Foraging behaviour of caterpillars given a choice of plant genotypes in the presence of insect predators

1. To examine the effects of predators and plant genotype on the behaviour, patterns of herbivory, growth, and survivorship of caterpillars, an experiment was conducted under semi‐natural conditions, with two host plant genotypes (low vs. high iridoid glycosides) of plantain Plantago lanceolata, two kinds of herbivores (noncryptic specialist Junonia coenia vs. cryptic generalist Pyrrharctia isabella), and two levels of caterpillar predation (with and without Podisus maculiventris stinkbugs). 2. Choice tests conducted in the laboratory showed that in three out of four cases, caterpillars preferred leaves from the low iridoid glycoside genotype. 3. In the field experiment, the presence of predators affected the amount of leaf material eaten per plot and the proportion of leaf material eaten by the caterpillars as expected, but it did not affect the use of plant genotypes by Junonia caterpillars within the plots. In contrast, the caterpillar density–predation treatments affected the proportion of leaves eaten by Pyrrharctia caterpillars that were of the low iridoid glycoside genotype, but not entirely as expected. The caterpillars used both genotypes equally when predators were present but the high iridoid glycoside genotype was used more by caterpillars at low density and without predators. 4. The message then is that on the scale that caterpillars could be choosy about intake of iridoid glycosides they were not choosy among plant genotypes; however they were choosy between leaves, which can differ in iridoid glycoside concentration via plant genotype and leaf age.     

Effects of genotype,habitat, and seasonal variation on iridoid glycoside content of Plantago lanceolata (Plantaginaceae) and the implications for insect herbivores

Summary We investigated the effects of genotype, habitat, and seasonal variation on production of the iridoid glycosides, aucubin and catalpol, in leaves of the common weed Plantago lanceolata. Two genotypes, one each from a lawn and an adjacent abandoned hayfield population, were clonally replicated in the greenhouse, and then planted back into the two habitats. One quarter of the plants from each treatment were harvested on each of four dates, at approximately two-week intervals. Over the course of the growing season, and in both habitats, we found a significant increase in the concentration of both aucubin and catalpol in P. lanceolata leaves. The genotypes differed in their response to environmental variation, both in time and between sites, as indicated by significant genotype x date and genotype x site interactions. Early in the season, habitat (lawn or field) had a greater effect on iridoid glycoside concentration than did plant genotype, but later in the season, plant genotype was more influential in determining the iridoid glycoside concentration. Thus, the relative palatability of Plantago genotypes to specialist and generalist herbivores may vary in time and space.     

Direct and correlated responses to selection on iridoid glycosides in Plantago lanceolata L.

Plantago lanceolata L. (ribwort plantain) produces two costly terpenoid secondary plant compounds, the iridoid glycosides aucubin and catalpol. We performed an artificial selection experiment to investigate direct and correlated responses to selection on the constitutive level of iridoid glycosides in the leaves for four generations. Estimated realized heritabilities (±SE) were 0.23 ± 0.07 and 0.23 ± 0.04 for upward and downward selection, respectively. The response to upward selection was caused by selection for a developmental pattern characterized by the production of fewer leaves that on average contain more iridoids, and by selection for a development‐independent increase in the level of these compounds. Significant correlated responses were observed for plant growth form. Upward selection resulted in plants with larger sized, but fewer leaves, fewer side rosettes, and fewer spikes, corresponding to a previously distinguished ‘hayfield’ ecotype, whereas downward selection produced the opposite pattern, corresponding to a ‘pasture’ ecotype. This indicates that the level of iridoid glycosides is genetically correlated with morphological traits in P. lanceolata, and is part of the complex of genetically correlated traits underlying the two ecotypes. The genetic association between iridoid level and growth forms suggests that there may be constraints to the simultaneous evolution of resistance to generalist insects (by iridoid glycosides) and to larger grazers (by a high production rate of prostrate leaves and inflorescences) in open grazed habitats where the ‘pasture’ ecotype is found.     

Host plant species affects the quality of the generalist Trichoplusia ni as a host for the polyembryonic parasitoid Copidosoma floridanum

Diet of herbivorous insects can influence both the herbivores and their natural enemies. We examined the direct and indirect effects of diet on the interactions between the polyphagous herbivore Trichoplusia ni Hübner (Lepidoptera: Noctuidae) and its polyembryonic parasitoid Copidosoma floridanum Ashmead (Hymenoptera: Encyrtidae). To determine how host plant species and host plant iridoid glycoside content affect host caterpillars and their parasitoids, parasitized and unparasitized T. ni were given leaves of either Plantago lanceolata L., which contains the iridoid glycosides aucubin and catalpol, Plantago major L. (Plantaginaceae), which contains only aucubin, or Taraxacum officinale F.H. Wigg (Asteraceae), which contains neither. Survival of unparasitized T. ni was much lower when fed P. major compared with the other two host plants, whereas pupae were smallest when fed T. officinale and developed most slowly when fed P. lanceolata as larvae. Neither aucubin nor catalpol were detected in intact Plantago‐fed T. ni larvae or their hemolymph, and only trace amounts of aucubin were detected in frass, suggesting that these compounds are mostly metabolized in the midgut and are not encountered by the parasitoid. Copidosoma floridanum clutch size was almost doubled when reared from P. lanceolata‐fed T. ni compared with T. officinale‐fed larvae and tripled compared with P. major‐fed larvae, although the percent of parasitoids surviving to adulthood was uniformly high regardless of host diet. The observed variation in C. floridanum fitness among host diets is likely mediated by the effect of the diets on host quality, which in turn may be influenced more by other factors in the host plants than their iridoid glycoside profiles. Interactions between plant metabolites, generalist herbivores like T. ni, and their parasitoids may be predominantly indirect.     

Behaviour of specialist and generalist caterpillars on plantain (Plantago lanceolata)

Abstract.

• 1 Foraging patterns of specialist (Junonia coenia Hubner: Nymphalidae) and generalist (Spilosoma congrua Wlk.: Arctiidae) caterpillars on five genotypes of plantain (Plantago lanceolata L.: Plantaginaceae) were examined in an experimental garden.
• 2 Movement by the specialists reflected declining availability of new leaves. When direct sunlight was available, these caterpillars were usually exposed to it.
• 3 Although the generalists also preferred new leaves, they spent less than 50% of their time on the plantain and changed location more frequently than the specialists. They often hid at the base of plants or under leaves.
• 4 Plant genotype influenced the apparency of the specialists and damage by the herbivores.

     

Effects of enhanced UV-B radiation on a weedy forb (Plantago lanceolata) and its interactions with a generalist and specialist herbivore

Assessments of potential impacts of global climate change often focus exclusively on plants; however, as the base of most food webs, plants generally experience abiotic stresses concomitantly with biotic stresses. Longleaf plantain, Plantago lanceolata L., is a cosmopolitan temperate perennial weed that experiences a wide range of environmental conditions throughout its range. We examined the impacts of elevated levels of exposure to shortwave (UV-B) radiation on this plant, on two herbivores associated with this plant, and on the plant-herbivore interaction. Plantains were grown at 6 and 12 kJ m^–2 d^–1 BE₃₀₀ UV-B radiation and concentrations of iridoid glycosides (aucubin and catalpol), verbascosides, and nitrogen were measured. In terms of plant impacts, we found that iridoid glycoside concentrations were unchanged by elevated UV-B radiation, whereas, in one experiment, the concentration of verbascosides in young leaves and levels of nitrogen in old leaves increased under elevated UV-B radiation. Variation in plant chemistry due to leaf age and maternal family was greater than variation due to UV-B exposure. When caterpillars were fed excised leaves from plants grown under elevated UV-B, growth and survivorship of the specialist herbivore, Precis coenia Hbn. (Lepidoptera: Nymphalidae), were unaltered and growth of the generalist herbivore, Trichoplusia ni (Hbn.) (Lepidoptera: Noctuidae), was accelerated. When the caterpillars were reared on potted plants at high and low levels of UV-B radiation, growth and survivorship of P. coenia were unchanged while growth of T. ni was significantly depressed by elevated UV-B. Elevated UV-B altered allocation patterns of above-ground biomass in these plants; masses of crowns and reproductive tissue were reduced. UV-B levels, however, did not affect distribution of damage to foliage inflicted by either species. In two additional experiments with artificial diet, designed to test the direct effect of UV-B radiation on caterpillars, growth and survivorship of P. coenia were unaltered while survivorship of T. ni was significantly depressed when caterpillars were exposed to elevated UV-B radiation. These studies collectively demonstrate that higher trophic level impacts of UV-B-induced changes in plants depend on the identity of the herbivore and its degree of adaptation not only to variation in hostplant quality but also variation in its light environment.     

Low rates of iridoid glycoside hydrolysis in two Longitarsus leaf beetles with different feeding specialization confer tolerance to iridoid glycoside containing host plants

Iridoid glycosides are plant defence compounds that are deterrent and/or toxic for unadapted herbivores but are readily sequestered by dietary specialists of different insect orders. Hydrolysis of iridoid glycosides by β‐glucosidase leads to protein denaturation. Insect digestive β‐glucosidases thus have the potential to mediate plant–insect interactions. In the present study, mechanisms associated with iridoid glycoside tolerance are investigated in two closely‐related leaf beetle species (Coleoptera: Chrysomelidae) that feed on iridoid glycoside containing host plants. The polyphagous Longitarsus luridus Scopoli does not sequester iridoid glycosides, whereas the specialist Longitarsus tabidus Fabricius sequesters these compounds from its host plants. To study whether the biochemical properties of their β‐glucosidases correspond to the differences in feeding specialization, the number of β‐glucosidase isoforms and their kinetic properties are compared between the two beetle species. To examine the impact of iridoid glycosides on the β‐glucosidase activity of the generalist, L. luridus beetles are kept on host plants with or without iridoid glycosides. Furthermore, β‐glucosidase activities of both species are examined using an artificial β‐glucosidase substrate and the iridoid glycoside aucubin present in their host plants. Both species have one or two β‐glucosidases with different substrate affinities. Interestingly, host plant use does not influence the specific β‐glucosidase activities of the generalist. Both species hydrolyse aucubin with a much lower affinity than the standard substrate. The neutral pH reduces the β‐glucosidase activity of the specialist beetles by approximately 60% relative to its pH optimum. These low rates of aucubin hydrolysis suggest that the ability to sequester iridoid glycosides has evolved as a key to potentially preventing iridoid glycoside hydrolysis by plant‐derived β‐glucosidases.     

Effects of cages,plant age and mechanical clipping on plantain chemistry

In plant-insect herbivore field studies, effects of cages, plant age, and mechanical clipping on host plant chemistry are often postulated but not well documented. We examined the effects of cages (for the purpose of restraining insects on experimental plots), plant age over the course of the experiment and mechanical clipping on plantain (Plantago lanceolata) chemistry. Leaf age affected the concentrations of nitrogen and iridoid glycosides (IGs; specifically aucubin and catalpol), with higher levels in newer leaves. Caged plants had higher levels of IGs and lower concentrations of nitrogen than uncaged plants. The IG concentrations were greater in new leaves of caged plants than uncaged plants, whereas the concentrations in mature leaves were unaffected by caging. Plants that were 5 weeks older had higher levels of IGs and lower nitrogen than plants harvested 5 weeks earlier. Comparison of three studies suggested that over the summer IG concentrations increase during dry years but decrease during wet years. Plants with above-ground parts clipped and then allowed to regrow for five weeks had similar concentrations of IGs and nitrogen compared to control plants; but the regrowth plants had a lower catalpol to total IG ratio. We conclude that cages and time can have significant positive effects on iridoid glycoside concentrations and significant negative effects on leaf nitrogen concentration. But our results also indicate that the direction and magnitude of the effects of cages, time and mechanical damage are not easily predicted. Therefore, it is advisable to determine and/or control for such effects in field experiments on plant-insect interactions.     

Impact of the dual defence system of Plantago lanceolata (Plantaginaceae) on performance,nutrient utilisation and feeding choice behaviour of Amata mogadorensis larvae (Lepidoptera,Erebidae)

Iridoid glycosides are plant defence compounds with potentially detrimental effects on non-adapted herbivores. Some plant species possess β-glucosidases that hydrolyse iridoid glycosides and thereby release protein-denaturing aglycones. To test the hypothesis that iridoid glycosides and plant β-glucosidases form a dual defence system, we used Plantago lanceolata and a polyphagous caterpillar species. To analyse the impact of leaf-age dependent differences in iridoid glycoside concentrations and β-glucosidase activities on insect performance, old or young leaves were freeze-dried and incorporated into artificial diets or were provided freshly to the larvae. We determined larval consumption rates and the amounts of assimilated nitrogen. Furthermore, we quantified β-glucosidase activities in artificial diets and fresh leaves and the amount of iridoid glycosides that larvae feeding on fresh leaves ingested and excreted. Compared to fresh leaves, caterpillars grew faster on artificial diets, on which larval weight gain correlated positively to the absorbed amount of nitrogen. When feeding fresh young leaves, larvae even lost weight and excreted only minute proportions of the ingested iridoid glycosides intact with the faeces, indicating that the hydrolysis of these compounds might have interfered with nitrogen assimilation and impaired larval growth. To disentangle physiological effects from deterrent effects of iridoid glycosides, we performed dual choice feeding assays. Young leaves, their methanolic extracts and pure catalpol reduced larval feeding in comparison to the respective controls, while aucubin had no effect on larval consumption. We conclude that the dual defence system of P. lanceolata consisting of iridoid glycosides and β-glucosidases interferes with the nutrient utilisation via the hydrolysis of iridoid glycosides and also mediates larval feeding behaviour in a concentration- and substance-specific manner.     

Occurrence of iridoid glycosides in in vitro cultures and intact plants of <Emphasis Type="Italic">Scrophularia nodosa</Emphasis> L.

Shoot, root, and callus cultures of Scrophularia nodosa L. (Scrophulariaceae) were established and cultivated in vitro. Iridoid glycosides, such as harpagoside, aucubin, and catalpol were identified by LC-ESI-MS and their contents determined by HPLC. For comparison intact plants of S. nodosa were analysed. In shoot cultures slightly lower amounts of detectable iridoid glycosides (4.36% dry weight) were determined than in the field grown plants (4.88%). Concentration of harpagoside was highest in leaves of field plants (1.05%) and in flowers of in vitro plantlets (1.10%). For aucubin the highest amount was found in the leaves of in vitro plantlets (1.67%) whereas the levels of aucubin in the leaves of field plants were remarkably lower. Catalpol was produced as a trace compound in intact plants and shoot cultures. Callus and root cultures were apparently not able to synthesise iridoid glycosides.     

Plant chemical defense against herbivores and pathogens: generalized defense or trade-offs?

Plants are often attacked by multiple enemies, including pathogens and herbivores. While many plant secondary metabolites show specific effects toward either pathogens or herbivores, some can affect the performance of both these groups of natural enemies and are considered to be generalized defense compounds. We tested whether aucubin and catalpol, two iridoid glycosides present in ribwort plantain (Plantago lanceolata), confer in vivo resistance to both the generalist insect herbivore Spodoptera exigua and the biotrophic fungal pathogen Diaporthe adunca using plants from P. lanceolata lines that had been selected for high- and low-leaf iridoid glycoside concentrations for four generations. The lines differed approximately three-fold in the levels of these compounds. Plants from the high-selection line showed enhanced resistance to both S. exigua and D. adunca, as evidenced by a smaller lesion size and a lower fungal growth rate and spore production, and a lower larval growth rate and herbivory under both choice and no-choice conditions. Gravimetric analysis revealed that the iridoid glycosides acted as feeding deterrents to S. exigua, thereby reducing its food intake rate, rather than having post-ingestive toxic effects as predicted from in vitro effects of hydrolysis products. We suggest that the bitter taste of iridoid glycosides deters feeding by S. exigua, whereas the hydrolysis products formed after tissue damage following fungal infection mediate pathogen resistance. We conclude that iridoid glycosides in P. lanceolata can serve as broad-spectrum defenses and that selection for pathogen resistance could potentially result in increased resistance to generalist insect herbivores and vice versa, resulting in diffuse rather than pairwise coevolution.     

Genetic variation in defensive chemistry in Plantago lanceolata (Plantaginaceae) and its effect on the specialist herbivore Junonia coenia (Nymphalidae)

To examine genetic variation in defensive chemistry within and between natural populations of Plantago lanceolata, we performed a greenhouse experiment using clonal replicates of 15 genotypes from each of two populations, from a mowed lawn and an abandoned hayfield. Replicates of each genotype were harvested for determinations of aboveground biomass and leaf chemical content either at the beginning of the experiment (initial controls), after exposure to herbivory by larvae of Junonia coenia, a specialist on P. lanceolata (herbivory treatment), or at the end of the experiment without exposure to herbivory (final controls). Allocation to the iridoid glycosides aucubin and catalpol and the phenylpropanoid glycoside verbascoside displayed significant genetic variation within and between populations, and differed with leaf age. Significant genotypextreatment interactions indicated genetic variation in response of leaf chemistry to the treatments. There was no evidence for a cost of allocation to chemical defense: genetic correlations within and between chemical pathways and between defensive chemicals and aboveground growth were positive or nonsignificant. Although iridoid glycosides are known to be qualitative feeding stimulants for J. coenia, multiple regression of larval survivorship on leaf chemical content and shoot biomass indicated that larvae had lower survivorship on P. lanceolata ge-notypes with higher concentrations of aucubin in the leaves. Larval survivorship was unaffected by levels of catalpol and verbascoside. Thus, although specialist herbivores may respond to defensive chemicals as qualitative feeding stimulants, they do not necessarily have higher fitness on plant genotypes containing higher concentrations of these chemicals.     

Effects of plant phenology, nutrients and herbivory on growth and defensive chemistry of plantain, Plantago lanceolata

To assess the combined effect of herbivory, nutrient availability and plant phenology on plant mass and defensive chemistry, we conducted a field experiment with plantain ( Plantago lanceolata : Plantaginaceae) using three levels of herbivory, three levels of fertilizer and two harvest dates. Shoot mass of the no-herbivory plants showed a nonlinear response to increased fertilizer such that mass with high fertilizer was no greater than that with low fertilizer. In contrast, shoot mass of the low-herbivory plants (12% damage) was not influenced by fertilizer, but for high-herbivory plants (23% damage), there was a positive linear response to increased fertilizer. Increasing nutrient levels caused a decrease in iridoid glycoside concentration. Herbivory did not induce higher iridoid glycoside concentration in leaves of any age. But increasing herbivory resulted in a decrease in the concentration of catalpol in new leaves. Another experiment assessed how leaf age and plant age affected plant defensive chemistry. Total iridoid glycosides increased over 5 weeks, but catalpol only increased in new leaves. Overall, the order of importance in determining variation in iridoid glycoside concentration was plant phenology, nutrient availability and, to a much lesser extent, herbivory.     

Selective sequestration of iridoid glycosides from their host plants in Longitarsus flea beetles

We investigated in eight species of the flea beetles genus Longitarsus (Coleoptera, Chrysomelidae) whether the beetles take up iridoid glycosides from their host plants of the Lamiaceae, Plantaginaceae, and Scrophulariaceae. Five of the beetle species, L. australis, L. lewisii, L. melanocephalus, L. nigrofasciatus, and L. tabidus, could be shown to sequester iridoid glycosides in concentrations between 0.40 and 1.55% of their dry weight. Eight different iridoid glycosides, acetylharpagide, ajugol, aucubin, catalpol, 8-epi-loganic acid, gardoside, geniposidic acid, and harpagide could be identified in the host plants, yet only aucubin and catalpol are sequestered by the beetles. No iridoid glycosides could be detected in the beetles if neither aucubin nor catalpol were present in the host plant, as in L. minusculus on Stachys recta (acetylharpagide only) and in L. salviae on Salvia pratensis (no iridoid glycosides). In one beetle species, L. luridus, we could not detect any iridoid glycosides although its field host, Plantago lanceolata, had considerable amounts of aucubin and catalpol plus two further iridoids. The five sequestering Longitarsus species differ in their capacity to store the compounds and in their affinity for catalpol relative to aucubin.     

Determination of aucubin and catalpol in Plantago species by micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC) was used for the separation and determination of two iridoid glycosides, aucubin and catalpol, in several Plantago species growing in Croatia: P. altissima L., P. argentea Chaix, P. coronopus L., P. holosteum Scop. (subsp. depauperata, subsp. holosteum and subsp. scopulorum), P. lagopus L., P. lanceolata L., and P. maritima L. Hot water extraction (HWE) was applied for the isolation of iridoid substances. Significant differences appeared between the iridoid contents in the examined species. The yield of aucubin and catalpol was up to 0.27% and 1.81% of the dry mass of the leaves, respectively. Besides aucubin and catalpol, two related compounds were determined in the plant samples.     

Influence of iridoid glycoside containing host plants on midgut β-glucosidase activity in a polyphagous caterpillar, Spilosoma virginica Fabricius (Arctiidae)

Iridoid glycosides are secondary plant compounds that have deterrent, growth reducing or even toxic effects on non-adapted herbivorous insects. To investigate the effects of iridoid glycoside containing plants on the digestive metabolism of a generalist herbivore, larvae of Spilosoma virginica (Lepidoptera: Arctiidae) were reared on three plant species that differ in their secondary plant chemistry: Taraxacum officinale (no iridoid glycosides), Plantago major (low iridoid glycoside content), and P. lanceolata (high iridoid glycoside content). Midguts of fifth instar larvae were assayed for the activity and kinetic properties of β-glucosidase using different substrates. Compared to the larvae on T. officinale, the β-glucosidase activity of larvae feeding on P. lanceolata was significantly lower measured with 4-nitrophenyl-β-d-glucopyranoside. Using the iridoid glycoside aucubin as a substrate, we did not find differences in the β-glucosidase activity of the larvae reared on the three plants. Heat inactivation experiments revealed the existence of a heat-labile and a more heat-stable β-glucosidase with similar Michaelis constants for 4-nitrophenyl-β-d-glucopyranoside. We discuss possible mechanisms leading to the observed decrease of β-glucosidase activity for larvae reared on P. lanceolata and its relevance for generalist herbivores in adapting to iridoid glycoside containing plant species and their use as potential host plants.     

Microorganisms and nematodes increase levels of secondary metabolites in roots and root exudates of Plantago lanceolata

Plant secondary metabolites play an important role in constitutive and inducible direct defense of plants against their natural enemies. While induction of defense by aboveground pathogens and herbivores is well-studied, induction by belowground organisms is less explored. Here, we examine whether soil microorganisms and nematodes can induce changes in levels of the secondary metabolites aucubin and catalpol (iridoid glycosides, IG) in roots and root exudates of two full-sib families of Plantago lanceolata originating from lines selected for low and high constitutive levels of IG in leaves. Addition of soil microorganisms enhanced the shoot and root biomass, and the concentration of aucubin in roots of both Plantago lines without affecting IG levels in the rhizosphere. By contrast, nematode addition tended to reduce the root biomass and enhanced the stalk biomass, and increased the levels of aucubin and catalpol in root exudates of both Plantago lines, without affecting root IG concentrations. The Plantago lines did not differ in constitutive levels of aucubin and total IG in roots, while the concentration of catalpol was slightly higher in roots of plants originally selected for low constitutive levels of IG in leaves. Root exudates of “high IG line” plants contained significantly higher levels of aucubin, which might be explained by their higher root biomass. We conclude that soil microorganisms can induce an increase of aucubin concentrations in the roots, whereas nematodes (probably plant feeders) lead to an enhancement of aucubin and catalpol levels in root exudates of P. lanceolata. A potential involvement of secondary metabolites in belowground interactions between plants and soil organisms is discussed.     

Light and Nutrient Dependent Responses in Secondary Metabolites of Plantago lanceolata Offspring Are Due to Phenotypic Plasticity in Experimental Grasslands

A few studies in the past have shown that plant diversity in terms of species richness and functional composition can modify plant defense chemistry. However, it is not yet clear to what extent genetic differentiation of plant chemotypes or phenotypic plasticity in response to diversity-induced variation in growth conditions or a combination of both is responsible for this pattern. We collected seed families of ribwort plantain (Plantago lanceolata) from six-year old experimental grasslands of varying plant diversity (Jena Experiment). The offspring of these seed families was grown under standardized conditions with two levels of light and nutrients. The iridoid glycosides, catalpol and aucubin, and verbascoside, a caffeoyl phenylethanoid glycoside, were measured in roots and shoots. Although offspring of different seed families differed in the tissue concentrations of defensive metabolites, plant diversity in the mothers'' environment did not explain the variation in the measured defensive metabolites of P. lanceolata offspring. However secondary metabolite levels in roots and shoots were strongly affected by light and nutrient availability. Highest concentrations of iridoid glycosides and verbascoside were found under high light conditions, and nutrient availability had positive effects on iridoid glycoside concentrations in plants grown under high light conditions. However, verbascoside concentrations decreased under high levels of nutrients irrespective of light. The data from our greenhouse study show that phenotypic plasticity in response to environmental variation rather than genetic differentiation in response to plant community diversity is responsible for variation in secondary metabolite concentrations of P. lanceolata in the six-year old communities of the grassland biodiversity experiment. Due to its large phenotypic plasticity P. lanceolata has the potential for a fast and efficient adjustment to varying environmental conditions in plant communities of different species richness and functional composition.     

Variable chemical defence in the checkerspot butterfly Euphydryas gillettii (Lepidoptera: NymphaIidae)

Abstract.

• 1 Like other checkerspots, Euphydryas gillettii butterflies may contain the defensive chemicals, iridoid glycosides, which are sequestered from their hostplants during larval feeding.
• 2 We analysed the iridoid glycoside content of E.gillettii adults from two different populations, Warm Lake, Idaho, and Granite Creek, Wyoming, that have different patterns of hostplant use.
• 3 Gas chromatographic analysis of thirty butterflies from the Wyoming population showed that they contained a mean of 1.27 (±0.19 SE) % dry weight iridoid glycosides. Notably, 20% of these butterflies contained no detectable iridoid glycosides.
• 4 In contrast, nineteen butterflies from the Idaho population contained a mean of 3.89 (±0.38 SE) % dry weight iridoid glycosides, and all butterflies contained iridoid glycosides.
• 5 These results illustrate how the chemical defence of herbivorous insects varies according to differential use of potential hostplants.

     

Prey species and prey diet affect growth of invertebrate predators

1. The effects of prey species and leaf age used by prey on performance of two generalist invertebrate predators were studied. The focal plant was Plantago lanceolata , which contains iridoid glycosides.
2. Diet of the herbivorous prey influenced their growth rate.
3. The generalist herbivore ( Vanessa cardui ) and the novel-plant feeder ( Manduca sexta ) contained very low levels of iridoid glycosides in their haemolymph, whereas the specialist ( Junonia coenia ) levels were 50–150-fold higher.
4. Predatory stinkbugs ( Podisus maculiventris ) fed either the novel-plant feeder or the specialist exhibited similar developmental rates. However, stinkbugs ate less of the generalist but grew faster. The growth rate of the stinkbugs was higher when the caterpillar species were raised on the new-leaf powder diet, which contained twice as much protein and iridoid glycosides as the mature-leaf powder diet.
5. Jumping spiders ( Phidippus audax ) ate more mealworms ( Tenebrio molitor ) than specialist J. coenia caterpillars, fed either new- or mature-leaf powder diets, and could not gain weight when fed J. coenia.
6. These results indicate that prey quality was not determined solely by the iridoid glycoside concentration in the diet.