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.     

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.     

Plant community diversity influences allocation to direct chemical defence in Plantago lanceolata

Background

Forecasting the consequences of accelerating rates of changes in biodiversity for ecosystem functioning requires a mechanistic understanding of the relationships between the structure of biological communities and variation in plant functional characteristics. So far, experimental data of how plant species diversity influences the investment of individual plants in direct chemical defences against herbivores and pathogens is lacking.

Methodology/Principal Findings

We used Plantago lanceolata as a model species in experimental grasslands differing in species richness and composition (Jena Experiment) to investigate foliar concentrations of the iridoid glycosides (IG), catalpol and its biosynthetic precursor aucubin. Total IG and aucubin concentrations decreased, while catalpol concentrations increased with increasing plant diversity in terms of species or functional group richness. Negative plant diversity effects on total IG and aucubin concentrations correlated with increasing specific leaf area of P. lanceolata, suggesting that greater allocation to light acquisition reduced the investment into these carbon-based defence components. In contrast, increasing leaf nitrogen concentrations best explained increasing concentrations of the biosynthetically more advanced IG, catalpol. Observed levels of leaf damage explained a significant proportion of variation in total IG and aucubin concentrations, but did not account for variance in catalpol concentrations.

Conclusions/Significance

Our results clearly show that plants growing in communities of varying species richness and composition differ in their defensive chemistry, which may modulate plant susceptibility to enemy attack and consequently their interactions with higher trophic level organisms.     

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.     

Neighbor species differentially alter resistance phenotypes in Plantago

In this study, we investigated how neighbors (i.e., competitors) altered resistance phenotypes, namely plant size and levels of secondary compounds (iridoid glycosides), of individual plants and specifically tested whether neighbor identity mattered. We conducted a greenhouse experiment with Plantago lanceolata and Plantago major (Plantaginaceae) in which each species served as focal plants as well as neighbors in a factorial design. In addition, we harvested plants six and nine weeks after transplantation to test whether effects changed as plants grew. In both species, competition reduced plant size, and this effect increased over time. Plantago lanceolata neighbors suppressed growth of both focal plant species more than P. major neighbors. Effects of competition on levels of secondary compounds were more complex. Concentrations of iridoid glycosides were increased by competition in both species at harvest one. By the second harvest, an effect of competition on iridoid glycosides was found only in P. major. Neighbor identity influenced levels of iridoid glycosides in P. lanceolata at harvest one; concentrations were higher in plants grown with P. lanceolata neighbors than in plants grown with P. major neighbors. We also tested whether there was a trade-off between growth (biomass) and defense (levels of iridoid glycosides). Biomass and iridoid glycoside content were significantly correlated only in plants grown with competition and harvested at nine weeks, and this relationship was positive in both species, indicating that there was no trade-off between growth and defense. This study suggests that neighbor identity could play an important role in interspecific interactions, including the interactions of plants with other trophic levels.     

Early Root Herbivory Impairs Arbuscular Mycorrhizal Fungal Colonization and Shifts Defence Allocation in Establishing Plantago lanceolata

Research into plant-mediated indirect interactions between arbuscular mycorrhizal (AM) fungi and insect herbivores has focussed on those between plant shoots and above-ground herbivores, despite the fact that only below-ground herbivores share the same part of the host plant as AM fungi. Using Plantago lanceolata L., we aimed to characterise how early root herbivory by the vine weevil (Otiorhynchus sulcatus F.) affected subsequent colonization by AM fungi (Glomus spp.) and determine how the two affected plant growth and defensive chemistry. We exposed four week old P. lanceolata to root herbivory and AM fungi using a 2×2 factorial design (and quantified subsequent effects on plant biomass and iridoid glycosides (IGs) concentrations. Otiorhynchus sulcatus reduced root growth by c. 64%, whereas plant growth was unaffected by AM fungi. Root herbivory reduced extent of AM fungal colonization (by c. 61%). O. sulcatus did not influence overall IG concentrations, but caused qualitative shifts in root and shoot IGs, specifically increasing the proportion of the more toxic catalpol. These changes may reflect defensive allocation in the plant against further attack. This study demonstrates that very early root herbivory during plant development can shape future patterns of AM fungal colonization and influence defensive allocation in the plant.     

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.     

Potential benefits of iridoid glycoside sequestration in Longitarsus melanocephalus (Coleoptera,Chrysomelidae)

Whenever potentially noxious plant compounds are taken up and recycled by herbivorous insects, a protective function of these sequestered compounds is assumed. The flea beetle Longitarsus melanocephalus sequesters iridoid glycosides from its host plant up to a concentration of 2% DW, yet so far it remained unknown whether the insects gain protection from natural enemies by sequestering plant compounds at these comparatively low concentrations. Here we tested whether iridoid glycosides might deter or inhibit various soil and litter-dwelling potential enemies and pathogens. In choice experiments presenting L. melanocephalus pupae together with Tribolium castaneum pupae, the predator Lithobius forficatus was deterred by the iridoid glycoside containing pupae, while Forficula auricularia as well as the nematode Heterorhabditis bacteriophora were not deterred. L. forficatus also avoided artificial baits doted with 2% iridoid glycosides while F. auricularia showed no aversion to iridoid glycosides at these concentrations and H. bacteriophora did not suffer any toxic effect. Of the pathogens tested, the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae were not inhibited in their growth by iridoid glycosides ranging up to 2%. However, an inhibitory effect could be observed against the entomopathogenic bacterium Bacillus thuringiensis, even at the relatively small concentrations that are common in L. melanocephalus. The antibacterial effect might thus be another important selective value of iridoid glycoside sequestration in this species.     

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.     

Early ontogenetic patterns in chemical defense in Plantago (Plantaginaceae): genetic variation and trade-offs

Predictions based on the plant age and growth-differentiation balance hypotheses of defense were tested in two congeneric species, Plantago lanceolata and P. major, by quantifying iridoid glycosides, defensive chemicals, in seeds and leaves during the first 6 wk of growth. Concentrations decreased from the seed to 2-wk-old seedling stage in P. lanceolata, but increased during this period in P. major. In both species, levels were similar for 2- and 4-wk-old plants, then significantly increased from 4 to 6 wk. Genetic variation in the ontogeny of iridoid glycoside production was significant in both species at the maternal family level and at the population level. To examine whether allocation costs could explain the low production of iridoid glycosides in seedlings, relationships between growth and defense (iridoid glycosides) were characterized. Growth and defense had a positive or null relationship in all age groups, indicating that there was no trade-off in these plants at any age. This study provides some support for the growth-differentiation balance hypothesis, but offers no support for the plant age hypothesis. Measuring how herbivory affects plant fitness at different ontogenetic stages may shed light on these patterns in Plantago and on the evolution of the ontogeny of defense.     

Effect of temperature and leaf age on growth versus moulting time of a generalist caterpillar fed plantain (Plantago lanceolata)

Abstract.

• 1 The simultaneous effects of daytime temperature (20°C versus 30°C) and leaf age (new versus intermediate-aged) on a generalist insect herbivore were examined. Fourth-instar Spilosoma congrua caterpillars were tested on plantain (Plantago lanceolata), one of this lepidopteran species’host plants, for which the major defensive chemicals, iridoid glycosides (aucubin and catalpol), could be quantified.
• 2 Cool temperature depressed amount of food eaten, amount of frass, and consumption and growth rates, and increased the proportion of time spent in the non-feeding period (from head-capsule slippage to ecdysis).
• 3 Average iridoid glycoside concentration was 2.8% dry weight (d.w.) in new leaves and 1.6% d.w. in intermediate-aged leaves. When fed new leaves, the caterpillars had a higher efficiency of conversion of ingested food to biomass and a higher growth rate than those fed intermediate-aged leaves. Furthermore, the proportion of time spent in the non-feeding period was prolonged by a diet of intermediate-aged leaves.
• 4 A second experiment showed that the percentage dry weight of aucubin, catalpol and total iridoid glycosides increased over 24 h in incubated, excised leaves, which meant that the caterpillars in the first experiment experienced somewhat higher iridoid glycoside concentrations than the levels in fresh leaves.
• 5 Overall, these results indicate that this generalist should prefer new plantain leaves over older leaves even though new leaves contain higher concentrations of iridoid glycosides.

     

Phenylethanoid glycosides from Lippia javanica

Lippia javanica (N.L.Burm.) Spreng. is an aromatic, multipurpose medicinal plant from which a number of volatile compounds have been identified, together with toxic triterpenoids and iridoid glycosides. Two additional phenylethanoid glycosides, verbascoside and isoverbascoside, were isolated from L. javanica and characterized. High performance liquid chromatography analyses of polar extracts of three other Lippia species (L. scaberrima, L. rehmannii and L. wilmsii), indigenous to South Africa, revealed the presence of both isomers. When compared to the other indigenous Lippia species, the leaves of L. javanica were found to contain the highest concentrations of both isomers. In addition, the intraspecies variation of the verbascoside/isoverbascoside content of L. javanica, harvested from the same and different localities, was investigated. The concentrations of the two phenylethanoids remained fairly consistent within and between different populations, even when geographically separated. While these compounds are produced by many genera, they may now be added to the list of iridoid glucosides employed as chemotaxonomic markers for Lippia species.     

Slow decomposition of very fine roots and some factors controlling the process: a 4-year experiment in four temperate tree species

Background and aims

Nitrate leaching from intensively cropped soils represents a huge environmental problem. In order to diversify the range of nitrogen management strategies, this investigation is focused on the effects of ribwort plantain, Plantago lanceolata L., and its allelochemicals on soil N mineralization.

Methods

High-performance liquid chromatography was used in this study for phytochemical analysis of the major allelochemicals aucubin, catalpol, and verbascoside. Soil incubation experiments demonstrated a significant suppression of soil N mineralization caused by the incorporation of the iridoid glycoside (IG) aucubin, leaf material of two varieties (P. lanceolata cv. Libor and cv. Arterner), and an aqueous extract of P. lanceolata leaves.

Results

Throughout the growing season, the two varieties conspicuously differed in aucubin and verbascoside contents as well as in leaf dry weight. In soil incubation experiments, incorporated leaf material of both varieties affected long-term low soil nitrate concentrations. Experimental aucubin application resulted in an inhibitory effect on soil N mineralization. This was not true for the IG catalpol. Furthermore, we observed a negative relationship between IG concentrations and inorganic soil nitrogen concentrations when the soil was incubated with aqueous P. lanceolata leaf extract of different concentrations.

Conclusion

This study enforced the hypothesis that allelochemicals of P. lanceolata have an active role in a suppression effect on soil N mineralization. Further research may be necessary to investigate the specific effects of P. lanceolata allelochemicals on the nitrogen cycle.     

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.     

The roles of foraging environment,host species,and host diet for a generalist pupal parasitoid

Even for parasitoids with a wide host range, not all host species are equally suitable, and host quality often depends on the plant the host feeds on. We compared oviposition choice and offspring performance of a generalist pupal parasitoid, Pteromalus apum (Retzius) (Hymenoptera: Pteromalidae), on two congeneric hosts reared on two plant species under field and laboratory conditions. The plants contain defensive iridoid glycosides that are sequestered by the hosts. Sequestration at the pupal stage differed little between host species and, although the concentrations of iridoid glycosides in the two plant species differ, there was no effect of diet on the sequestration by host pupae. The rate of successful parasitism differed between host species, depending on the conditions they were presented in. In the field, where plant‐associated cues are present, the parasitoid used Melitaea cinxia (L.) over Melitaea athalia (Rottemburg) (Lepidoptera: Nymphalidae), whereas more M. athalia were parasitised in simplified laboratory conditions. In the field, brood size, which is partially determined by rate of superparasitism, depended on both host and plant species. There was little variation in other aspects of offspring performance related to host or plant species, indicating that the two host plants are of equal quality for the hosts, and the hosts are of equal quality for the parasitoids. Corresponding to this, we found no evidence for associative learning by the parasitoid based on their natal host, so with respect to these host species they are truly generalist in their foraging behaviour.     

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.     

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.     

Adaptive transgenerational plasticity in the perennial Plantago lanceolata

Phenotypes of plants, and thus their ecology and evolution, can be affected by the environmental conditions experienced by their parents, a phenomenon called parental effects or transgenerational plasticity. However, whether such effects are just passive responses or represent a special type of adaptive plasticity remains controversial because of a lack of solid tests of their adaptive significance. Here, we investigated transgenerational effects of different nutrient environments on the productivity, carbon storage and flowering phenology of the perennial plant Plantago lanceolata, and whether these effects are influenced by seasonal variation in the maternal environment. We found that maternal environments significantly affected the offspring phenotype, and that plants consistently produced more biomass and had greater root carbohydrate storage if grown under the same environmental conditions as experienced by their mothers. The observed transgenerational effects were independent of the season in which seeds had matured. We therefore conclude that transgenerational effects on biomass and carbon storage in P. lanceolata are adaptive regardless of the season of seed maturation.     

The interplay between toxin-releasing β-glucosidase and plant iridoid glycosides impairs larval development in a generalist caterpillar, Grammia incorrupta (Arctiidae)

Herbivores with polyphagous feeding habits must cope with a diet that varies in quality. One of the most important sources of this variation in host plant suitability is plant secondary chemistry. We examined how feeding on plants containing one such group of compounds, the iridoid glycosides, might affect the growth and enzymatic activity in a polyphagous caterpillar that feeds on over 80 plant species in 50 different families. Larvae of the polyphagous arctiid, Grammia incorrupta, were reared exclusively on one of two plant species, one of which contains iridoid glycosides (Plantago lanceolata, Plantaginaceae) while the other does not (Taraxacum officinale, Asteraceae). Larval weight was measured on the two host plants, and midgut homogenates of last instar larvae were then assayed for activity and kinetic properties of β-glucosidases, using both a standard substrate, 4-nitrophenyl-β-D-glucose (NPβGlc), and the iridoid glycoside aucubin, one of the two main iridoid glycosides in P. lanceolata. Larvae feeding on P. lanceolata weighed significantly less and developed more slowly compared to larvae on T. officinale. While the larval midgut β-glucosidase activity determined with NPβGlc was significantly decreased when fed on P. lanceolata, aucubin was substantially hydrolyzed and the larval β-glucosidase activity towards both substrates correlated negatively with larval weight. Our results demonstrate that host plants containing high concentrations of iridoid glycosides have a negative impact on larval development of this generalist insect herbivore. This is most likely due to the hydrolysis of plant glycosides in the larval midgut which results in the release of toxic aglycones. Linking the reduced larval weight to the toxin-releasing action of an iridoid glycoside cleaving β-glucosidase, our results thus support the detoxification limitation hypothesis, suggesting fitness costs for the larvae feeding solely on P. lanceolata. Thus, in addition to the adaptive regulation of midgut β-glucosidase activity, host plant switching as a behavioral adaptation might be a prerequisite for generalist herbivores that allows them to circumvent the negative effects of plant secondary compounds.     

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.