Synergistic Effects of Amides from Two <Emphasis Type="Italic">Piper</Emphasis> Species on Generalist and Specialist Herbivores

Plants use a diverse mix of defenses against herbivores, including multiple secondary metabolites, which often affect herbivores synergistically. Chemical defenses also can affect natural enemies of herbivores via limiting herbivore populations or by affecting herbivore resistance to parasitoids. In this study, we performed feeding experiments to examine the synergistic effects of imides and amides (hereafter “amides”) from Piper cenocladum and P. imperiale on specialist (Eois nympha, Geometridae) and generalist (Spodoptera frugiperda, Noctuidae) lepidopteran larvae. Each Piper species has three unique amides, and in each experiment, larvae were fed diets containing different concentrations of single amides or combinations of the three. The amides from P. imperiale had negative synergistic effects on generalist survival and specialist pupal mass, but had no effect on specialist survival. Piper cenocladum amides also acted synergistically to increase mortality caused by parasitoids, and the direct negative effects of mixtures on parasitoid resistance and pupal mass were stronger than indirect effects via changes in growth rate and approximate digestibility. Our results are consistent with plant defense theory that predicts different effects of plant chemistry on generalist versus adapted specialist herbivores. The toxicity of Piper amide mixtures to generalist herbivores are standard bottom-up effects, while specialists experienced the top-down mediated effect of mixtures causing reduced parasitoid resistance and associated decreases in pupal mass.     

Trade-offs in Antiherbivore Defenses in Piper cenocladum: Ant Mutualists Versus Plant Secondary Metabolites

Ant–plant mutualisms may provide indirect evidence for costs of antiherbivore defenses when plants demonstrate trade-offs between allocating resources and energy into ant attractants versus chemical defenses. We tested the hypothesis that ecological trade-offs in defenses are present in Piper cenocladum. This plant possesses two distinct defenses: food bodies that attract predatory ants that destory herbivore eggs and amides that deter herbivores. Previous studies have demonstrated that the food bodies in P. cenocladum are an effective defense because the ants deter herbivory by specialist herbivores. Amides in other Piper species have been shown to have toxic qualities, but we tested the additional hypothesis that these amides have an actual defensive function in P. cenocladum. To test for ecological trade-offs between the two putative defenses, fragments of P. cenocladum were examined for the presence of amides both when the plant was producing food bodies and when it was not producing food bodies. Plants with active ant colonies had redundant defenses, producing food bodies and high levels of amides at the same time, but we detected a trade-off in that they had significantly lower levels of amides than did plants with no ants. To test for the defensive value of P. cenocladum amides, we used an ant bioassay and we examined herbivory results from previous experiments with plants that had variable levels of amides. These tests demonstrated that amides are deterrent to omnivorous ants, leaf cutting ants, and orthopterans. In contrast, the resident Pheidole bicornis ants are effective at deterring herbivory by specialist herbivores that oviposit eggs on the plant but not at deterring herbivory by nonresident omnivores. We concluded that although both amides and food body production appear to be costly, redundancy in defenses is necessary to avoid damage by a complex suit of herbivores.     

Synergistic Effects of Three Piper Amides on Generalist and Specialist Herbivores

The tropical rainforest shrub Piper cenocladum, which is normally defended against herbivores by a mutualistic ant, contains three amides that have various defensive functions. While the ants are effective primarily against specialist herbivores, we hypothesized that these secondary compounds would be effective against a wider range of insects, thus providing a broad array of defenses against herbivores. We also tested whether a mixture of amides would be more effective against herbivores than individual amides. Diets spiked with amides were offered to five herbivores: a naïve generalist caterpillar (Spodoptera frugiperda), two caterpillar species that are monophagous on P. cenocladum (Eois spp.), leaf-cutting ants (Atta cephalotes), and an omnivorous ant (Paraponera clavata). Amides had negative effects on all insects, whether they were naïve, experienced, generalized, or specialized feeders. For Spodoptera, amide mixtures caused decreased pupal weights and survivorship and increased development times. Eois pupal weights, larval mass gain, and development times were affected by additions of individual amides, but increased parasitism and lower survivorship were caused only by the amide mixture. Amide mixtures also deterred feeding by the two ant species, and crude plant extracts were strongly deterrent to P. clavata. The mixture of all three amides had the most dramatic deterrent and toxic effects across experiments, with the effects usually surpassing expected additive responses, indicating that these compounds can act synergistically against a wide array of herbivores.     

Prevalence of Chemical Defenses among Freshwater Plants

Although macrophyte–herbivore interactions in freshwater systems were generally disregarded for many years, recent data suggest that herbivory can be intense and important in structuring freshwater communities. This has led to the hypothesis that chemical defenses should be common among freshwater plants, but few studies have reported such chemical defenses, and no previous studies have assessed the frequency of chemical defenses among a substantial number of freshwater plant species. In a study of 21 macrophyte species co-occurring with the omnivorous crayfish Procambarus acutus in a southeastern USA wetland environment, we found that extracts of 11 species (52%) deterred feeding by P. acutus when tested in artificial foods at natural concentrations. Of these 11 chemically defended species, one species, Eupatorium capillifolium, consistently had a more unpalatable extract following mechanical damage to plant tissue, indicative of an activated chemical defense. Because herbivores are commonly nitrogen-limited and select food based on several plant traits, including plant nutritional value, it might be expected that chemical defenses would be especially important for protein-rich plants. However, we found no relationship between soluble protein concentration and deterrence of plant extracts.     

Plant–herbivore–carnivore Interactions in Cotton, <Emphasis Type="Italic">Gossypium hirsutum</Emphasis>: Linking Belowground and Aboveground

Most studies on plant–herbivore interactions focus on either root or shoot herbivory in isolation, but above- and belowground herbivores may interact on a shared host plant. Cotton (Gossypium spp.) produces gossypol and a variety of other gossypol-like terpenoids that exhibit toxicity to a wide range of herbivores and pathogens. Cotton plants also can emit herbivore-induced volatile compounds at the site of damage and systemically on all tissues above the site of damage. As these volatile compounds attract natural enemy species of the herbivore, they are thought to represent an indirect plant defense. Our study quantified gossypol and gossypol-like compounds in cotton plants with foliage feeding (Heliocoverpa zea), root feeding (Meloidogyne incognita), or their combination. Cotton plants with these treatments were studied also with respect to induced local and systemic volatile production and the attraction of the parasitic wasp Microplitis croceipes to those plants. We also evaluated whether foliage or root feeding affected foliar nitrogen levels in cotton. After 48 hr of leaf feeding and 5 wk of root feeding, local and systemic induction of volatiles (known to attract parasitoids such as M. croceipes) occurred with herbivore damage to leaves, and it increased in levels when root herbivory was added. Nevertheless, M. croceipes were equally attracted to plants with both leaf and root damage and leaf damage only. In contrast to previous studies in cotton, production of gossypol and gossypol-like compounds was not induced in leaf and root tissue following foliage or root herbivory, or their combination. We conclude that root feeding by M. incognita has little influence on direct and indirect defenses of Gossypium hirsutum against insect herbivory.     

The Effects of Arbuscular Mycorrhizal Fungi on Direct and Indirect Defense Metabolites of Plantago lanceolata L.

Arbuscular mycorrhizal fungi can strongly influence the metabolism of their host plant, but their effect on plant defense mechanisms has not yet been thoroughly investigated. We studied how the principal direct defenses (iridoid glycosides) and indirect defenses (volatile organic compounds) of Plantago lanceolata L. are affected by insect herbivory and mechanical wounding. Volatile compounds were collected and quantified from mycorrhizal and non-mycorrhizal P. lanceolata plants that underwent three different treatments: 1) insect herbivory, 2) mechanical wounding, or 3) no damage. The iridoids aucubin and catalpol were extracted and quantified from the same plants. Emission of terpenoid volatiles was significantly higher after insect herbivory than after the other treatments. However, herbivore-damaged mycorrhizal plants emitted lower amounts of sesquiterpenes, but not monoterpenes, than herbivore-damaged non-mycorrhizal plants. In contrast, mycorrhizal infection increased the emission of the green leaf volatile (Z)-3-hexenyl acetate in untreated control plants, making it comparable to emission from mechanically wounded or herbivore-damaged plants whether or not they had mycorrhizal associates. Neither mycorrhization nor treatment had any influence on the levels of iridoid glycosides. Thus, mycorrhizal infection did not have any effect on the levels of direct defense compounds measured in P. lanceolata. However, the large decline in herbivore-induced sesquiterpene emission may have important implications for the indirect defense potential of this species.     

Polymorphism in Plant Defense Against Herbivory: Constitutive and Induced Resistance in Cucumis sativus

Theory predicts that plant resistance to herbivores is determined by both genetic and environmentally induced components. In this study, we demonstrate that the phenotypic expression of plant resistance to spider mite herbivory in Cucumis sativus is determined by genetic and environmental factors and that there is an interaction between these factors. Previous feeding by spider mites induced systemic resistance to subsequent attack over several spatial scales within plants, reducing the population growth of mites compared to that on control plants. Effects of induction were effective locally over the short term, but resulted in local increased susceptibility to spider mite attack after several days. However, this local induced susceptibility on the damaged leaf was associated with induced resistance on newer leaves. Induced resistance was correlated with increases in cucurbitacin content of leaves, but was not associated with changes in the density of leaf trichomes. Induced resistance to herbivory was not detected in plants of a genotype lacking constitutive expression of cucurbitacins, which were in general highly susceptibile to mite attack. Allocation trade-offs between growth and defense are often invoked to explain the maintenance of variation in the levels of plant resistance. Contrary to current thinking, neither constitutive nor herbivore-induced plant resistance were associated with reductions in plant allocation to root and shoot growth. However, plants that had high levels of induced resistance to spider mites were the most susceptible to attack by a specialist beetle. Such ecological trade-offs between resistance to generalist herbivores and susceptibility to specialist herbivores may be important in the maintenance of variation of plant resistance traits. In summary, C. sativus exhibits strong genetic variation for constitutive and induced resistance to spider mites, and this variation in resistance is associated with ecological trade-offs.     

Specialist Weevil, Rhyssomatus lineaticollis, Does Not Spatially Avoid Cardenolide Defenses of Common Milkweed by Ovipositing into Pith Tissue

Rhyssomatus lineaticollis is a milkweed specialist whose larvae feed upon pith parenchyma in ramet stems of the common milkweed, Asclepias syriaca. Compared with other specialist insect herbivores on milkweeds, this curculionid beetle is unusual in that it is cryptically colored and does not sequester cardenolides characteristic of milkweed chemical defense. Based upon optimal defense theory, we predicted that pith tissue would be low in defensive compounds and that oviposition into the pith would spatially avoid cardenolides. We rejected this hypothesis because we found that pith tissue has a relatively high cardenolide concentration compared to cortex, epidermis, and leaf tissues. Moreover, we found total plant cardenolide concentration was lower in plants that contained the beetle eggs. Cardenolide concentrations were different among tissues in intact stems without the pith herbivore compared to stems where it was present. Furthermore, the overall polarity of the cardenolides present varied among plant tissues and between plants with and without R. lineaticollis eggs. Although we found lower concentrations of cardenolide in piths where the eggs were present, the cardenolides present in the pith contained more nonpolar forms, indicating that the plant may be responding to herbivory by increasing toxic efficacy of cardenolide defenses while lowering the total concentration. We suggest that preoviposition behavior by female beetles, which includes feeding on new leaves of the plant, is a mechanism by which females manipulate plant chemistry and assess quantitative and qualitative changes in cardenolide chemistry in response to herbivory prior to oviposition.     

Palatability of Macroalgae that Use Different Types of Chemical Defenses

This study compared algal palatability and chemical defenses from subtropical green algae that may use different types of defense systems that deter feeding by the rock-boring sea urchin Echinometra lucunter. The potential defense systems present include (1) the terpenoid caulerpenyne and its activated products from Caulerpa spp., and (2) dimethylsulfoniopropionate (DMSP)-related defenses in Ulva spp. Secondary metabolites from these chemical groups have been shown to deter feeding by various marine herbivores, including tropical and temperate sea urchins. Live algal multiple-choice feeding assays and assays incorporating algal extracts or isolated metabolites into an artificial diet were conducted. Several green algae, including Ulva lactuca, Caulerpa prolifera, and Cladophora sp., were unpalatable. Nonpolar extracts from U. lactuca deterred feeding, whereas nonpolar extracts from C. prolifera had no effect on feeding. Polar extracts from both species stimulated feeding. Caulerpenyne deterred feeding at approximately 4% dry mass; however, dimethyl sulfide and acrylic acid had no effect at natural and elevated concentrations. E. lucunter is more tolerant than other sea urchins to DMSP-related defenses and less tolerant to caulerpenyne than many reef fish. Understanding the chemical defenses of the algae tested in this study is important because they, and related species, frequently are invasive or form blooms, and can significantly modify marine ecosystems.     

Defense Suppression through Interplant Communication Depends on the Attacking Herbivore Species

In response to herbivory, plants emit volatile compounds that play important roles in plant defense. Herbivore-induced plant volatiles (HIPVs) can deter herbivores, recruit natural enemies, and warn other plants of possible herbivore attack. Following HIPV detection, neighboring plants often respond by enhancing their anti-herbivore defenses, but a recent study found that herbivores can manipulate HIPV-interplant communication for their own benefit and suppress defenses in neighboring plants. Herbivores induce species-specific blends of HIPVs and how these different blends affect the specificity of plant defense responses remains unclear. Here we assessed how HIPVs from zucchini plants (Cucurbita pepo) challenged with different herbivore species affect resistance in neighboring plants. Volatile “emitter” plants were damaged by one of three herbivore species: saltmarsh caterpillars (Estigmene acrea), squash bugs (Anasa tristis), or striped cucumber beetles (Acalymma vittatum), or were left as undamaged controls. Neighboring “receiver” plants were exposed to HIPVs or control volatiles and then challenged by the associated herbivore species. As measures of plant resistance, we quantified herbivore feeding damage and defense-related phytohormones in receivers. We found that the three herbivore species induced different HIPV blends from squash plants. HIPVs induced by saltmarsh caterpillars suppressed defenses in receivers, leading to greater herbivory and lower defense induction compared to controls. In contrast, HIPVs induced by cucumber beetles and squash bugs did not affect plant resistance to subsequent herbivory in receivers. Our study shows that herbivore species identity affects volatile-mediated interplant communication in zucchini, revealing a new example of herbivore defense suppression through volatile cues.

     

Effects of large mammalian herbivores and ant symbionts on condensed tannins of Acacia drepanolobium in Kenya

Condensed tannins have been considered to be important inducible defenses against mammalian herbivory. We tested for differences in condensed tannin defenses in Acacia drepanolobium in Kenya over two years among different large mammalian herbivore treatments [total exclusion, antelope only, and megaherbivore (elephants and giraffes) + antelope] and with four different ant symbiont species on the trees. We predicted that (1) condensed tannin concentrations would be lowest in the mammal treatment with the lowest level of herbivory (total exclusion), (2) trees occupied by mutualist ants that protect the trees most aggressively would have lower levels of tannins, and (3) if chemical defense production is costly, there would be a trade-off between tannin concentrations, growth, and mechanical defenses. Mean tannin concentrations increased from total exclusion treatments to wildlife-only treatments to megaherbivore + antelope treatments. In 1997, condensed tannin concentrations were significantly lower in trees occupied by the ant Crematogaster nigriceps, the only ant species that actively removed axillary buds. Contrary to our prediction, trees occupied by ant species that protect the trees more aggressively against mammalian herbivores did not have lower overall levels of condensed tannins. There was no consistent evidence of a trade-off between tannin concentrations and growth rate, but there was a positive correlation between mean thorn length and mean tannin concentrations across species of ant inhabitants and across herbivore treatments in 1997. Contrary to our expectation, trees had higher tannin concentrations in the upper parts of the canopy where there is little herbivory by mammals.     

Geographic Variation in Alkaloid Production in <Emphasis Type="Italic">Conium maculatum</Emphasis> Populations Experiencing Differential Herbivory by <Emphasis Type="Italic">Agonopterix alstroemeriana</Emphasis>

Conium maculatum, a Eurasian weed naturalized in North America, contains high concentrations of piperidine alkaloids that act as chemical defenses against herbivores. C. maculatum was largely free from herbivory in the United States, until approximately 30 yr ago, when it was reassociated via accidental introduction with a monophagous European herbivore, the oecophorid caterpillar Agonopterix alstroemeriana. At present, A. alstroemeriana is found in a continuum of reassociation time and intensities with C. maculatum across the continent; in the Pacific Northwest, A. alstroemeriana can cause severe damage, resulting in some cases in complete defoliation. Studies in biological control and invasion biology have yet to determine whether plants reassociated with a significant herbivore from the area of indigeneity increase their chemical defense investment in areas of introduction. In this study, we compared three locations in the United States (New York, Washington, and Illinois) where C. maculatum experiences different levels of herbivory by A. alstroemeriana to determine the association between the intensity of the interaction, as measured by damage, and chemical defense production. Total alkaloid production in C. maculatum was positively correlated with A. alstroemeriana herbivory levels: plants from New York and Washington, with higher herbivory levels, invested two and four times more N to alkaloid synthesis than did plants from Illinois. Individual plants with lower concentrations of alkaloids from a single location in Illinois experienced more damage by A. alstroemeriana, indicative of a preference on the part of the insect for plants with less chemical defense. These results suggest that A. alstroemeriana may act either as a selective agent or inducing agent for C. maculatum and increase its toxicity in its introduced range.     

Inter- and Intraspecific Variations in Production of Spines and Phenols in Prosopis caldenia and Prosopis flexuosa

In central Argentina the leguminous tree Prosopis caldenia is more abundant in sites of relatively high productivity (lowlands), whereas P. flexuosa is more abundant in sites of relatively low productivity (slopes and uplands). Based upon current antiherbivore defense theory, we predicted: (1) a higher investment in defenses in P. flexuosa than in P. caldenia, and (2) that limitations in resources would result in an increase of the defenses in both species. Our approach for testing these predictions was to estimate leaf phenol concentration and spinescence in adults (field study) and seedlings (greenhouse study) of both species growing at different levels of resource availability. In adult plants, the concentration of phenols was higher (P < 0.01) in P. flexuosa than in P. caldenia, but the opposite relationship was observed in seedlings. The amount of biomass invested in spines was similar (P > 0.10) in both species, whereas spine density was higher (P < 0.05) in P. caldenia than in P. flexuosa. In both species, limitations in resources did not result (P > 0.05) in increases in leaf phenol concentration, amount of biomass invested in spines, spine length, or spine density, except for the increase (P < 0.05) of spine density in seedlings of P. caldenia at low water and nutrient availability. In general, our results did not support current hypotheses on the production of antiherbivore defenses. It is argued that factors such as herbivore behavior (e.g., habitat selection, trampling, branch breakage) and alternative sinks for carbon (e.g., N₂ fixation, carbohydrate reserves), in addition to resource availability in evolutionary and ecological time, should be considered for a more complete understanding of the inter- and intraspecific variations in the production of both physical and chemical antiherbivore defenses.     

Effects of Herbivore Damage and Nutrient Level on Induction of Iridoid Glycosides in Plantago lanceolata

Damage by larvae of the buckeye butterfly (Junonia coenia) resulted in removal of 15–25% of Plantago lanceolata leaf area. Plants grown under high nutrients were larger than those grown under low nutrients. Twenty-eight days after herbivory, plants grown under high nutrients were still larger than those grown under low nutrients, and plants exposed to herbivores were significantly smaller than those not exposed to herbivores, regardless of the nutrient treatment. Damage by larvae also increased the iridoid glycoside content in the leaves and reproductive tissues of these Plantago lanceolata relative to undamaged controls. Whether damaged or undamaged, the iridoid glycoside content of P. lanceolata was highest in the reproductive tissues and lowest in the roots. Although initial concentrations of iridoid glycosides were significantly higher in plants grown under low nutrient conditions than in plants grown under high nutrient conditions, nutrient availability did not alter the phytochemical response of plants to herbivore damage. These results provide additional support for the defensive role of the iridoid glycosides in Plantago lanceolata by demonstrating that phytochemical variation is not always an incidental effect of nutrient stress but can be a direct response to damage by herbivores.     

Cotton Plant, Gossypium hirsutum L., Defense in Response to Nitrogen Fertilization

Plants respond to insect herbivory by producing dynamic changes in an array of defense-related volatile and nonvolatile secondary metabolites. A scaled response relative to herbivory levels and nutrient availability would be adaptive, particularly under nutrient-limited conditions, in minimizing the costs of expressed defensive pathways and synthesis. In this study, we investigated effects of varying nitrogen (N) fertilization (42, 112, 196, and 280 ppm N) on levels of cotton plant (Gossypium hirsutum) phytohormones [jasmonic acid (JA) and salicylic acid (SA)], terpenoid aldehydes (hemigossypolone, heliocides H₁, H₂, H₃, and H₄), and volatile production in response to beet armyworm (Spodoptera exigua) herbivory. Additional bioassays assessed parasitoid (Cotesia marginiventris) host-searching success in response to cotton plants grown under various N fertilizer regimes. At low N input (42 ppm N), herbivore damage resulted in significant increases in local leaf tissue concentrations of JA and volatiles and in systemic accumulation of terpenoid aldehydes. However, increased N fertilization of cotton plants suppressed S. exigua-induced plant hormones and led to reduced production of various terpenoid aldehydes in damaged mature leaves and undamaged young leaves. While increased N fertilization significantly diminished herbivore-induced leaf volatile concentrations, the parasitism of S. exigua larvae by the parasitoid C. marginiventris in field cages did not differ among N treatments. This suggests that, despite significant N fertilization effects on herbivore-induced plant defenses, at short range, the parasitoids were unable to differentiate between S. exigua larvae feeding on physiologically different cotton plants that share large constitutive volatile pools releasable when damaged by herbivores.     

Phenylpropenoid phenolics in sweetbay magnolia as chemical determinants of host use in saturniid silkmoths (Callosamia)

Host plant chemistry can play an important role in determining the evolution of host use patterns in herbivorous insects by influencing host selection, consumption, and assimilation of foliage. We used a comparative approach to test the hypothesis that specialist herbivores of sweetbay magnolia (Magnolia virginiana) possess adaptations that allow them to overcome chemical deterrents or toxins that prevent herbivory by unadapted herbivores. The three silkmoth species in the genusCallosamia can be collectively regarded as specialists on magnoliaceous hosts; however, only the monophagousC. securifera is able to complete development on sweetbay magnolia, its natural host. In laboratory assays with intact foliage, bothC. angulifera and the polyphagousC. promethea fed readily on sweetbay but were unable to survive past the third instar. Two neolignan compounds, magnolol and a biphenyl ether, were found to reduce neonate growth and survival of unadapted herbivore species when painted on acceptable host leaves at concentrations similar to those found in sweetbay foliage. Both compounds significantly reduced neonate growth ofC. angulifera andC. promethea but had no effect on the sweetbay specialist,C. securifera, indicating that the latter species possesses the unique ability in the genus to tolerate, metabolize, or otherwise circumvent the phytochemical defenses of this host.     

The Iridoid Glucoside,Antirrhinoside, from <Emphasis Type="Italic">Antirrhinum majus</Emphasis> L. has Differential Effects on Two Generalist Insect Herbivores

The iridoid glucoside, antirrhinoside, is constitutively distributed throughout Antirrhinum majus L. in a manner consistent with its possible role as an allelochemical, but there is no evidence that it has a defensive function with respect to insect herbivory. To address this question, two generalist herbivores, Lymantria dispar L. (gypsy moth) and Trichoplusia ni Hübner (cabbage looper) were chosen for feeding trials on excised whole leaves of A. majus and in artificial diet assays. In leaf excision feeding trials, fourth instar gypsy moth rejected, without sampling, the leaves of A. majus regardless of what node the leaf was excised from. In contrast, fourth instar cabbage looper readily fed on the excised leaves, and antirrhinoside was not found in their bodies or feces (frass) as determined by thin layer and high-pressure liquid chromatography. In the leaf and diet assays, a second major leaf iridoid in A. majus, antirrhide, was found in both cabbage looper and gypsy moth frass. In diet feeding assays, the growth of gypsy moth and cabbage looper were not inhibited by methanol extracts, iridoid fractions, or pure antirrhinoside at concentrations of 0.6% in diet, but cabbage looper growth was enhanced. At an antirrhinoside concentration of 3.3% in diet, gypsy moth growth was reduced, whereas cabbage looper growth again increased significantly relative to the control. It is likely that antirrhinoside functions as defense against herbivory for one generalist insect herbivore but also, at low concentrations, enhances the growth of another.     

Evaluation of the Evolution of Increased Competitive Ability (EICA) Hypothesis: Loss of Defense Against Generalist but not Specialist Herbivores

The evolution of increased competitive ability (EICA) hypothesis predicts that invasive plant species may escape their specialized natural enemies in their introduced range and subsequently evolve with a decrease in investment in anti-herbivore chemical defenses relative to native conspecifics. We compared the chemical profile of 10 populations of US native and 20 populations of European invasive Solidago gigantea. To test for differences in inducibility between native and invasive populations, we measured secondary chemistry in both damaged and undamaged plants. We also performed bioassays with three specialist and two generalist insect herbivores from four different feeding guilds. There was no evidence that invasive populations had reduced concentrations of sesquiterpenes, diterpenes, or short-chain hydrocarbons (SCH), although significant variation among populations was detected. Sesquiterpene and diterpene concentrations were not influenced by damage to the host plant, whereas SCH concentrations were decreased by damage for both native and invasive plants. Performance of the three specialist insects was not affected by the continental origin of the host plant. However, larval mass of the generalist caterpillar Spodoptera exigua was 37% lower on native plants compared to invasive plants. The other generalist insect, a xylem-tapping spittlebug that occurs on both continents, performed equally well on native and invasive plants. These results offer partial support for the defense predictions of the EICA hypothesis: the better growth of Spodoptera caterpillars on European plants shows that some defenses have been lost in the introduced range, even though our measures of secondary chemistry did not detect differences between continents. Our results show significant variation in chemical defenses and herbivore performance across populations on both continents and emphasize the need for analysis across a broad spatial scale and the use of multiple herbivores.     

Volatile Emissions from an Odorous Plant in Response to Herbivory and Methyl Jasmonate Exposure

Induced volatile terpenes have been commonly reported among diverse agricultural plant species, but less commonly investigated in odorous plant species. Odorous plants synthesize and constitutively store relatively large amounts of volatiles, and these may play a role in defense against herbivores. We examined the effect of herbivory and methyl jasmonate (MeJA) exposure on the release of volatile organic compounds (VOCs) in the marsh elder, Iva frutescens, which contains numerous constitutive VOCs, mainly mono- and sesquiterpenes. Our specific goal was to test for the presence of inducible VOCs in a naturally occurring plant already armed with VOCs. The abundant, native specialist leaf beetle Paria aterrima was used in herbivore induction trials. VOCs were sampled from herbivore wounded and unwounded, and from MeJA treated and untreated I. frutescens. Total VOC emissions were significantly greater in response to herbivory and MeJA treatment compared to unwounded controls. Herbivore wounding caused a substantial shift in the emission profile (42 VOCs from wounded, compared to 8 VOCs from unwounded I. frutescens), and MeJA had a similar yet less substantial influence on the emission pattern (28 VOCs from MeJA treated compared to 8 VOCs from untreated I. frutescens). Constitutive VOC emissions predominated, but some VOCs were detected only in response to herbivory and MeJA treatment, suggesting de novo synthesis. Several VOCs exhibited a delayed emission profile in contrast to the rapid release of constitutive VOCs, and principal components analysis revealed they were not associated with constitutive emissions. While I. frutescens contains many constitutive VOCs that are released immediately in response to herbivory, it also produces novel VOCs in response to feeding by the specialist P. aterrima and MeJA treatment.     

Glucosinolate and Trichome Defenses in a Natural Arabidopsis lyrata Population

Glucosinolates (GS) and trichomes contribute to plant resistance against insect herbivores in the model Arabidopsis thaliana. The functional and genetic characteristics of herbivore defense, however, can differ even between closely related species. In a quantitative genetic experiment with the out-crossing perennial Arabidopsis lyrata spp. petraea, we measured constitutive GS composition, trichome density, leaf thickness, and plant resistance in four different herbivore interactions. In a single population of A. lyrata, we found heritable variation for trichome density as well as GS amount and carbon side-chain elongation ratios associated with activity in methylthioalkylmalate synthase (MAM). Unexpectedly, heritabilities for indole GS in A. lyrata were high and less affected by differences in plant age and environment than aliphatic GS. We found significant heritability in plant resistance to the specialist Plutella xylostella and generalist Trichoplusia ni, but not to the specialists Pieris brassicae and Phyllotreta cruciferae. Analyses of phenotypic and genetic correlations between candidate defense traits and insect resistance suggested that A. lyrata resistance was conferred by a combination of indole GS amount and trichome density, and, to a lesser extent, aliphatic GS ratios and leaf thickness. Variation in the most abundant compound, the aliphatic 3-hydroxypropyl GS, had little impact on A. lyrata herbivore resistance. The contribution of defense traits to resistance depended on the experimental herbivory context, and resistances were weakly correlated. A diversified defense strategy is likely to be important for long-lived individuals of A. lyrata that are subject to attack by many different herbivores in nature.