Stinging hairs on the Japanese nettle Urtica thunbergiana have a defensive function against mammalian but not insect herbivores

Thorns and hairs of plants can serve as defenses against herbivores, although they may not have evolved under selection by herbivory. Japanese nettles, Urtica thunbergiana, in Nara Park, Nara Prefecture, Japan, where sika deer have been protected for 1200 years, bear many more stinging hairs than those in areas with few or no deer. Previous studies suggested that such hairy nettles evolved under natural selection imposed by intense deer browsing, because stinging hairs deterred deer browsing and because among-population variation in hair density was associated with deer abundance. To confirm this hypothesis, we examined (1) whether stinging hairs affected oviposition and feeding preferences of herbivorous insects and (2) the degree to which they deterred deer via laboratory and field experiments with hairy nettles from Nara Park and with almost-hairless nettles from another area. A specialist butterfly, Indian red admiral, showed no oviposition or larval feeding preferences for either hairy or hairless nettles. Insect damage levels did not significantly differ between the two variants. In contrast, deer browsed hairless nettles more heavily than hairy ones. In hairy nettles, however, the level of deer browsing was not proportional to stinging-hair density, presumably because the hairy nettle population had reached a plateau for resistance as a result of long-term strong directional selection for stinging hairs. These results corroborate the hypothesis that hairy nettles in Nara Park evolved through natural selection under intense deer browsing.     

The evolution of nettle resistance to heavy deer browsing

We examined whether heavy browsing by sika deer, Cervus nippon Temminck, changed morphological characteristics of a Japanese nettle, Urtica thunbergiana Sieb. et Zucc., in Nara Park, where a large population of sika deer has been maintained for more than 1,200 years. Wild nettles of Nara Park exhibited smaller leaf area, 11–223 times more stinging hairs per leaf, and 58–630-times higher stinging hair densities than those of other areas where there was no evidence of sika deer browsing. There were no significant differences in stinging hair length between the areas. Nettles from Nara Park that were cultivated from seeds in a greenhouse retained a larger number and higher density of stinging hairs. In the field, nettles of Nara Park were less frequently browsed by sika deer and showed higher survivorship than nettles that were transplanted from an unbrowsed area into Nara Park. These results indicate that: (1) the U. thunbergiana population of Nara Park has an extremely high stinging hair density compared with those of unbrowsed areas; (2) this characteristic has a genetic basis, and (3) stinging hairs serve as a defensive structure against sika deer, contributing to an increase in survivorship. Thus, we conclude that a U. thunbergiana population in Nara Park, with extremely high stinging hair densities, has evolved through natural selection due to heavy browsing by sika deer.     

Variation in Cnidoscolus texanus in relation to herbivory

Summary Twelve Oklahoma populations of Cnidoscolus texanus were investigated for morphological variation between and within populations, and between site classes with contrasting histories of use by domesticated livestock. Characters scored were numbers of stinging hairs on petioles and midribs, and depth of leaf lobing. Leaf lobing was not significantly variable within or among populations or grazing categories. Stinging hair numbers showed significant differences within and among populations. Petiole stinging hairs were more numberous in heavily grazed sites, suggesting population differentiation in response to the selective pressures of herbivory.     

Induced defense in Nicotiana attenuata (Solanaceae) fruit and flowers

Plants protect themselves against herbivory using a continuum of strategies, ranging from constitutive defenses to intermittent induced responses. Induced defenses may not provide immediate and maximum protection, but could be advantageous when continuous defense is either energetically or ecologically costly. As such, induced defenses in flowers could help defend relatively valuable tissue while keeping reproductive structures accessible and attractive to pollinators. Thus far, no one has demonstrated the efficacy of induced defenses against floral herbivores (florivores) in the field. Here we show that mechanical leaf damage in wild tobacco, Nicotiana attenuata (Solanaceae), reduced both flower and fruit herbivory in the field and that exogenous application of methyl jasmonate, a potent elicitor of induced responses, reduced both leaf and floral damage in natural populations. This result is consistent with a survey of damage in the field, which showed a negative relationship between leaf damage and flower and fruit damage. Although optimal defense theory predicts that induced defenses should be rare in reproductive tissues, owing to their high fitness value, our results suggest otherwise. Induced defenses in leaves and reproductive tissues may allow plants to respond effectively to the concomitant pressures of defending against herbivory and attracting pollinators.     

How to induce defense responses in wild plant populations? Using bilberry (Vaccinium myrtillus) as example

Inducible plant defense is a beneficial strategy for plants, which imply that plants should allocate resources from growth and reproduction to defense when herbivores attack. Plant ecologist has often studied defense responses in wild populations by biomass clipping experiments, whereas laboratory and greenhouse experiments in addition apply chemical elicitors to induce defense responses. To investigate whether field ecologists could benefit from methods used in laboratory and greenhouse studies, we established a randomized block‐design in a pine‐bilberry forest in Western Norway. We tested whether we could activate defense responses in bilberry (Vaccinium myrtillus) by nine different treatments using clipping (leaf tissue or branch removal) with or without chemical treatment by methyljasmonate (MeJA). We subsequently measured consequences of induced defenses through vegetative growth and insect herbivory during one growing season. Our results showed that only MeJA‐treated plants showed consistent defense responses through suppressed vegetative growth and reduced herbivory by leaf‐chewing insects, suggesting an allocation of resources from growth to defense. Leaf tissue removal reduced insect herbivory equal to the effect of the MeJa treatments, but had no negative impact on growth. Branch removal did not reduce insect herbivory or vegetative growth. MeJa treatment and clipping combined did not give an additional defense response. In this study, we investigated how to induce defense responses in wild plant populations under natural field conditions. Our results show that using the chemical elicitor MeJA, with or without biomass clipping, may be a better method to induce defense response in field experiments than clipping of leaves or branches that often has been used in ecological field studies.     

COSTS OF INDUCED RESPONSES AND TOLERANCE TO HERBIVORY IN MALE AND FEMALE FITNESS COMPONENTS OF WILD RADISH

Theory predicts that plant defensive traits are costly due to trade-offs between allocation to defense and growth and reproduction. Most previous studies of costs of plant defense focused on female fitness costs of constitutively expressed defenses. Consideration of alternative plant strategies, such as induced defenses and tolerance to herbivory, and multiple types of costs, including allocation to male reproductive function, may increase our ability to detect costs of plant defense against herbivores. In this study we measured male and female reproductive costs associated with induced responses and tolerance to herbivory in annual wild radish plants (Raphanus raphanistrum). We induced resistance in the plants by subjecting them to herbivory by Pieris rapae caterpillars. We also induced resistance in plants without leaf tissue removal using a natural chemical elicitor, jasmonic acid; in addition, we removed leaf tissue without inducing plant responses using manual clipping. Induced responses included increased concentrations of indole glucosinolates, which are putative defense compounds. Induced responses, in the absence of leaf tissue removal, reduced plant fitness when five fitness components were considered together; costs of induction were individually detected for time to first flower and number of pollen grains produced per flower. In this system, induced responses appear to impose a cost, although this cost may not have been detected had we only quantified the traditionally measured fitness components, growth and seed production. In the absence of induced responses, 50% leaf tissue removal, reduced plant fitness in three out of the five fitness components measured. Induced responses to herbivory and leaf tissue removal had additive effects on plant fitness. Although plant sibships varied greatly (49–136%) in their level of tolerance to herbivory, costs of tolerance were not detected, as we did not find a negative association between the ability to compensate for damage and plant fitness in the absence of damage. We suggest that consideration of alternative plant defense strategies and multiple costs will result in a broader understanding of the evolutionary ecology of plant defense.     

Among-population variation in resistance traits of a nettle and its relationship with deer habitat use frequency

Because leaf hairs serve as resistance against herbivores, among-population variation in hair production may arise from adaptation to local herbivore communities. It is possible that Japanese nettle (Urtica thunbergiana) shows among-population variation in stinging hair abundance that is associated with the frequency of habitat use by sika deer (Cervus nippon). We examined 31–32 individuals of each of 19 populations for leaf area, stinging hair number (/leaf) and stinging hair density (per square centimeter) in and away from Nara Park (6.6 km²), where many deer have been protected for 1,200 years. At each site we also measured deer habitat use frequency, light intensity and soil fertility as environmental factors potentially affecting leaf traits. We analyzed our hierarchical data at the levels of individuals and populations using multilevel structural equation modeling. Leaf area had a positive direct effect on stinging hair number at the individual level but no significant effect at the population level. At the population level, deer habitat use frequency had a negative direct effect on leaf area and positive direct effects on stinging hair number and density, generating a negative indirect correlation between leaf area and stinging hair number. Light intensity had a negative direct effect on leaf area, while soil fertility had no significant effect on any trait. These results suggest that the relationships between leaf area and stinging hair number at the two levels do not align. We discussed what processes were involved in the effects of environmental factors on leaf traits.     

Natural and artificial floral damage induces resistance in Nemophila menziesii (Hydrophyllaceae) flowers

Resistance to leaf herbivory is well-documented in plants. In contrast, resistance to herbivory in flowers has received very little attention, even though reproductive tissues are often essential for plant reproduction. Plants may protect reproductive tissues with a range of defenses from constitutive to induced, although ecological costs associated with constitutive defense or resistance are expected to be higher than costs associated with induced responses. Induced responses in flowers may be effective against floral herbivores while minimizing the negative impacts of resistance on pollinators. This study examines induced responses in Nemophila menziesii (Hydrophyllaceae), a plant that frequently receives high levels of floral herbivory. I report that natural caterpillar herbivory increased levels of resistance against caterpillars later in the season. Similarly, artificial clipping to flowers consistently reduced natural damage to flowers vs unclipped controls over two years. Neither whole-plant nor individual seed set was affected by the reduction of floral damage. Induced resistance in reproductive tissues may benefit plants that are exposed to both floral herbivory and pollinator activity and can be an important link between plant antagonists and plant mutualists.     

Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium

Plants have considerable ability to respond to herbivory, both with (above-ground) regrowth and with increased defense. We simulated both leaf and shoot herbivory in controlled, replicated experiments on individuals of Acacia drepanolobium in Laikipia, Kenya. These experiments were carried out on individuals that had experienced different, experimentally controlled histories of large mammalian herbivory. Both forms of simulated herbivory were associated with compensatory regrowth. Branches whose shoots had been removed grew significantly more over the next year than paired control branches, fully compensating for the lost shoot length. Branches whose leaves were removed both grew faster and had more leaves one year later than did control branches. Shoot removal, but not leaf removal, increased the production of side shoots. However, because past herbivore pressure was negatively associated with net shoot growth, there may be a long-term cost of herbivory even when plants appear to fully compensate for herbivory in the short term. In contrast to the effects on growth, simulated herbivory did not significantly increase physical (spines) or chemical (tannins) defenses, and there were no significant negative correlations between compensatory growth and plant defense.     

Differential induced response to generalist and specialist herbivores by Lindera benzoin (Lauraceae) in sun and shade

Theoretically, induced defenses should be prevalent within low resource environments like the forest understory where constitutive defenses would be costly. Also, the induced response should be stronger when the herbivore is a generalist rather than a specialist, which often have mechanisms to avoid or overcome plant defenses. These ideas have been previously tested for herbaceous species, and we examined these predictions in Lindera benzoin (spicebush), a common woody shrub of the eastern deciduous forest. Lindera benzoin plants in contrasting light environments served as control plants or were subjected to one of four treatments: application of jasmonic acid, clipping, herbivory by the specialist Epimecis hortaria (tulip tree beauty) and herbivory by the generalist Spodoptera exigua (beet armyworm). Following treatment, we assessed induced responses by measuring leaf chemistry (C/N ratio, protein content, and peroxidase activity), and by using insect bioassays with E. hortaria larvae. We found no difference in peroxidase activity between light environments in controls, plants treated with clipping or jasmonic acid. In plants subject to insect herbivory, peroxidase activity was greater in shade plants than in sun plants. The magnitude of this increase in the shade varied between the herbivores, with a 32 fold increase in plants exposed to the generalist S. exigua and a 9 fold increase in plants exposed to the specialist E. hortaria . Leaves from shade plants had more protein and lower C/N ratios than leaves from sun plants, regardless of induction treatment. In control plants, E. hortaria larvae consumed more leaf biomass and achieved greater final weights in the sun than in the shade, but these differences disappeared with induction treatments were applied. These results are among the first to show rapid induction in a woody plant, and different levels of induction with light environments and with specialist versus generalist herbivores.     

Induction of extrafloral nectar depends on herbivore type in invasive and native Chinese tallow seedlings

Although induced defenses are widespread in nature, and a potentially important strategy used by invasive plants, it is unclear how induced defenses vary among populations and whether the intensity and duration of induced defenses depends on herbivore type. For invasive plants, low herbivore loads in their introduced ranges can lead to differences in herbivore defense compared to their native ranges, but we currently know little about how induced defenses vary among native and invasive populations. We conducted a greenhouse experiment to examine variation in one type of induced defense, extrafloral nectar (EFN) production, among native and invasive populations of Chinese tallow tree, Triadica sebifera. We experimentally manipulated herbivory from an exotic generalist scale insect, a native generalist caterpillar, both herbivores, or neither and then examined EFN production by Triadica. Damage from leaf-chewing caterpillars resulted in strongly induced EFN in both native and invasive populations while damage from phloem-feeding scales did not. Extrafloral nectar production and dissolved solute content peaked 4 days after caterpillar herbivory for both native and invasive populations. Number and proportion of leaves producing EFN, EFN volume and concentration of dissolved solutes were similar among native and invasive populations. These results suggest that selection for indirect defenses may be different than selection for other defenses in the introduced ranges of invasive plants, as constitutive and induced EFN production is retained in invasive populations.     

Prickly Poppies Can Get Pricklier: Ontogenetic Patterns in the Induction of Physical Defense Traits

Plant ontogeny is a common source of variation in defense and herbivory. Yet, few studies have investigated how the induction of physical defense traits changes across plant ontogeny. Physical defense traits are costly to produce, and thus, it was predicted that induction as a cost-saving strategy would be particularly favorable for seedlings, leading to ontogenetic declines in the inducibility of these traits. We tested for induction of three different physical defense traits (prickles, latex and leaf toughness) in response to mechanical defoliation and jasmonic acid application using prickly poppies (Argemone glauca and A. mexicana, Papaveraceae) as a model system. Genetic variation in the induction of physical defenses was tested using maternal sib-ships sampled from multiple populations. Both species induced higher densities of laminar prickles, although the magnitude of induction was much higher in the endemic Hawaiian prickly poppy, A. glauca, than in the cosmopolitan A. mexicana. The magnitude of prickle induction was also higher in young compared to older juvenile plant stages in A. glauca, demonstrating a strong role of ontogeny. Neither latex exudation nor leaf toughness was induced in either species. Although significant genetic variation was detected within and among populations for constitutive expression of physical defense traits in Argemone, there was no evidence for genetic variation in the induction of these traits. This study provides the first evidence for the induction of physical defenses in prickly poppies, emphasizing how an ontogenetically explicit framework can reveal new insights into plant defense. Moreover, this study illustrates how sister species comparisons between island vs. continental plants can provide new insights into plant functional and evolutionary ecology, highlighting a fruitful area for future research on more species pairs.     

Plant responses and herbivory following simulated browsing and stem cutting of Combretum apiculatum

Abstract. Time and mode of herbivory on savanna trees and their subsequent responses are dependent on, among other things, earlier herbivory and fire. We used clipping (simulated browsing) and stem cutting (simulated heavy browsing and to some extent simulated fire) to evaluate such interactions. Study organisms were a deciduous, broad‐leaved tree species, Combretum apiculatum (Combretaceae), browsing large herbivores and leaf‐eating insects. The treatments were done in the late dry season before bud break. Late in the following wet season, we recorded plant responses to treatment and browsing. The treated trees, especially the cut ones, responded by producing larger and fewer annual shoots. Compared to control trees, there was a slight increase in shoot biomass of clipped trees and a strong reduction of cut ones. Leaf area increased in clipped trees, but decreased in cut ones. A marked increase in the number of browsed trees was recorded amongst treated trees. Number of bites, consumption and utilization also increased with severity of treatment. In contrast, insect herbivory was reduced on both clipped and cut trees. The observed patterns are discussed in relation to current ideas on plant ‐ herbivore interactions.     

Are Defenses of Wild Radish Populations Well Matched with Variability and Predictablity of Herbivory?

Theory predicts that plants should employ constitutive (fixed) defenses when herbivory is consistently strong among years and induced (plastic) defenses when herbivory varies among years but is predictable within a season. We tested this theory by examining the herbivore species and damage censused over three seasons for 20 populations of wild radish in northern California. We conducted assays of constitutive resistance by challenging undamaged plants from these 20 populations with their common herbivores in the greenhouse. We assayed induced resistance by comparing the performance of herbivores on plants that had been experimentally damaged to undamaged plants from the same populations. Following damage, plants generally became more resistant to chewing herbivores (caterpillars) but more susceptible to sucking herbivores (aphids). Constitutive resistance to caterpillars was not stronger for populations that had high levels of herbivory that varied little among years, contrary to theory. Induced resistance may be stronger for plants from populations where herbivory varied more among years, consistent with expectations, although low power makes this conclusion equivocal. Induced resistance was not stronger for populations where early herbivory was a good predictor of late season herbivory. This lack of support for theory could have been caused by inadequacies with the experimental tests or with the theory and its assumptions. The theory assumes a coevolutionary equilibrium; however, high gene flow that has been reported for wild radish could disrupt matches between risk of herbivory and plant defense. The theory also assumes that resistance traits evolved as defenses against herbivory although these traits also serve other functions. Finally, the correlation we measured between early and late season herbivory may be at a temporal scale that is irrelevant since wild radish appears to adjust its defenses very rapidly.     

Induction and persistence of allelochemicals in the foliage of balsam fir seedlings following simulated browsing

The intensity, pattern, and timing of browsing disturbances influence the mobilization of chemical defenses (allelochemicals) and subsequent growth of conifers such as balsam fir, an important food source for various insect and mammalian herbivores. The objective of this study was to examine the induction and persistence of allelochemicals in foliage of balsam fir seedlings as affected by the pattern and timing of browsing disturbance. We conducted a simulated browsing study in a greenhouse environment using four browsing patterns and three persistence times. Total phenols were induced shortly after simulated browsing and persisted for at least 1 month, whereas condensed tannins exhibited a delayed induction but were more persistent, remaining above background levels 2 months after simulated browsing. The chemistry of non-browsed seedlings revealed that a seasonal pattern was evident for both total phenols and condensed tannins. As the experiment progressed through the growing season, background levels of total phenols decreased while those of condensed tannins increased with the two allelochemicals groups appearing to show a negative relationship. These trends suggest that total phenols, which are assumed to have a lower biosynthetic cost and are more quickly mobilized than condensed tannins, are a first line of herbivory defense followed by condensed tannins which take longer to mobilize but have a longer persistence time or that total phenols act as a primary defense against insect herbivores which disturb plants in the early growing season while condensed tannins are mobilized in the late season to defend against mammalian browsers of balsam fir in winter.     

Developmental Changes in Direct and Indirect Defenses in the Young Leaves of the Neotropical Tree Genus Inga (Fabaceae)

Plant fitness is affected by herbivory, and in moist tropical forests, 70 percent of herbivore damage occurs on young leaves. Thus, to understand the effects of herbivory on tropical plant fitness, it is necessary to understand how tropical young leaves survive the brief, but critical, period of susceptibility. In this study, we surveyed three species of Inga during young leaf expansion. Three classes of toxic secondary metabolites (phenolics, saponins, and tyrosine), extrafloral nectar production, leaf area, and extrafloral nectary area were measured at randomly assigned young leaf sizes. In addition, all defenses were compared for potential trade‐offs during leaf expansion. No trade‐offs among defenses were found, and the concentration of all defenses, except tyrosine, decreased during leaf expansion. We suggest that plants continued to increase phenolic and saponin content, but at a rate that resulted in decreasing concentrations. In contrast, tyrosine content per leaf steadily increased such that a constant concentration was maintained regardless of young leaf size. Nectar production remained constant during leaf expansion, but, because young leaf area increased by tenfold, the investment in extrafloral nectar per leaf area significantly decreased. In addition, nectary area did not change during leaf expansion and therefore the relative size of the nectary significantly decreased during young leaf expansion. These results support the predictions of the optimal defense hypothesis and demonstrate that the youngest leaves have the highest investment in multiple defenses, most likely because they have the highest nitrogen content and are most susceptible to a diversity of herbivores.     

Expression and costs of induced defense traits in Alliaria petiolata,a widespread invasive plant

We used jasmonic acid to induce first-year plants of Alliaria petiolata, a European invader that largely escapes herbivory in North America, to examine continental, population, and environmental variation in the expression and costs of induced defense traits. While absolute levels varied among populations, the induction of trypsin inhibitor activity was strong and largely uniform across five native and seven invasive populations. Trichome densities varied across populations, were absent in two of them, and only tended to be inducible by jasmonic acid. Jasmonate induction was substantially costly to leaf growth and dry biomass production, the magnitude of which varied little among populations. Continental origin of the populations explained an insignificant amount of variation in any trait. Trypsin inhibitor activity was strongly inducible across a nutrient gradient, but induction was more costly to leaf growth at low soil nutrient levels. Our results show that A. petiolata displays defense traits that are strongly inducible by jasmonic acid across populations, that jasmonate induction is substantially costly to growth with little variation among populations, and that costs of induction increase with decreased soil nutrient availability. Escaping the need to express induced defense traits and their costs in the face of reduced herbivory in introduced habitats may benefit fitness of invasive plants even in the absence of any evolutionary change in resistance in these plants.     

Plant responses to variable timing of aboveground clipping and belowground herbivory depend on plant age

Aims Plants use different types of responses such as tolerance and induced defense to mitigate the effects of herbivores. The direction and magnitude of both these plant responses can vary with plant age. However, most studies have focused on aboveground herbivory, whereas important feeding occurs belowground. Here, we tested the hypothesis that plant tolerance and defense following shoot damage or root herbivory depends on plant age.     

Leaf compensatory growth as a tolerance strategy to resist herbivory in Pancratium sickenbergeri

We examined the effects of leaf herbivory by the dorcas gazelle, Gazella dorcas, on the compensatory growth of the geophyte Pancratium sickenbergeri (Amaryllidaceae) in the Negev desert, Israel. In three populations exposed to different levels of herbivory, we removed different amounts of photosynthetic leaf area from plants in five clipping treatments: 0, 25, 50%-dispersed over all leaves, 50%-entire area of half the leaves, 100%. The population with the lowest level of herbivory showed the lowest relative regrowth rate after clipping. In the population with a constantly high level of herbivory, plants in intermediate-clipping treatments overcompensated in leaf area after clipping. For all the populations, clipped plants produce more new leaves than unclipped plants. In the population with the highest level of herbivory, clipping treatments did not have a significant effect on the number of fruits per plant. In addition, we did not find a trade-off between investments in growth and reproduction in this population. Our results indicated that, in the desert lily, herbivores may select for plant mechanisms that compensate after damage as a tolerant strategy to maintain fitness.     

Herbivory-induced overcompensation and resource-dependent production of extrafloral nectaries in Luffa cylindrica (Cucurbitaceae)

Extrafloral nectaries (EFNs) are nectar secretory structures involved in the indirect defense of plants. In the sponge gourd (Luffa cylindrica), EFNs commonly occur on the lower surface of leaf blades and stipules and remain functional until leaf senescence. To test the hypothesis that the development of EFNs is influenced by herbivore damage and resource availability, we grew Luffa cylindrica under different concentrations of Hoagland's nutrient solution (nutrient-poor conditions: 10%, 50%; and control condition: 100%) and two herbivory treatments (damaged and undamaged leaves). We collected ten leaves from treated plants to quantify leaf area and EFN density. Overall, leaf area increased and EFN decreased in damaged plants, but this significantly depended on nutritional status. In undamaged plants, EFN density tended to remain constant, whereas foliar area increased with nutrient input. Under herbivory, foliar area increased at 10% but decreased at 50 and 100% of nutrients in relation to undamaged plants, whereas EFN density tended to increase with nutrient availability to exceed undamaged plants under control concentrations. Plants under nutrient-poor conditions subjected to herbivory exhibited an increased foliar area, characterizing a compensatory mechanism. Our results suggest that herbivore-induced indirect defense is a damage- and resource-dependent response in Luffa cylindrica. These findings contribute to understanding the factors that modulate indirect defenses and plant-herbivore-environment interactions.