Leaf Colour as a Signal of Chemical Defence to Insect Herbivores in Wild Cabbage (Brassica oleracea)

Leaf colour has been proposed to signal levels of host defence to insect herbivores, but we lack data on herbivory, leaf colour and levels of defence for wild host populations necessary to test this hypothesis. Such a test requires measurements of leaf spectra as they would be sensed by herbivore visual systems, as well as simultaneous measurements of chemical defences and herbivore responses to leaf colour in natural host-herbivore populations. In a large-scale field survey of wild cabbage (Brassica oleracea) populations, we show that variation in leaf colour and brightness, measured according to herbivore spectral sensitivities, predicts both levels of chemical defences (glucosinolates) and abundance of specialist lepidopteran (Pieris rapae) and hemipteran (Brevicoryne brassicae) herbivores. In subsequent experiments, P. rapae larvae achieved faster growth and greater pupal mass when feeding on plants with bluer leaves, which contained lower levels of aliphatic glucosinolates. Glucosinolate-mediated effects on larval performance may thus contribute to the association between P. rapae herbivory and leaf colour observed in the field. However, preference tests found no evidence that adult butterflies selected host plants based on leaf coloration. In the field, B. brassicae abundance varied with leaf brightness but greenhouse experiments were unable to identify any effects of brightness on aphid preference or performance. Our findings suggest that although leaf colour reflects both levels of host defences and herbivore abundance in the field, the ability of herbivores to respond to colour signals may be limited, even in species where performance is correlated with leaf colour.     

Plant stages with biotic,indirect defences are more palatable and suffer less herbivory than their undefended counterparts

Plants have evolved several anti‐herbivory strategies, including direct defences, such as mechanical and chemical defences, and indirect or biotic defences, such as the recruitment of defending animals. We examined whether the investment plants make in direct defences differs between those which do and do not invest in biotic defences, by comparing standing herbivory and palatability of congeneric species with and without indirect defences at two ontogenetic stages: before and after the onset of indirect defences. We used Cordia alliodora and Croton suberosus as the species with indirect defences and Cordia elaeagnoides and Croton pseudoniveus as the species without indirect defences. We predicted that herbivores would prefer to eat species and stages with indirect defences to those without them. As predicted, we found that herbivores preferred species and ontogenetic stages with indirect defences in all cases. Overall, however, natural levels of herbivory were lower in species with indirect defences. We conclude that indirect defences offer effective protection against herbivores and posit that their recruitment allows plants to reduce investment in other defence mechanisms. Our results support the notion that plants trade‐off between direct and indirect defensive strategies. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 536–543.     

Leaf mines as visual defensive signals to herbivores

Leaf‐mining insects produce conspicuous and distinct leaf mines on various types of plant leaves. The diversity of leaf‐mine morphology has typically been explained by several factors, such as selective feeding on plant tissues, improvement of microclimate, faecal disposal, reduction in the efficiency of parasitoid search behaviour and leafminer phylogeny. Although these factors are certainly associated with mining patterns, masking the mines, rather than making them conspicuous, appears to be more advantageous for deterring parasitoids and predators of leafminers. However, here, I propose that prominent leaf mines may serve to signal or cue herbivores to avoid feeding on the mined leaves. Because most leafminers are sessile and complete their development within a single leaf, herbivory of mined leaves is detrimental to leafminer survival. Other herbivores appear to avoid consuming mined leaves for a variety of reasons: leaf mines mimic leaf variegation or mottling; mined leaves induce chemical and physical defences against herbivores; and leaf mines mimic fungal infection, animal excrement, and necrosed plant tissues. Hence, natural selection may have favoured leafminers that produce conspicuous mines because of the increased survival and fecundity of thereby reducing herbivory on mined leaves.     

Elm leaves ‘warned’ by insect egg deposition reduce survival of hatching larvae by a shift in their quantitative leaf metabolite pattern

Plants may take insect eggs on their leaves as a warning of future herbivory and intensify their defence against feeding larvae. Responsible agents are, however, largely unknown, and little knowledge is available on this phenomenon in perennial plants. We investigated how egg deposition affects the anti‐herbivore defence of elm against the multivoltine elm leaf beetle. Prior egg deposition caused changes in the quality of feeding‐damaged leaves that resulted in increased larval mortality and reduced reproductive capacity of the herbivore by harming especially female larvae. Chemical analyses of primary and secondary leaf metabolites in feeding‐damaged, egg‐free (F) and feeding‐damaged, egg‐deposited (EF)‐leaves revealed only small differences in concentrations when comparing metabolites singly. However, a pattern‐focused analysis showed clearly separable patterns of (F) and (EF)‐leaves because of concentration differences in especially nitrogen and phenolics, of which robinin was consumed in greater amounts by larvae on (EF) than on (F)‐leaves. Our study shows that insect egg deposition mediates a shift in the quantitative nutritional pattern of feeding‐damaged leaves, and thus might limit the herbivore's population growth by reducing the number of especially female herbivores. This may be a strategy that pays off in a long run particularly in perennial plants against multivoltine herbivores.     

Within‐plant distribution of 1,4‐benzoxazin‐3‐ones contributes to herbivore niche differentiation in maize

Plant defences vary in space and time, which may translate into specific herbivore‐foraging patterns and feeding niche differentiation. To date, little is known about the effect of secondary metabolite patterning on within‐plant herbivore foraging. We investigated how variation in the major maize secondary metabolites, 1,4‐benzoxazin‐3‐one derivatives (BXDs), affects the foraging behaviour of two leaf‐chewing herbivores. BXD levels varied substantially within plants. Older leaves had higher levels of constitutive BXDs while younger leaves were consistently more inducible. These differences were observed independently of plant age, even though the concentrations of most BXDs declined markedly in older plants. Larvae of the well‐adapted maize pest Spodoptera frugiperda preferred and grew better on young inducible leaves irrespective of plant age, while larvae of the generalist Spodoptera littoralis preferred and tended to grow better on old leaves. In BXD‐free mutants, the differences in herbivore weight gain between old and young leaves were absent for both species, and leaf preferences of S. frugiperda were attenuated. In contrast, S. littoralis foraging patterns were not affected. In summary, our study shows that plant secondary metabolites differentially affect performance and foraging of adapted and non‐adapted herbivores and thereby likely contribute to feeding niche differentiation.     

Chrysanthemum expressing a linalool synthase gene ‘smells good’, but ‘tastes bad’ to western flower thrips

Herbivore‐induced plant volatiles are often involved in direct and indirect plant defence against herbivores. Linalool is a common floral scent and found to be released from leaves by many plants after herbivore attack. In this study, a linalool/nerolidol synthase, FaNES1, was overexpressed in the plastids of chrysanthemum plants (Chrysanthemum morifolium). The volatiles of FaNES1 chrysanthemum leaves were strongly dominated by linalool, but they also emitted small amount of the C11‐homoterpene, (3E)‐4,8‐dimethyl‐1,3,7‐nonatriene, a derivative of nerolidol. Four nonvolatile linalool glycosides in methanolic extracts were found to be significantly increased in the leaves of FaNES1 plants compared to wild‐type plants. They were putatively identified by LC‐MS‐MS as two linalool–malonyl–hexoses, a linalool–pentose–hexose and a glycoside of hydroxy–linalool. A leaf‐disc dual‐choice assay with western flower thrips (WFT, Frankliniella occidentalis) showed, initially during the first 15 min of WFT release, that FaNES1 plants were significantly preferred. This gradually reversed into significant preference for the control, however, at 20–28 h after WFT release. The initial preference was shown to be based on the linalool odour of FaNES1 plants by olfactory dual‐choice assays using paper discs emitting pure linalool at similar rates as leaf discs. The reversal of preference into deterrence could be explained by the initial nonvolatile composition of the FaNES1 plants, as methanolic extracts were less preferred by WFT. Considering the common occurrence of linalool and its glycosides in plant tissues, it suggests that plants may balance attractive fragrance with ‘poor taste’ using the same precursor compound.     

Gone with the wind: trembling leaves may deter herbivory

Plants employ various defensive tactics against herbivores but are rarely considered to use rapid movements to resist predation. However, the aboveground parts of plants are often forcefully moved by wind and rain. This passive movement has been overlooked as an anti‐herbivore trait. The leaves of many plant species, such as aspens, Indian sacred fig, bamboos, and palms, tremble even in a slight breeze. Leaves that are easily moved by gentle winds can sometimes resist strong winds and may have other benefits as well. In the present study, it is proposed that the movement of such plant leaves physically deters arthropod herbivory and pathogen infection by repelling colonization and oviposition by herbivorous insects. This leads to herbivores and pathogens being dislodged from the plants, and the ensuing death of the herbivores on the ground or at least their recolonization to other plants, as well as the interruption of feeding, intraspecific communication and the mating behaviour of herbivores, thus lowering their performance on the plant or increasing enemy attack of the herbivores. In addition, passive leaf movements may undermine herbivore camouflage and expose them to predation, and may also allow plant volatiles to diffuse efficiently to repel herbivores and attract natural enemies. Thus, the mechanistic properties of these leaves may have anti‐herbivore effects in the wind and rain. This hypothesis can also be applied to aquatic plants that tremble in gentle water currents. In addition, genetic manipulation of the tendency for leaf movement may be beneficial for the management of pest insects and pathogens with reduced pesticides in forestry and agriculture. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 738–747.     

Impacts of urbanization on insect herbivory and plant defences in oak trees

Systematic comparisons of species interactions in urban versus rural environments can improve our understanding of shifts in ecological processes due to urbanization. However, such studies are relatively uncommon and the mechanisms driving urbanization effects on species interactions (e.g. between plants and insect herbivores) remain elusive. Here we investigated the effects of urbanization on leaf herbivory by insect chewers and miners associated with the English oak Quercus robur by sampling trees in rural and urban areas throughout most of the latitudinal distribution of this species. In performing these comparisons, we also controlled for the size of the urban areas (18 cities) and gathered data on CO₂ emissions. In addition, we assessed whether urbanization affected leaf chemical defences (phenolic compounds) and nutritional traits (phosphorus and nitrogen), and whether such changes correlated with herbivory levels. Urbanization significantly reduced leaf chewer damage but did not affect leaf miners. In addition, we found that leaves from urban locations had lower levels of chemical defences (condensed and hydrolysable tannins) and higher levels of nutrients (nitrogen and phosphorus) compared to leaves in rural locations. The magnitude of urbanization effects on herbivory and leaf defences was not contingent upon city size. Importantly, while the effects of urbanization on chemical defences were associated with CO₂ emissions, changes in leaf chewer damage were not associated with either leaf traits or CO₂ levels. These results suggest that effects of urbanization on herbivory occur through mechanisms other than changes in the plant traits measured here. Overall, our simultaneous assessment of insect herbivory, plant traits and abiotic correlates advances our understanding of the main drivers of urbanization effects on plant–herbivore interactions.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

A long‐standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long‐lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant–herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.     

‘Branching’ with complex coarse woody debris reduces herbivory on recovering erosion scalds

Coarse woody debris (CWD) has many benefits in restoration, including protecting seedlings from herbivory, and it is often reintroduced where CWD is depauperate. At Calperum Station on the Murray River floodplain, artificially applied CWD (‘branching’) is being trialled to restore erosion scalds. At two erosion scalds, we applied 900 m² of branching and created a 900 m² branching‐free control. We predicted that branching would protect seedlings from browsing by mammalian herbivores. We planted Nitre Goosefoot (Chenopodium nitrariaceum) seedlings in each treatment and measured the number of terminal ends affected by herbivory and plant height for 8 weeks. Branching reduced the number of terminal ends affected by herbivory by ~80% and plant height loss by ~60%. Remote cameras detected some browsing by kangaroos, the most abundant herbivores on the Calperum floodplain, although cameras were too few to directly observe much of the browsing that occurred. We did not detect an effect of site browsing pressure on overall herbivory or the effectiveness of the branching. Despite the small sample size, our results suggest that branching can effectively reduce herbivory. It is likely that the effectiveness of branching in reducing herbivory is conditional on a range of factors, such as browsing pressure (i.e. scat density), so a large‐scale trial is required to clarify the role branching has on reducing herbivory in relation to these complex interactions.     

Plant defence responses to volatile alert signals are population‐specific

Herbivore‐induced volatiles are widespread in plants. They can serve as alert signals that enable neighbouring leaves and plants to pre‐emptively increase defences and avoid herbivory damage. However, our understanding of the factors mediating volatile organic compound (VOC) signal interpretation by receiver plants and the degree to which multiple herbivores affect VOC signals is still limited. Here we investigated whether plant responses to damage‐induced VOC signals were population specific. As a secondary goal, we tested for interference in signal production or reception when plants were subjected to multiple types of herbivore damage. We factorially crossed the population sources of paired Phaseolus lunatus plants (same versus different population sources) with a mechanical damage treatment to one member of the pair (i.e. the VOC emitter, damaged versus control), and we measured herbivore damage to the other plant (the VOC receiver) in the field. Prior to the experiment, both emitter and receiver plants were naturally colonized by aphids, enabling us to test the hypothesis that damage from sap‐feeding herbivores interferes with VOC communication by including emitter and receiver aphid abundances as covariates in our analyses. One week after mechanical leaf damage, we removed all the emitter plants from the field and conducted fortnightly surveys of leaf herbivory. We found evidence that receiver plants responded using population‐specific ‘dialects’ where only receivers from the same source population as the damaged emitters suffered less leaf damage upon exposure to the volatile signals. We also found that the abundance of aphids on both emitter and receiver plants did not alter this volatile signalling during both production and reception despite well‐documented defence crosstalk within individual plants that are simultaneously attacked by multiple herbivores. Overall, these results show that plant communication is highly sensitive to genetic relatedness between emitter and receiver plants and that communication is resilient to herbivore co‐infestation.     

Evolutionary irony: evidence that ‘defensive’ plant spines act as a proximate cue to attract a mammalian herbivore

Many plants produce structural defenses to deter feeding by herbivores. However, many previous studies testing whether spines are effective at defending against mammalian herbivores have produced equivocal results. These ambiguous results are hypothesized to be due to herbivore counter‐adaptations. We investigated potential counter‐adaptations in a population of white‐throated woodrats Neotoma albigua that specialize on cactus by investigating feeding behavior and preference for cacti varying in spinescence. Neotoma albigula exhibited a unique behavior of clipping cactus spines, which renders these defenses ineffective. Strikingly, these woodrats chose to collect spiny cacti over experimentally de‐spined cacti, demonstrating that spines act as a proximal cue that attracts woodrats. This attraction is likely due to the higher protein and lower fiber content of spiny cacti compared to naturally non‐spiny cacti. Thus, the ‘defensive’ spines of cacti are ineffective against a specialist herbivore and instead serve as an indicator of nutritional quality that promotes herbivory. Our results support the ‘rule‐of‐thumb’ hypothesis of foraging, which states that herbivores forage according to obvious visual cues that are indicative of nutritional content, rather than sampling nutrient composition of plants. We propose that specialist herbivores are unique systems in which to study other counter‐adaptations to structural defenses and ‘rule‐of‐thumb’ foraging strategies.     

Defensive masquerade by plants

The defensive strategy known as masquerade, or camouflage without crypsis (a type of deception that partly overlaps mimicry) has received little scientific attention in animals, and concerning plants even less. Moreover, when cases of masquerade were described in plants, they were considered as camouflage or other types of defence through mimicry. Masquerade (including in plants) may operate not only through vision, but also via other senses. Here I review several types of published cases of masquerade in plants, although they were not defined as such when published, and propose that there are two different types of masquerade in plants: (1) non‐plant‐mimicking defensive masquerade, in which they look (or smell) like uninteresting objects to herbivores (look like a stone or an animal, or smell like droppings or carrion, etc.), and (2) plant‐mimicking defensive masquerade, in which plants or plant parts do not look appealing for herbivores, not being green, looking dead or old, harbouring insects, already attacked, less nutritious, etc. Defensive masquerade by plants may in many cases be non‐exclusive, but serve additional physiological and defensive functions or operate simultaneously with other defences. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 1162–1166.     

Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community

We tested the hypothesis that selective feeding by insect herbivores in an old‐field plant community induces a shift of community structure towards less palatable plant species with lower leaf and litter tissue quality and may therefore affect nutrient cycling. Leaf palatability of 20 herbaceous plant species which are common during the early successional stages of an old‐field plant community was assayed using the generalist herbivores Deroceras reticulatum (Mollusca: Agriolomacidae) and Acheta domesticus (Ensifera: Gryllidae). Palatability was positively correlated with nitrogen content, specific leaf area and water content of leaves and negatively correlated with leaf carbon content and leaf C/N‐ratio. Specific decomposition rates were assessed in a litter bag experiment. Decomposition was positively correlated with nitrogen content of litter, specific leaf area and water content of living leaves and negatively correlated with leaf C/N‐ratio. When using phylogentically independent contrasts the correlations between palatability and decomposition versus leaf and litter traits remained significant (except for specific leaf area) and may therefore reflect functional relationships. As palatability and decomposition show similar correlations to leaf and litter traits, the correlation between leaf palatability and litter decomposition rate was also significant, and this held even in a phylogenetically controlled analysis. This correlation highlights the possible effects of invertebrate herbivory on resource dynamics. In a two‐year experiment we reduced the density of above‐ground and below‐ground insect herbivores in an early successional old‐field community in a two‐factorial design by insecticide application. The palatability ranking of plants showed no relationship with the specific change of cover abundance of plants due to the reduction of above‐ or below‐ground herbivory. Thus, changes in the dominance structure as well as potentially associated changes in the resource dynamics are not the result of differences in palatability between plant species. This highlights fundamental differences between the effects of insect herbivory on ecosystems and published results from vertebrate‐grazing systems.     

Cascading effects of induced terrestrial plant defences on aquatic and terrestrial ecosystem function

Herbivores induce plants to undergo diverse processes that minimize costs to the plant, such as producing defences to deter herbivory or reallocating limited resources to inaccessible portions of the plant. Yet most plant tissue is consumed by decomposers, not herbivores, and these defensive processes aimed to deter herbivores may alter plant tissue even after detachment from the plant. All consumers value nutrients, but plants also require these nutrients for primary functions and defensive processes. We experimentally simulated herbivory with and without nutrient additions on red alder (Alnus rubra), which supplies the majority of leaf litter for many rivers in western North America. Simulated herbivory induced a defence response with cascading effects: terrestrial herbivores and aquatic decomposers fed less on leaves from stressed trees. This effect was context dependent: leaves from fertilized-only trees decomposed most rapidly while leaves from fertilized trees receiving the herbivory treatment decomposed least, suggesting plants funnelled a nutritionally valuable resource into enhanced defence. One component of the defence response was a decrease in leaf nitrogen leading to elevated carbon : nitrogen. Aquatic decomposers prefer leaves naturally low in C : N and this altered nutrient profile largely explains the lower rate of aquatic decomposition. Furthermore, terrestrial soil decomposers were unaffected by either treatment but did show a preference for local and nitrogen-rich leaves. Our study illustrates the ecological implications of terrestrial herbivory and these findings demonstrate that the effects of selection caused by terrestrial herbivory in one ecosystem can indirectly shape the structure of other ecosystems through ecological fluxes across boundaries.     

Above‐ and belowground herbivory jointly impact defense and seed dispersal traits in Taraxacum officinale

Plants are able to cope with herbivores by inducing defensive traits or growth responses that allow them to reduce or avoid the impact of herbivores. Since above‐ and belowground herbivores differ substantially in life‐history traits, for example feeding types, and their spatial distribution, it is likely that they induce different responses in plants. Moreover, strong interactive effects on defense and plant growth are expected when above‐ and belowground herbivores are jointly present. The strengths and directions of these responses have been scarcely addressed in the literature. Using Taraxacum officinale, the root‐feeding nematode Meloidogyne hapla and the locust Schistocerca gregaria as a model species, we examined to what degree above‐ and belowground herbivory affect (1) plant growth responses, (2) the induction of plant defensive traits, that is, leaf trichomes, and (3) changes in dispersal‐related seed traits and seed germination. We compared the performance of plants originating from different populations to address whether plant responses are conserved across putative different genotypes. Overall, aboveground herbivory resulted in increased plant biomass. Root herbivory had no effect on plant growth. Plants exposed to the two herbivores showed fewer leaf trichomes than plants challenged only by one herbivore and consequently experienced greater aboveground herbivory. In addition, herbivory had effects that reached beyond the individual plant by modifying seed morphology, producing seeds with longer pappus, and germination success.     

A surface microrelief on the leaves of Glossophyllum florinii (?Ginkgoales) from the Upper Triassic of Lunz,Austria

The cuticles of many extant seed plants display distinct surface microreliefs, which represent adaptations to certain habitat conditions or mechanical defences against herbivores and phytopathogenic microorganisms. Although microreliefs have variously been noted in fossil cuticles, hypotheses relating to the effectiveness of these structures in fossil plants have not been advanced to date. A surface microrelief composed of longitudinally orientated idiocuticular striae occurs on the leaves of the enigmatic Carnian (Late Triassic) gymnosperm (?ginkgophyte) Glossophyllum florinii Kräusel from the Northern Calcareous Alps of lower Austria. Most striae originate from the tips of the papillae on the stomatal subsidiary cells. The G. florinii surface microrelief may have (1) reduced leaf wettability, (2) produced or enhanced the self‐cleaning effect of the leaf, (3) prevented the formation of a water film on the leaf surface, and/or (4) mechanically stabilized the leaf. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 153 , 87–95.     

Additive and non‐additive effects of birch genotypic diversity on arthropod herbivory in a long‐term field experiment

Herbivores are important drivers of plant population dynamics and community composition in natural and managed systems. Intraspecific genetic diversity of long‐lived plants like trees might shape patterns of herbivory by different guilds of herbivores that trees experience through time. However, previous studies on plant genetic diversity effects on herbivores have been largely short‐term. We investigated how tree genotypic variation and diversity influence herbivory of silver birch Betula pendula in a long‐term field experiment. Using clones of eight genotypes, we constructed experimental plots consisting of one, two, four or eight genotypes, and measured damage by five guilds of arthropod herbivores twice a year over three different years (four, six and nine years after the experiment was established). Genotypes varied significantly for most types of herbivore damage, but genotype resistance rankings often shifted over time, and none of the clones was more resistant than all others to all types of herbivores. At the plot level, birch genotypic diversity had significant positive additive effect on leaf rollers and negative non‐additive effects on chewing herbivores and gall makers. In contrast, leaf‐mining and leaf‐tying damage was not influenced by birch genotypic diversity. Within diverse plots, the direction of genotypic diversity effects varied depending on birch genotype, some having lower and some having higher herbivory in mixed stands. This research highlights the importance of long‐term studies including different feeding guilds of herbivores to understand the effects of plant genetic diversity on arthropod communities. Different responses of various feeding guilds to genotypic diversity and shifts in resistance of individual genotypes over time indicate that genotypic mixtures are unlikely to result in overall reduction in herbivory over time.     

The role of leaf defensive traits in oaks on the preference and performance of a polyphagous herbivore,Orgyia vetusta

1. Leaves possess traits that mediate the preference and performance of herbivores. Most evidence for the importance of leaf traits as defences against herbivory comes from studies of few model plant species. 2. In a phylogenetically explicit comparison, I explain the differences in preference and performance of tussock moth (Orgyia vetusta Boisduval) larvae on leaves of 27 oak (Quercus) species using nine putative leaf defences. 3. The preference for an oak species correlated positively with the survival of caterpillars. The correlation between preference and performance did not differ between oak species native to the range of tussock moth versus those from outside the herbivore's range. 4. The first principal component of leaf traits predicted survival of caterpillars on oak leaves but only marginally predicted their preference between oak species. A multiple regression model showed that evergreenness, toughness, and condensed tannin content were the best predictors of caterpillar survival, and leaf toughness was the best predictor of host preference. 5. Generalist caterpillars may accurately assess the value of novel food sources. Moreover, many leaf traits that have been found to affect herbivory within a plant species can also be used to predict the fitness of a generalist herbivore between species.