Calluna.

Summary 1This study examined the indirect impacts of leaf-mining insects, Eriocrania spp. Zeller (Lepidoptera: Eriocraniidae) on a phloem-feeding insect, Euceraphis betulae Koch (Homoptera: Drepanosiphinae). While many insect herbivores affect one another through changes to host plant chemical composition, Eriocrania also has the potential to affect E. betulae through structural modification of a shared leaf. 2Euceraphis betulae mortality was higher when caged on leaves with Eriocrania leaf-miners. Mortality was not affected by the amount of leaf mined or elevated phenolic compound concentrations in mined leaves, but leaf-miner induced damage to the midrib was strongly correlated with poor aphid survival. In field surveys, E. betulae was significantly less abundant on mined leaves with midrib damage than on mined leaves with just lamina damage, or mine-free leaves. 3Experiments simulating leaf-miner damage on B. pendula leaves pinpointed midrib damage as being associated with higher E. betulae mortality, whereas lamina damage had no effect on aphid mortality. Disruption of phloem hydraulics is proposed as the mechanism underpinning the negative impacts on the aphid. 4Eriocrania larvae mining leaves with manually damaged midribs weighed more than those in which the midrib was intact. There was also a trend towards higher nitrogen concentrations in leaves in which Eriocrania had damaged the midrib. There could therefore be a selective advantage to leaf-miners that damage the midrib if severance improves leaf nutritional quality, in addition to rendering the leaves unsuitable to potential competitors.

1. A potential host-mediated interaction between the birch-feeding aphid Euceraphis betulae Koch (Homoptera: Aphididae) and Operophtera brumata L. (Geometridae: Lepidoptera) was investigated, by measuring survival and feeding preference responses of E. betulae to leaves damaged recently by O. brumata larvae. 2. Euceraphis betulae survival was considerably lower on damaged foliage than on undamaged foliage, both in field experiments using cages and on potted saplings in laboratory tests. 3. Euceraphis betulae did not avoid damaged foliage in field experiments although they did avoid damaged foliage in laboratory preference tests using individual leaves. A link between aphid preference and performance on damaged foliage could therefore be demonstrated in the greenhouse/laboratory but not in the field. Multiple factors may influence feeding preference in the more complex field environment. 4. These results suggest that host-mediated competition may occur between O. brumata and E. betulae, especially in outbreak years when winter moth populations are very high.

Plants can have a significant impact on the fitness and efficacy of natural enemies. These interactions are widespread and suggest that the influences on the population dynamics of insect herbivores cannot be simply divided into "bottom up" and "top down". Several questions remain little studied in this field. Firstly, to what extent can plants affect the interactions between insects and their pathogens? Secondly, what are the effects of variation within natural enemy species on host/enemy/plant interactions? Finally, if plant/pathogen interactions can occur, do pathogens have increased fitness on the locally abundant food plant of their host? This study explored the influence of three host plant species of the polyphagous winter moth, Operophtera brumata, on infections caused by two geographic isolates of the winter moth nucleopolyhedrovirus (NPV) collected from distinct winter moth habitats. Insects were infected on excised leaf tissue of common oak, Quercus robur, Sitka spruce, Picea sitchenis, and heather, Calluna vulgaris. Parameters fundamental to the basic reproductive rate of the pathogen were estimated: these being infectivity, speed of kill and the yield of virus per insect. Leaf nitrogen and phenolic content were measured as indicators of host plant quality for the three plant species: oak had the highest levels of nitrogen and also the highest levels of phenolic compounds. Heather had higher levels of phenolic compounds than Sitka spruce. Host plant did not affect the infectivity of either isolate but insects that ingested virus on oak foliage died sooner and yielded more virus than insects that ingested virus on Sitka spruce or heather. The effect of host plant species on pathogen yield varied between the two isolates of the NPV but not as predicted by our adaptive hypothesis. The interactions between virus and food plant are discussed in relation to host and pathogen population dynamics.

Endophytic fungi live asymptomatically within plants. They are usually regarded as non-pathogenic or even mutualistic, but whether plants respond antagonistically to their presence remains unclear, particularly in the little-studied associations between endophytes and nong-raminoid herbaceous plants. \ud \ud We investigated the effects of the endophyte Chaetomium cochlioides on leaf chemistry in Cirsium arvense. Plants were sprayed with spores; leaf material from both subsequent new growth and the sprayed leaves was analysed 2 wk later. Infection frequency was 91% and63% for sprayed and new growth, respectively, indicating that C. cochlioides rapidly infects new foliage.\ud \ud Metabolomic analyses revealed marked changes in leaf chemistry with infection, especially in new growth. Changes in several novel oxylipin metabolites were detected, including arabi-dopsides reported here for the first time in a plant species other than Arabidopsis thaliana,and a jasmonate-containing galactolipid.\ud \ud The production of these metabolites in response to endophyte presence, particularly in newly infected foliage, suggests that endophytes elicit similar chemical responses in plants to those usually produced following wounding, herbivory and pathogen invasion. Whether en-dophytes benefit their hosts may depend on a complex series of chemically mediated interactions between the plant, the endophyte, other microbial colonists and natural enemies.

Summary • The relationship is examined here between Nardus stricta root length colonized (RLC) by arbuscular mycorrhizal (AM) fungi (% RLC) and vegetation composition in a heathland community. • Two approaches were taken: a field survey of AM colonization of Nardus at two contrasting locations (Glen Clunie and Glen Shee, UK) in relation to vegetation composition; and an 11-month bioassay of AM inoculum potential in Calluna vulgaris and Nardus swards using uncolonized Nardus transplants. • In the field survey, % RLC of Nardus was positively related to Nardus density. This relationship was strongest in Glen Clunie, probably because overall colonization levels were lower at the more fertile Glen Shee site. In the bioassay, Nardus transplants within Calluna swards developed little or no AM colonization, whereas those within Nardus swards had a mean RLC of 23%. • This is the first report that degree of AM colonization is related to host density in the field. Isolated Nardus plants in Calluna swards might be disadvantaged compared with Nardus plants in Nardus swards because of lower of AM colonization.

Summary Experiments were carried out to determine the effects of elevated atmospheric carbon dioxide (CO2) on phenolic biosynthesis in four plant species growing over three generations for nine months in a model plant community. Results were compared to those obtained when the same species were grown individually in pots in the same soils and controlled environment. In the model herbaceous plant community, only two of the four species showed any increase in biomass under elevated CO2, but this occurred only in the first generation for Spergula arvensis and in the second generation for Poa annua. Thus, the effects of CO2 on plant biomass and carbon and nitrogen content were species- and generation-specific. The activity of the principle phenolic biosynthetic enzyme, phenylalanine ammonia lyase (PAL), increased under elevated CO2 in Senecio vulgaris only in Generation 1, but increased in three of the four plant species in Generation 2. There were no changes in the total phenolic content of the plants, except for P. annua in Generation 1. Lignin content decreased under elevated CO2 in Cardamine hirsuta in Generation 1, but increased in Generation 2, whilst the lignin content of P. annua showed no change, decreased, then increased in response to elevated CO2 over the three generations. When the species were grown alone in pots, elevated CO2 increased PAL activity in plants grown in soil taken from the Ecotron community after nine months of plant growth, but not in plants grown in the soil used at the start of the experiment (‘initial' soil). In P. annua, phenolic biosynthesis decreased under elevated CO2 in initial soil, and in both P. annua and S. vulgaris there was a significant interaction between effects of soil type and CO2 level on PAL activity. In this study, plant chemical composition altered more in response to environmental factors such as soil type than in response to carbon supply. Results were species-specific and changed markedly between generations.

Some theories of plant allocation to defence predict that chemical or structural defences against herbivores should be maximised when browsing is most likely to occur. However, plants are constrained by a trade-off between growth and defence such that slow-growing plants usually have higher levels of secondary compounds, such as phenolics and tannins, than faster-growing ones. Thus, it is possible that the selection for increased yield and growth rate that occurs when plants are domesticated, may cause a reduction in allocation to these compounds. We tested this hypothesis using wild (Olea europaea L. var. sylvestris Brot.) and cultivated (O. europaea L. var. europaea) olive growing in an area with high densities of ungulates. In our study, olives outside fences excluding ungulates were heavily browsed. However, browsing induced an increase in the phenolic content of olives of both varieties in winter but not in spring. In spring, new leaves of both varieties had generally higher levels of phenolics and nitrogen than old leaves, but new leaves in both varieties exposed to browsing had a lower nitrogen content compared to controls. Browsing in both olive varieties caused leaf and shoot density to increase and leaf and shoot length to decrease, but in wild olives browsed shoots lost their leaves and became similar to spines. Structural responses to browsing occurred in spring during regrowth, whilst chemical changes were more obvious in winter, in both varieties. We suggest that olive may exhibit both morphological and chemical responses to browsing, depending on the different resource allocation priorities at different times of year. In spring, independently of browsing, cultivated olive had generally longer shoots and lower levels of phenolics than wild olive. We speculate that domestication may have selected for faster growth, at the expense of allocation to secondary compounds.

Summary 1. Competitive interactions between Eriocrania spp. Zeller (Lepidoptera: Eriocraniidae) leaf-miners and other folivorous guilds (free-living Lepidoptera and phloem feeding aphids) of silver birch (Betula pendula Roth) were examined experimentally. 2. Measurements of the foliage quality were taken from the experimental trees in order to discover whether variation in nitrogen and secondary compounds (specifically total phenolics), had an effect on the strength of any competitive interactions between the guilds. 3. There was evidence for indirect interactions within the leaf-mining guild mediated by the host plant: miners in aggregations had double the mortality of single miners. Interactions with leaf-chewing Lepidoptera were limited to interference and resource removal since only damage to the leaf being mined had an effect on the miner performance. There were no interactions between Eriocrania and aphids, despite the presence of aphids reducing the quantity of foliage available by 33%. 4. The strength of the competitive interactions, either within or between guilds, was not modified by the quality of the host trees. However, biomass of emerging larvae was negatively related to increased phenolic content of host trees and positively related to increased nitrogen content. Furthermore, larvae emerged later from trees where foliage had higher phenolic content. 5. The longevity of feeding events by competitive folivore guilds and the proximity of the feeding to the miners were identified as the key factors influencing the effects of foliage damage on the performance of Eriocrania.

1. Interactions between early season leaf-miners from the genus Eriocrania Zeller (Eriocraniidae: Lepidoptera), and the leaf-chewing guild of silver birch (Betula pendula Roth) were investigated. 2. Field observations indicated that a negative association between damage caused by Eriocrania, and leaf-chewers could be observed under certain conditions, namely at higher damage densities in the final week of mine development on trees with relatively high densities of mines. 3. Experimental manipulation of Operophtera brumata L. (Geometridae: Lepidoptera) densities in the field confirmed the negative association between established mines of Eriocrania, and this species of free-living folivore. 4. The behaviour of O. brumata was modified further in the presence of mines. On branches where mined leaves were present, O. brumata larvae consumed a larger proportion of leaf tissue, by continuing to feed on the same leaf longer rather than shifting to a new leaf. Increased consumption may indicate some modification of leaf quality beyond the leaves occupied by Eriocrania. Alternatively, it may indicate an association between Eriocrania, and higher quality leaf material.

Calluna vulgaris L. (Hull) is not one of the usual hosts of the winter moth, Operophtera brumata L., but outbreaks have caused extensive damage to heather moorland in Scotland in recent years. This study investigated the potential role of environmental change in such outbreaks by rearing O. brumata larvae on C. vulgaris plants grown in open-top chambers for 20 months with enriched CO2 (600 ppm) and nitrogen supply (average 52·5 kg N ha−1 yr−1) in factorial combination. This prolonged exposure to elevated CO2 caused no change in shoot growth, photosynthesis or foliar C∶N ratio of C. vulgaris, even with increased N supply, indicating that the absence of response was not due to N limitation. Increased N supply itself resulted in increased shoot growth and a decrease in tissue C∶N ratio. Phenolic content did not change in response to either CO2 or N enrichment, contrary to the predictions of the carbon/nutrient balance hypothesis. In line with the absence of plant response, there was no effect of CO2 on the development of Operophtera brumata on C. vulgaris, and so continued increase in atmospheric CO2 concentration is unlikely to affect directly O. brumata outbreaks on heather moorland. Operophtera brumata showed increased larval development, growth rate and pupal weight on N-treated plants, correlated both to the decrease in foliar C∶N ratio, and to the increase in shoot extension which was predictive of survivorship. Thus, increased atmospheric N deposition, or increased rates of mineralization in a warmer environment, might increase the severity of O. brumata outbreaks on C. vulgaris. Since the combination of high N availability and disturbance of heather canopy by herbivory is known to result in increased dominance of grasses, it is suggested that this could lead to further degradation of moorland in upland Britain.

Three year-old Sitka spruce saplings, which had received one of two levels of fertiliser addition (high or low) in one year, were either browsed by red deer or left undamaged in a subsequent year. After a further year of growth, the selection and browsing behaviour of red deer on damaged and undamaged trees were studied in a controlled experiment. Morphological and chemical analysis of ten trees from each treatment combination showed that damaged trees were significantly smaller after one year than undamaged trees but had longer needles. Chemical composition was not affected by browsing. High fertiliser addition increased most size parameters and increased the nutritive value of trees. The preference for previously damaged and undamaged trees was compared in a series of thirty two trials. On eight separate occasions, four red deer hinds were individually offered the choice of two damaged and two undamaged trees in an experimental enclosure. During each trial, biomass removed, number of bites removed, duration of browsing, intake rate, and mean bite weight at each tree were recorded. Biomass removed, bites taken and time spent at trees were significantly greater for previously undamaged trees than for damaged trees. All three variables were also positively related to the height of each tree and the number of lateral branches on each tree. The greater foraging effort on undamaged trees was not fully accounted for by the morphology of trees or their chemical composition. It is suggested that increased needle length caused animals to stop feeding sooner and hence led to less browsing damage in previously damaged trees. The results imply that previously observed localised browsing damage in forestry plantations is unlikely to be the result of changes to the plant induced by previous browsing damage.

The chemical composition of galled and ungalled plant tissue was compared in a series of experiments. Gall and adjacent plant tissue was analysed for 20 species of gall-former on 11 different plant species. There were clear differences between galled and ungalled tissue in levels of nutrients and secondary compounds. Gall tissue generally contained lower levels of nitrogen and higher levels of phenolic compounds than ungalled plant tissue. The gall tissue produced by the same plant in response to different species of gall-former differed in chemical composition, as did the gall-tissue from young and mature galls of the same species. The chemical differences between gall and plant tissues were studied in more detail in two field manipulations. Firstly, the seasonal changes in phenolic biosynthesis in Pontania proxima and P. pedunculi (Hymenoptera: Tenthredinidae) gall tissue were compared to those of their host plants, Salix alba and S. caprea. In both types of gall tissue, phenolic levels declined as the season progressed, but levels in the surrounding plant tissue increased. When the gall insects were killed with insecticide, phenolic levels in the galled tissue dropped to the same level as those in adjacent plant tissue. Secondly, the density of Cynips divisa (Hymenoptera: Cynipidae) galls on Quercus robur leaves was reduced by removing half the galls present, either those from the central region of the leaf or those from the edge. Decreasing gall density increased the size of the remaining galls and the weight of the insects, but these effects were most marked when the galls remaining were growing centrally on the leaf, i.e. when the galls from the edge had been removed. Decreasing gall density increased the nitrogen content of the remaining galls, again to a greater extent in galls growing centrally on the leaf. The results of these studies suggest that the levels of nutrients and secondary compounds in gall tissue are usually markedly different to those of surrounding plant tissue, and that gall-formers may produce species-specific and temporally variable changes in the chemical composition of gall tissue.

Grasses are becoming more abundant in areas in NE Scotland which until recently were dominated by heather (Calluna vulgaris). However, it is not clear if grasses are aggressive competitors which are now able to outcompete the dwarf shrub due to changes in environmental factors (such as grazing pressure and increasing nutrient inputs), or just opportunistic invaders, occupying gaps in the canopy which occur when heather reaches the degenerate stage. Experiments in turves and in field plots were carried out in order to investigate the performance of three grass species, Nardus stricta, Deschampsia cespitosa and Deschampsia flexuosa growing in competition with heather. These three species were selected because they differ in their nutrient requirements, palatability to herbivores and tolerance of shading. The grasses were planted in heather canopies of different structure, either turves of heather of different height and age, or moorland plots with or without heavy grazing by sheep and deer. Fertiliser (NPK) was applied to half the experimental plants. The growth of the grass species and the heather in response to the fertiliser and grazing treatments was measured, together with the light levels penetrating the canopy and received by the grass plants.

Summary 1. In addition to exhibiting preferences for particular plant species, vertebrate herbivores select particular individuals of these species whilst leaving others undamaged. This pattern of diet selection may reflect differences in the chemical composition (and hence nutritional quality) between individual plants, and/or variability in the physical constraints on intake rate, such as plant structure. 2. An experiment was conducted to test the effects of environmental manipulations on the morphology and chemical composition of Sitka Spruce saplings, and to evaluate the consequences for herbivory by Red Deer. Fertilizing the trees increased tree height, branch span and leader length, twig width and needle width, and decreased the concentrations of total phenolics, condensed tannins, fibre and lignin but monoterpene content was not altered. Shading also reduced phenolic and tannin concentrations. 3. When the fertilized and shaded trees were offered to deer in feeding trials, the probability of a tree being visited by a deer and the biomass removed were influenced by tree morphological variables, as were the bite rate and intake rate of the deer. More biomass was removed from larger trees. 4. Once the effects of tree morphology had been taken into account, there was no effect of the fertilizer and shade treatments on deer browsing behaviour that could be attributed to changes in chemical composition of the trees. The relationship between intake rate and bite size at each tree varied between individual deer, but the functional response relationship between intake rate and bite size was not influenced by the treatments applied to the trees. 5. Tree morphology may have a larger influence on deer feeding behaviour than tree chemical composition.

This study aimed to establish the relevance of the carbon: nutrient balance hypothesis (CNB) to the browsing behaviour of red deer on Sitka spruce. The responses of Sitka spruce saplings to the four factorial combinations of neutral shade/no shade and high/low fertilizer application were examined. The effects of the resultant chemical and growth responses of the trees on feeding behaviour by red deer hinds were then experimentally tested. As predicted by CNB, high soil nutrient levels increased a range of growth characteristics and total nitrogen concentrations of shoots whilst the levels of condensed tannins, and fibre components were reduced. These growth effects were depressed by the simultaneous application of shade. In contrast, there were no treatment effects on leaf monoterpene content. In 36 trials, hinds were offered 16 saplings deriving from combinations of two of the four treatments. All six combinations of pairs of treatments were tested on each of six hinds. Larger trees or those with more lateral branches were more likely to be browsed by deer, were browsed for longer and suffered greater offtake than smaller or less bushy trees. Contrary to expectation, addition of the shade and fertilizer treatment into the regression model did not explain any additional variation in the probability of attack, offtake from trees or time spent at trees. In a further experiment to test the effects of the volatile monoterpenes on feeding behaviour, greater tree size was again associated with higher probability of attack while chemical parameters had no effect. However, those trees with greater leaf monoterpene concentrations suffered less offtake by deer when browsed, suggesting that these compounds only affect feeding behaviour after the tree was attacked. The hinds' prior assessment of trees as a food patch appeared to be limited to morphological assessment. Although several chemical constituents of Sitka spruce responded to shade and fertilizer according to the CNB predictions, no effect of these treatments on deer feeding behaviour was detected. In contrast, monoterpenes did not obey CNB predictions but had a negative effect on foraging behaviour. Whilst CNB may be able to predict some aspects of trees' secondary chemistry, these responses may not necessarily influence herbivores' browsing behaviour.