Showing papers on "Plant tolerance to herbivory published in 2010"

The Ontogeny of Plant Defense and Herbivory: Characterizing General Patterns Using Meta‐Analysis

Abstract: Defense against herbivores often changes dramatically as plants develop. Hypotheses based on allocation theory and herbivore selection patterns predict that defense should increase or decrease, respectively, across ontogeny, and previous research partly supports both predictions. Thus, it remains unclear which pattern is more common and what factors contribute to variability among studies. We conducted a meta-analysis of 116 published studies reporting ontogenetic patterns in plant defense traits and herbivory. Patterns varied depending on plant life form (woody, herbaceous, grass), type of herbivore (insect, mollusk, mammal), and type of defense trait (secondary chemistry, physical defense, tolerance). In woody plants, chemical defense increased during the seedling stage, followed by an increase in physical defenses during the vegetative juvenile stage. Mammalian herbivores showed a strong preference for mature compared to juvenile tissues in woody plants. Herbs experienced a significant increase in secondary chemistry across the entire ontogenetic trajectory, although the magnitude of increase was greatest during the seedling stage. Correspondingly, mollusks preferred young compared to older herbs. Future research investigating growth/defense trade-offs, allometry, herbivore selection patterns, and ecological costs would shed light on the mechanisms driving the ontogenetic patterns observed.

A meta-analysis of insect pest behavioral manipulation with plant volatiles

Abstract: Many insect pests utilize plant volatiles for host location and untangling the mechanisms of this process can provide tools for pest management. Numerous experimental results have been published on the effect of plant volatiles on insect pests. We used a meta-analysis to summarize this knowledge and to look for patterns. Our goal was to identify herbivore and plant traits that might explain the herbivores’ behavioral response to plant volatiles in field applications. We scored a total of 374 unique plant volatile-insect herbivore interactions obtained from 34 published studies investigating 50 herbivore pest species. Attractants had a significant effect on insect herbivore abundance but repellents did not; this latter result could be a result of the comparatively small number of field studies that tested plant volatiles as repellents (3%). Females were significantly more attracted to plant volatile baits than males. The diet breadth of herbivores was independent of a behavioral response to plant volatiles, but more case studies show effects of volatiles on chewers, followed by wood-borers and sap-feeders. There are more demonstrations of attraction to plant volatiles in Lepidoptera than in Thysanoptera. The method of plant volatile application had a significant effect on herbivore abundance and increasing the number of chemicals in individual baits attracted more herbivores. The magnitude of the response of herbivores to plant volatiles in forest and agricultural habitats was similar. We explore consistent patterns and highlight areas needing research in using plant volatiles to manage insect pests.

Resource allocation to defence and growth are driven by different responses to generalist and specialist herbivory in an invasive plant

Abstract: Summary 1. Invasive plants often have novel biotic interactions in their introduced ranges. These interactions, including less frequent herbivore attacks, may convey a competitive advantage over native plants. Invasive plants may vary in defence strategies (resistance vs. tolerance) or in response to the type of herbivore (generalists vs. specialists), but no study to date has examined this broad set of traits simultaneously. 2. Here, we examined resistance and tolerance of Chinese tallow (Triadica sebifera) populations from the introduced and native ranges to generalist (Cnidocampa flavescens) and specialist herbivores (Gadirtha inexacta) in the native range. 3. In a field common-garden test of resistance, caterpillars of each species were raised on plants from native and invasive populations. We found the specialist grew larger on and consumed more mass of invasive plant populations than native populations, while the generalist showed the same performance between them. The results were consistent with our laboratory bioassay using excised leaves. Chemical analyses showed that the invasive plants had lower tannin content and higher ratio of carbohydrate to protein than those of their native counterparts, suggesting that plants from invasive populations have altered chemistry that has a larger impact on specialist than on generalist resistance. 4. To test for differences in herbivore tolerance, plants were first defoliated by specialist or generalist herbivory and then allowed to regrow for 100 days in a field common garden. We found that plants from invasive populations had greater herbivore tolerance than native populations, especially for tolerance to generalists. They also grew more rapidly than native counterparts in the absence of herbivory. 5. Synthesis. The results of these experiments indicate that differences in selective pressures between ranges have caused dramatic reductions in resistance to specialist herbivores and those changes in plant secondary chemistry likely underlie these differences. The greater tolerance of invasive populations to herbivory appears to at least partly reflect an increase in growth rate in the introduced range. The greater tolerance to generalist herbivores suggests the intriguing possibility of selection for traits that allow plants to tolerate generalist herbivores more than specialist herbivores.

Support from the underground: Induced plant resistance depends on arbuscular mycorrhizal fungi

Abstract: Summary 1. Mycorrhizal symbiosis is thought to affect interactions between plants and herbivores by its influence on plant growth, nutrition and the plants defence system. Moreover, arbuscular mycorrhizal fungi (AMF) may enhance the inducibility of resistance responses. Until now, induction of plant resistance has not been considered to be a mechanism affecting the outcome of mycorrhization for plant-herbivore interactions. 2. Here, we test the hypothesis that the resistance of plants against herbivores depends on the induction of plant resistance by previous herbivory and mycorrhization. With a full factorial experiment in a greenhouse, we examined responses in growth of seven herbaceous plant species to AMF and the induction of resistance. To evaluate whether induced resistance is higher in plants with AMF we analyzed the combined effects of AMF and induction on herbivory, using bioassay caterpillars (Spodoptera littoralis). 3. Across all species, mycorrhization increased growth of plants and performance of the bioassay herbivore feeding on them. If, however, we induced resistance by allowing a caterpillar to feed for a short period on the plants, mycorrhization did not increase plant growth and performance of a subsequent herbivore that fed on the plant. This suggests that the increased plant resistance after induction was dependent on the symbioses with AMF. 4. Our results indicate that induction interacts with the allocation of the additional resources provided by mycorrhization towards plant growth and plant resistance. Therefore, mycorrhiza may play an important but hitherto overlooked role in the induction of plant resistance against herbivores.

Herbivore-induced plant responses in Brassica oleracea prevail over effects of constitutive resistance and result in enhanced herbivore attack.

Abstract: 1. Plant responses to herbivore attack may have community-wide effects on the composition of the plant-associated insect community. Thereby, plant responses to an early-season herbivore may have profound consequences for the amount and type of future attack. 2. Here we studied the effect of early-season herbivory by caterpillars of Pieris rapae on the composition of the insect herbivore community on domesticated Brassica oleracea plants. We compared the effect of herbivory on two cultivars that differ in the degree of susceptibility to herbivores to analyse whether induced plant responses supersede differences caused by constitutive resistance. 3. Early-season herbivory affected the herbivore community, having contrasting effects on different herbivore species, while these effects were similar on the two cultivars. Generalist insect herbivores avoided plants that had been induced, whereas these plants were colonised preferentially by specialist herbivores belonging to both leaf-chewing and sap-sucking guilds. 4. Our results show that community-wide effects of early-season herbivory may prevail over effects of constitutive plant resistance. Induced responses triggered by prior herbivory may lead to an increase in susceptibility to the dominant specialists in the herbivorous insect community. The outcome of the balance between contrasting responses of herbivorous community members to induced plants therefore determines whether induced plant responses result in enhanced plant resistance.

Selenium hyperaccumulation offers protection from cell disruptor herbivores.

Abstract: Background Hyperaccumulation, the rare capacity of certain plant species to accumulate toxic trace elements to levels several orders of magnitude higher than other species growing on the same site, is thought to be an elemental defense mechanism against herbivores and pathogens. Previous research has shown that selenium (Se) hyperaccumulation protects plants from a variety of herbivores and pathogens. Selenium hyperaccumulating plants sequester Se in discrete locations in the leaf periphery, making them potentially more susceptible to some herbivore feeding modes than others. In this study we investigate the protective function of Se in the Se hyperaccumulators Stanleya pinnata and Astragalus bisulcatus against two cell disrupting herbivores, the western flower thrips (Frankliniella occidentalis) and the two-spotted spider mite (Tetranychus urticae).

Short signalling distances make plant communication a soliloquy

Abstract: Plants respond to attack by herbivores or pathogens with the release of volatile organic compounds. Neighbouring plants can receive these volatiles and consecutively induce their own defence arsenal. This ‘plant communication’, however, appears counterintuitive when it benefits independent and genetically unrelated receivers, which may compete with the emitter. As a solution to this problem, a role for volatile compounds in within-plant signalling has been predicted. We used wild-type lima bean (Phaseolus lunatus) to quantify under field conditions the distances over which volatile signals move, and thereby determine whether these cues will mainly trigger resistance in other parts of the same plant or in independent plants. Independent receiver plants exhibited airborne resistance to herbivores or pathogens at maximum distances of 50 cm from a resistance-expressing emitter. In undisturbed clusters of lima bean, over 80 per cent of all leaves that were located around a single leaf at this distance were other leaves of the same plant, whereas this percentage dropped below 50 per cent at larger distances. Under natural conditions, resistance-inducing volatiles of lima bean move over distances at which most leaves that can receive the signal still belong to the same plant.

Sex-related differences in reproductive allocation, growth, defense and herbivory in three dioecious neotropical palms.

Abstract: Background Frequently, in dioecious plants, female plants allocate more resources to reproduction than male plants. Therefore it is expected that asymmetrical allocation to reproduction may lead to a reproduction-growth tradeoff, whereby female plants grow less than male plants, but invest more in defenses and thus experience lower herbivory than male plants. Methodology/Principal Findings We tested these expectations by comparing resource allocation to reproduction, growth and defense and its consequences on herbivory in three sympatric dioecious Chamaedorea palms (C. alternans, C. pinnatifrons and C. ernesti-augusti) using a pair-wise design (replicated male/female neighboring plants) in a Mexican tropical rain forest. Our findings support the predictions. Biomass allocation to reproduction in C. pinnatifrons was 3-times higher in female than male plants, consistent with what is known in C. alternans and C. ernesti-augusti. Growth (height and leaf production rate and biomass production) was higher in male plants of all three species. Female plants of the three species had traits that suggest greater investment in defense, as they had 4–16% tougher leaves, and 8–18% higher total phenolic compounds concentration. Accordingly, female plants sustained 53–78% lower standing herbivory and 49–87% lower herbivory rates than male plants. Conclusions/Significance Our results suggests that resource allocation to reproduction in the studied palms is more costly to female plants and this leads to predictable intersexual differences in growth, defense and herbivory. We conclude that resource allocation to reproduction in plants can have important consequences that influence their interaction with herbivores. Since herbivory is recognized as an important selective force in plants, these results are of significance to our understanding of plant defense evolution.

Fungal endophyte negative effects on herbivory are enhanced on intact plants and maintained in a subsequent generation

Abstract: 1. Fungal endophytes are ubiquitous associates of virtually all plant species. Although many studies have focused on the role of these microorganisms as mediators of plant–herbivore interactions, these studies have usually been conducted using short-term experiments. 2. Truly effective defences against herbivores may require normal functioning of the plant, as excised leaves may be less resistant as compared with those still attached to the plant. Yet, most studies investigating possible effects of endophytes in conferring host resistance to herbivores have been conducted with plant parts rather than intact plants. 3. Using the root endophytic fungus (Acremonium strictum)—broad bean (Vicia faba)—generalist herbivore (Helicoverpa armigera) model, we conducted experiments to examine whether endophyte effects on herbivory would depend on the experimental setting used in the investigation and whether they would translate into a subsequent generation of the herbivore. 4. Acremonium strictum negative effects on the fitness of H. armigera first generation were more evident when the larvae foraged freely on inoculated intact whole plants than when offered leaf discs of inoculated plants. Furthermore, these effects were carried over into H. armigera second generation reared on an artificial diet. 5. Acremonium strictum could not be re-isolated from V. faba leaves; hence direct contact between the endophyte and the insect could be excluded. Alternatively, loss of volatiles or inhibitory effects of compounds that were stronger in situ might have caused changes in larval feeding and performance on leaf discs as compared with intact plants, regardless of infection status. 6. We suggest that the reduction in fitness parameters of H. armigera across two generations is caused indirectly via an endophyte-triggered reduction in plant quality.

A tritrophic signal that attracts parasitoids to host-damaged plants withstands disruption by non-host herbivores.

Abstract: Volatiles emitted by herbivore-infested plants are highly attractive to parasitoids and therefore have been proposed to be part of an indirect plant defense strategy. However, this proposed function of the plant-provided signals remains controversial, and it is unclear how specific and reliable the signals are under natural conditions with simultaneous feeding by multiple herbivores. Phloem feeders in particular are assumed to interfere with plant defense responses. Therefore, we investigated how attack by the piercing-sucking cicadellid Euscelidius variegatus influences signaling by maize plants in response to the chewing herbivore Spodoptera littoralis. The parasitoid Cotesia marginiventris strongly preferred volatiles of plants infested with its host S. littoralis. Overall, the volatile emissions induced by S. littoralis and E. variegatus were similar, but higher levels of certain wound-released compounds may have allowed the wasps to specifically recognize plants infested by hosts. Expression levels of defense marker genes and further behavioral bioassays with the parasitoid showed that neither the physiological defense responses nor the attractiveness of S. littoralis infested plants were altered by simultaneous E. variegatus attack. Our findings imply that plant defense responses to herbivory can be more robust than generally assumed and that ensuing volatiles convey specific information about the type of herbivore that is attacking a plant, even in complex situations with multiple herbivores. Hence, the results of this study support the notion that herbivore-induced plant volatiles may be part of a plant's indirect defense stratagem.

Herbivory enhances positive effects of plant genotypic diversity.

Abstract: Both plant diversity and vertebrate herbivores can impact plant fitness and ecosystem functioning, however their interactions have not been explicitly tested. We manipulated plant genotypic diversity of the native plant Oenothera biennis and monitored its survivorship and lifetime fitness with and without one of its major vertebrate consumers, white-tailed deer Odocoileus virginianus. Intense but unmanipulated herbivory by meadow voles Microtus pennsylvanicus killed over 70% of nearly 4000 experimental plants. However, plants grown in genotypically diverse patches suffered fewer vole attacks and had higher survival and reproductive output than plants in monoculture. Moreover, positive effects of genotypic diversity were enhanced by the presence of deer, indicating a non-additive interaction between diversity and trophic-level complexity. Genetic selection analyses showed that the selective value of ecologically important traits depended on plant diversity and exposure to deer, demonstrating that community complexity can promote fitness through multiple ecologically and evolutionarily important feedbacks.

Root herbivores and detritivores shape above-ground multitrophic assemblage through plant-mediated effects.

Abstract: P>1. Indirect effects mediated by changes in plant traits are the main mechanism by which above- and below-ground herbivores affect each other and their enemies. Only recently the role of decomposers in the regulation of such plant-based systems has been considered. We hypothesized that: (i) below-ground organisms, both herbivores (negative effect on plants) and detritivores (positive effect on plants), will have a profound effect on the interactions among above-ground arthropods; (ii) floral herbivores will negatively affect other above-ground herbivores associated with the plant; and (iii) not only above- and below-ground herbivores, but also detritivores will affect the production of secondary metabolites, i.e. glucosinolates, in the plants. 2. We manipulated the presence of above-ground herbivores, below-ground herbivores and below-ground detritivores on the Brassicaceae Moricandia moricandioides in the field to disentangle their individual and combined effects on other organism groups. We also investigated their effects on the plant's chemical defence to evaluate potential mechanisms. 3. Our results show that not only above- and below-ground herbivores, but also detritivores affected other herbivores and parasitoids associated with the host plant. Most effects were not additive because their strength changed when other organisms belonging to different functional groups or food web compartments were present. Moreover, below-ground herbivore and detritivore effects on above-ground fauna were related to changes in glucosinolate concentrations and in quantity of resources. 4. This study indicates that multitrophic interactions in plant-based food webs can dramatically change by the action of below-ground organisms. One of the most important and novel results is that detritivores induced changes in plant metabolites, modifying the quality and attractiveness of plants to herbivores and parasitoids under field conditions.

Genetic variation in herbivore resistance and tolerance: the role of plant life-history stage and type of damage.

Abstract: Information of the patterns of genetic variation in plant resistance and tolerance against herbivores and genetic trade-offs between these two defence strategies is central for our understanding of the evolution of plant defence. We found genetic variation in resistance to two specialist herbivores and in tolerance to artificial damage but not to a specialist leaf herbivore in a long-lived perennial herb. Seedlings tended to have genetic variation in tolerance to artificial damage. Genetic variation in tolerance of adult plants to artificial damage was not consistent in time. Our results suggest that the level of genetic variation in tolerance and resistance depends on plant life-history stage, type of damage and timing of estimating the tolerance relative to the occurrence of the damage, which might reflect the pattern of selection imposed by herbivory. Furthermore, we found no trade-offs between resistance and tolerance, which suggests that the two defence strategies can evolve independently.

Variation in arbuscular mycorrhizal fungi colonization modifies the expression of tolerance to above-ground defoliation

Abstract: Summary 1. Plant association with arbuscular mycorrhizal fungi (AMF) has been considered a factor increasing plant tolerance to herbivory. However, this positive effect could decrease with colonization density if the benefit : cost ratio of the AMF–plant association changes. We measured plant performance and tolerance to defoliation across a gradient of commercial AMF (Glomus sp.) inoculum concentration. 2. Six genetic families of Datura stramonium were grown under greenhouse conditions and subjected to five increasing levels of AMF inoculum concentration and to defoliation treatments, i.e. the presence/absence of 50% artificial damage, following a full-factorial design. 3. AMF colonization increased linearly with inoculum concentration while foliar area, root mass, flowering phenology and seed production expressed nonlinear functions. Plant genetic variation in the benefit function of AMF colonization was also detected. We show a negative interaction between AMF concentration and plant tolerance to defoliation. 4. Synthesis. The negative correlation between plant tolerance and AMF concentration suggests that defoliation can reduce AMF benefits and that natural variations in AMF can limit the evolution of optimum levels of tolerance. Moreover, genetic variation in the shape of the reaction norms to AMF in the presence/absence of defoliation suggests that plants may evolve in response to variation in densities of AMF and herbivores.

Varying responses of insect herbivores to altered plant chemistry under organic and conventional treatments

Abstract: The hypothesis that plants supplied with organic fertilizers are better defended against insect herbivores than those supplied with synthetic fertilizers was tested over two field seasons. Organic and synthetic fertilizer treatments at two nitrogen concentrations were supplied to Brassica plants, and their effects on the abundance of herbivore species and plant chemistry were assessed. The organic treatments also differed in fertilizer type: a green manure was used for the low-nitrogen treatment, while the high-nitrogen treatment contained green and animal manures. Two aphid species showed different responses to fertilizers: the Brassica specialist Brevicoryne brassicae was more abundant on organically fertilized plants, while the generalist Myzus persicae had higher populations on synthetically fertilized plants. The diamondback moth Plutella xylostella (a crucifer specialist) was more abundant on synthetically fertilized plants and preferred to oviposit on these plants. Glucosinolate concentrations were up to three times greater on plants grown in the organic treatments, while foliar nitrogen was maximized on plants under the higher of the synthetic fertilizer treatments. The varying response of herbivore species to these strong differences in plant chemistry demonstrates that hypotheses on defence in organically grown crops have over-simplified the response of phytophagous insects.

Zoophytophagous pentatomids feeding on plants and implications for biological control

Abstract: Zoophytophagous insects can feed on a variety of prey, plants and plant products. By studying the interactions between predatory hemipterans and plants harbouring the prey of these insects, scientists have started to establish two potential outcomes: (1) positive effects like the enhancement of their life history characteristics by acquiring plant contents; and (2) negative effects mediated by plant resistance to herbivores or prey ingesting secondary plant metabolites. Despite this research, there is a lack of information about the feeding sites of predatory hemipterans on their host plants, what they ingest from plants, and whether they cause damage to their host plants. The results presented here indicate that the xylem is one of the feeding sites of predatory hemipterans on plants. The dissection of predators that fed on plants with marked vessels and testing insects for the presence of Cry protein constitutively expressed in the cytoplasm of plant cells revealed that bugs are not able to acquire cytoplasm contents from the plant cell. In addition, we demonstrate that systemic insecticide circulating inside plants from soil applications contaminates these predators. Our results are discussed in the context of zoophytophagous feeding behaviour exhibited by predatory hemipterans and the use of systemic insecticides for the conservation of natural enemies. This interaction contradicts the concept of ecological selectivity obtained for natural enemies through the placement of systemic insecticide in the soil as a selective method of deploying chemical control and predatory hemipteran conservation within the integrated pest management framework.

Interactions between root and shoot herbivores of Ammophila arenaria in the laboratory do not translate into correlated abundances in the field

Abstract: Over the past decades a growing body of literature has presented proof of the possible interactions between foliar and root herbivores. These effects can be positive, negative or neutral in either direction, depending on the species and the involved mechanism. Most of these studies however concern experiments under controlled conditions. Whether these interactions affect the distribution of herbivores under natural conditions still largely remains an open question. This study examined interactions between root feeding nematodes and shoot feeding aphids on Ammophila arenaria in the laboratory. We subsequently addressed the question whether expectations from this experiment are reflected in correlations between plant related variables and the abundance of both herbivores in the field. We demonstrated that nematodes and aphids can negatively affect each other in a controlled microcosm. In the field however no significant correlations between nematode and aphid abundances could be detected. There, shorter plants with a more vital leaf set and a higher root density supported the highest numbers of aphids. Plants with a lower root density and higher root vitality held more migratory endoparasitic nematodes, while more nematode cysts were found among roots with a low vitality. A certain plant property can furthermore affect above- and belowground herbivores in the opposite direction, such as root density in this case. This study suggests that effects of root herbivores on foliar herbivores or vice versa seem to be blurred in a field situation where other variables related to plant vitality and water content structure the herbivore populations. Therefore, caution should be used in generalising the prevalence of these interactions between the above- and belowground fauna, based solely on laboratory experiments.

Exotic vertebrate and invertebrate herbivores differ in their impacts on native and exotic plants: a meta-analysis

Abstract: Herbivores modify various ecological processes including interactions between native and exotic plants that may affect invasion success by the exotic plants It is unknown whether different types of exotic herbivores have similar effects on native and exotic plants Using two distinct data sets, we ran meta-analyses to compare exotic vertebrate and invertebrate herbivore preferences for, and effects on performance and population sizes of native and exotic plants We found that exotic vertebrate herbivores have positive effects on exotic plant performance and population sizes, and no significant effects on native plants Exotic invertebrates have significant negative effects on performance and population sizes of both exotic and native plants Vertebrates prefer to feed on native plants relative to exotic plants, while invertebrates prefer the exotic plants to native plants Thus the exotic vertebrate herbivores may aid invasiveness of exotic plants, in accordance with the invasional meltdown hypothesis, while exotic invertebrate herbivores probably have no net effect on invasion process of the exotic plants Invertebrate herbivore preferences for exotic plants support the biotic resistance hypothesis, as the native plants probably resist the invertebrate herbivory We also tested an evolutionary logic that posits that herbivores with similar evolutionary history as plants will affect the plants less negatively than plants with which they have not co-evolved Our results indicate that there is no consistent pattern in effects of exotic vertebrate and invertebrate herbivores on exotic plants with or without which they have co-evolved

Effects of resource availability on tolerance of herbivory in the invasive Alternanthera philoxeroides and the native Alternanthera sessilis

Abstract: Sun Y, Ding J & Frye MJ (2010). Effects of resource availability on tolerance of herbivory in the invasive Alternanthera philoxeroides and the native Alternanthera sessilis. Weed Research 50, 527–536. Summary Resource availability is known to affect herbivore selectivity and the ability of plants to respond to herbivores. However, little information is available for the performance of invasive plants subject to insect herbivory and limited resources availability. We conducted a glasshouse experiment using both the invasive Alternanthera philoxeroides and its native congener Alternanthera sessilis, to compare the effects of resource availability on plant tolerance to herbivory. The results suggest that water availability affects plant tolerance to herbivory, such that stressful water conditions promoted greater herbivore tolerance for A. philoxeroides and decreased herbivore tolerance for A. sessilis. The addition of fertiliser also affected the plant’s ability to respond to herbivory. Fertilised plants of both species generated more biomass and had greater tolerance to herbivory than unfertilised plants. Adding fertiliser appeared to increase the compensation capacity to herbivory for A. philoxeroides. In unfertilised conditions, moisture had no effect on A. philoxeroides tolerance to herbivory, and plants performed better under drought conditions. The results of this study may help to further our understanding of why biological control efforts against A. philoxeroides have been successful in some aquatic environments, but not others. Our study suggests that control of A. philoxeroides in high-moisture and nutrient poor environments should be more effective than control under nutrient rich and low-moisture soils. Understanding the mechanism of invasive plant compensation under different environmental conditions will be important for improving and predicting management efficiency.

Associations of plant fitness, leaf chemistry, and damage suggest selection mosaic in plant-herbivore interactions.

Abstract: The geographic mosaic theory of coevolution states that variation in species interactions forms the raw material for coevolutionary processes, which take place over large geographic scales. One key assumption underlying the process of coevolution in plant–herbivore interactions is that herbivores exert selection on their host plants and that this selection varies among plant populations. We examined spatial variation in the existence and strength of phenotypic selection on host plant resistance exerted by specialist herbivores in 17 archipelago populations of the perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). In these highly fragmented populations, V. hirundinaria is consumed by the larvae of two specialist herbivores: the folivorous moth Abrostola asclepiadis and the seed predator Euphranta connexa. Selection imposed on host plants by these herbivores was examined by analyzing the associations between levels of herbivory, plant fitness, and contents of a number of leaf chemicals reflecting pl...

Spatially complex neighboring relationships among grassland plant species as an effective mechanism of defense against herbivory.

Abstract: Close spatial relationships between plant species are often important for defense against herbivory. The associational plant defense may have important implications for plant community structure, species diversity, and species coexistence. An increasing number of studies have focused on associational plant defense against herbivory at the scale of the individual plant and its nearest neighbors. However, the average neighborhood effects between plant species at the scale of whole plant communities have received almost no attention. The aims of this study were to determine patterns of spatial relationship between different plant species that can provide effective defense against herbivory. We conducted a manipulative experiment using sheep and three native plant species with different palatability. Consumption of palatable plants by herbivores was largest when the three plant species were isolated in three patches and independent of each other. A homogenous and spatially equal neighbor relationship between the three species did not reduce the risk of herbivory of palatable species compared to isolation of these species, but it reduced the total intake of all plant species. The palatable species was subject to less herbivory in a complex spatial neighborhood of several plant species. High complexity of spatial neighborhood resulted in herbivores passively reducing selectivity, thereby reducing the probability of damage to palatable species in the community, or making inaccurate judgments in foraging selectivity between and within patches, thereby reducing the vulnerability of palatable plants and even the whole plant community. We conclude that compelling herbivores to passively reduce the magnitude of foraging selectivity by establishing spatially complex neighborhoods between plant species is a compromise and optimal spatial strategy by plants to defend themselves again herbivory. This may contribute not only to maintenance of plant species diversity but also to a stable coexistence between herbivores and plants in grassland ecosystems.

Plant defenses against parasitic plants show similarities to those induced by herbivores and pathogens

Abstract: Herbivores and pathogens come quickly to mind when one thinks of the biotic challenges faced by plants. Important but less appreciated enemies are parasitic plants, which can have important consequences for the fitness and survival of their hosts. Our knowledge of plant perception, signaling, and response to herbivores and pathogens has expanded rapidly in recent years, but information is generally lacking for parasitic species. In a recent paper we reported that some of the same defense responses induced by herbivores and pathogens--notably increases in jasmonic acid (JA), salicylic acid (SA), and a hypersensitive-like response (HLR)--also occur in tomato plants upon attack by the parasitic plant Cuscuta pentagona (field dodder). Parasitism induced a distinct pattern of JA and SA accumulation, and growth trials using genetically-altered tomato hosts suggested that both JA and SA govern effective defenses against the parasite, though the extent of the response varied with host plant age. Here we discuss similarities between the induced responses we observed in response to Cuscuta parasitism to those previously described for herbivores and pathogens and present new data showing that trichomes should be added to the list of plant defenses that act against multiple enemies and across Kingdoms.

Herbivores affect natural selection for floral-sex ratio in a field population of horsenettle, Solanum carolinense

Abstract: Although aspects of a plant's breeding system are generally believed to have evolved in response to selection for effective pollination, herbivores may also play a selective role. Here we report on a field experiment involving 960 transplanted ramets of the andromonoecious herb Solanum carolinense in which the pattern of natural selection for an important breeding-system trait was influenced by naturally occurring herbivores. As the level of flower and fruit herbivory increased, the pattern of selection on floral-sex ratio went from stabilizing (with an optimum of 29% male flowers), to directional toward a lower proportion of males, with the optimum bottoming at 0% male. This pattern of selection likely helped generate the broad-sense genetic correlation (r = 0.42) between floral-sex ratio and resistance to herbivory. These results contribute to the growing awareness that herbivores can be important influences not only on plant resistance traits, but also on the evolution of their hosts' breeding system.

The resource regulation hypothesis and positive feedback loops in plant–herbivore interactions

Abstract: Resource regulation occurs when herbivory maintains or increases plant susceptibility to further herbivory by the same species. A review of the literature indicates it is a widespread plant–animal interaction involving a diverse array of herbivores. At least three mechanisms can produce this positive feedback cycle. First, phytophagous insect and mammalian herbivore damage can stimulate dormant buds to produce vigorous juvenile growth, which is preferred for further attack. Juvenilization cycles may have repeatedly evolved because herbivores are able to take advantage of a generalized plant compensatory response to any type of damage. Second, herbivores can manipulate plant source–sink relationships to attain more resources, and this alteration of plant growth may benefit subsequent herbivore generations. Third, herbivory can alter plant nutrition or defensive chemistry in a way that makes a plant susceptible to more herbivory. Resource regulation probably occurs because damage to resources preferred by the herbivores induces a generalized plant response that produces more preferred resources. Alternatively, manipulation of plant resources to induce resource regulation may have evolved in herbivores with a high degree of philopatry due to selection to alter plant resources to benefit their offspring. Resource regulation can stabilize insect population dynamics by maintaining a supply of high-quality plant resources. It can also increase the heterogeneity of host-plant resources for herbivores by altering the physiological age structure and the distribution of resources within plants. Resource regulation may have strong plant-mediated effects on other organisms that use that host plant, but these effects have not yet been explored.

Environmental context determines within- and potential between-generation consequences of herbivory

Abstract: Plant tolerance to herbivory may depend on local environmental conditions. Models predict both increased and decreased tolerance with increasing resources. Transgenerational effects of herbivory may result in cross-generation tolerance. We evaluated within- and potential between-generation consequences of deer browsing in light-gap and understory habitats in the forest-edge herb, Campanulastrum americanum. Plants were assigned to deer-browsed, simulated-herbivory, and control (undamaged) treatments in the two light environments. In light gaps, plants were eaten earlier, more frequently, and had less vegetative recovery relative to uneaten plants than in the understory. As a result, browsed light-gap plants had a greater reduction in flowers and fruit than understory plants. This reduced tolerance was in part because deer browsing damaged plants in light gaps more than those in the understory. However, in the simulated herbivory treatment, where damage levels were similar between light habitats, plants growing in high-resource light gaps also had reduced tolerance of herbivory relative to those in the forest understory. C. americanum's reproductive phenology was delayed by reduced light and the loss of the apical meristem. As a result, deer-browsed plants in the light gap flowered slightly later than uneaten plants in the understory. C. americanum has a polymorphic life history and maternal flowering time influences the frequency of annual and biennial offspring. The later flowering of deer-browsed plants in light gaps will likely result in a reduced frequency of high-fitness annual offspring and an increase in lower fitness biennial offspring. Therefore, additional between-generation costs of herbivory are expected relative to those predicted by fruit number alone.

The potential for modifying plant volatile composition to enhance resistance to arthropod pests

Abstract: Plant volatiles provide herbivorous arthropods with information that allows them to discriminate between host and non-host plants. Volatiles may also indicate plant stress status, and natural enemies can use herbivore-induced plant volatiles as cues for prey location. Neighbouring plants may also make use of volatile cues to prepare for herbivore attack. Since both constitutive and inducible plant volatile emissions can be modified by plant breeding, the possibility exists to improve plant resistance against important pests both directly and indirectly via improved biological control. So far this approach has been tested only in the realm of research, predominantly using transgenic Arabidopsis with modified composition of terpenoids or C6 green leaf compounds. However several studies have shown that it is indeed possible both to reduce herbivory and enhance natural enemy attraction simultaneously. If such effects can be translated into increased and more stable yields in important crops, this strategy might be explored by the plant breeding industry and eventually become available to plant growers in the form of resistant cultivars. There are however ecological challenges associated with this approach, and the modified plant volatile composition should preferably be inducible specifically by the target pests, or by field application of specific elicitors based on forecasts of pest attack.

Impacts of belowground herbivory on oviposition decisions in two congeneric butterfly species

Abstract: Root-feeding insects can affect the performance of aboveground insect herbivores when they are forced to feed on the same host plant. Here we explored whether the oviposition behaviour of two closely related herbivorous species (cabbage butterflies; Lepidoptera: Pieridae) is influenced by root-feeding insects, when they are given the chance to choose between host plants with and without root herbivores. Considering that egg load is an important physiological factor influencing the foraging behaviour of insects, we also examined whether root-feeding insects differentially influence oviposition preference in butterflies with low and high egg loads. Oviposition preference in both butterfly species with low and high egg loads was monitored using host plants with and without root herbivores. To ascertain the status of butterfly age with low and high egg loads, the oviducts of a separate group of butterflies was dissected to record the number of immature and mature eggs in butterflies of various ages. Pieris brassicae L. butterflies with low egg loads preferred plants without root herbivores over plants with root herbivores, and laid more egg clutches on the leaves of plants that were not attacked by root herbivores. Butterflies with comparatively high egg loads also selected a larger proportion of plants without root herbivores, but laid a similar number of egg clutches on the plant shoots independent of the presence or absence of root herbivores belowground. Independent of the age and egg load, Pieris rapae L. butterflies selected a larger proportion of plants not attacked by root herbivores to lay eggs, but the number of eggs laid was similar in plants with and without root herbivores. This study shows that belowground insects can influence behavioural decisions of aboveground insect herbivores. Interestingly, the strength of these interactions depends on the physiological state of the insects which is probably correlated with their perception of environmental quality.