CONSTRAINTS ON CHEMICAL COEVOLUTION: WILD PARSNIPS AND THE PARSNIP WEBWORM

The parsnip webworm (Depressaria pastinacella) and the wild parsnip (Pastinaca sativa) together represent a potentially “coevolved” system in that throughout their ranges the plant has relatively few other herbivores and the insect has virtually no other hosts. Individual wild parsnip plants within a central Illinois population vary in their content and composition of furanocoumarins, secondary compounds with insecticidal properties. Half-sib and parent-offspring regression estimates of the heritability of furanocoumarins demonstrate that this variation is genetically based. Wild parsnip plants also vary in their resistance to damage by the parsnip webworm, which feeds on flowers and developing seeds. In an experimental garden, seed production in the primary umbel ranged from 0 to 1,664 seeds among individuals, and mean seed production of half-sib families ranged from 3.7 seeds to 446.0 seeds. Approximately 75% of the variation in resistance among half-sib families to D. pastinacella was attributable to four furanocoumarin characteristics—resistance is positively related to the proportion of bergapten and the amount of sphondin in seeds, and negatively related to the amount of bergapten and the proportion of sphondin in leaves. Each of the four resistance factors had significant heritability. Thus, resistance in wild parsnip to the parsnip webworm is to a large extent chemically based and genetically controlled. Genetic correlations among fitness and resistance characters, however, tend to limit coevolutionary responses between herbivore and plant. In greenhouse plants protected from herbivory, several of the resistance factors have negative genetic correlations with potential seed production. Ostensibly, highly resistant plants in the absence of herbivory would be at a competitive disadvantage in the field. The selective impact of the herbivore is also limited in this population by a negative genetic correlation among resistance factors. Selection to increase one resistance factor (e.g., the proportion of bergapten in the seed) would at the same time decrease the amount of a second resistance factor (e.g., the amount of sphondin in the seed). The wild parsnip and the parsnip webworm, then, appear to have reached an evolutionary “stalemate” in the coevolutionary arms race.     

GENETICS OF PHYSIOLOGICAL AND BEHAVIORAL RESISTANCE TO HOST FURANOCOUMARINS IN THE PARSNIP WEBWORM

Depressaria pastinacella, the parsnip webworm, feeds almost exclusively on the flowers and fruits of Pastinaca sativa, the wild parsnip. Resistance to webworms in wild parsnip populations is largely attributable to genetically based variation in furanocoumarin chemistry; by differentially reducing fruit set among chemical phenotypes, parsnip webworms may act as selective agents on wild parsnip populations. To determine whether wild parsnip chemistry can act as a selective agent on webworm populations, it is necessary to establish that resistance mechanisms in the webworm to furanocoumarins are genetically based. In this study, we estimated the amount of genetic variation in behavioral and physiological responses of webworms to parsnip furanocoumarins. Virtually no variation was found among webworm families for feeding preferences for diets varying as much as fourfold in furanocoumarin content. Nor was significant variation found for mean furanocoumarin intake over the assay period, except in one case, in which maternal effects may account for differences among families. In contrast, substantial familial variation existed for cytochrome P450–mediated metabolism of bergapten and xanthotoxin, two host furanocoumarins. The presence of additive genetic variation in metabolism, and the absence of such variation in discriminative feeding behavior, suggests that adaptation to changes in furanocoumarin chemistry, resulting either from changes in the distribution of chemical phenotypes in parsnip populations or from shifts to new chemically different host plants, is likely to be facilitated by physiological rather than behavioral means.     

Host plant, host plant chemistry and the polyembryonic parasitoid Copidosoma sosares: indirect effects in a tritrophic interaction

Host plant identity and host plant chemistry have often been shown to influence host finding and acceptance by natural enemies but comparatively less attention has been paid to the tritrophic effects of host plant and host plant chemistry on other natural enemy fitness correlates, such as survivorship, clutch size, body size, and sex ratio. Such studies are central to understanding both the selective impact of plants on natural enemies as well as the potential for reciprocal selective impact of natural enemies on plant traits. We examined the effects of host plant and host plant chemistry in a tritrophic system consisting of three apiaceous plants (Pastinaca sativa, Heracleum sphondylium and H. mantegazzianum), the parsnip webworm (Depressaria pastinacella) and the polyembryonic parasitic wasp Copidosoma sosares. All of these plants produce furanocoumarins, known resistance factors for parsnip webworms. Furanocoumarin concentrations were correlated neither with the presence nor the number of webworms on a given plant. Concentrations of two furanocoumarins were negatively associated with C. sosares fitness correlates: isopimpinellin with the likelihood that a given webworm would be parasitized and xanthotoxin with both within‐brood survivorship (of all‐male and mixed‐sex broods) and clutch size. Brood sex ratio and body sizes of individual wasps were not correlated with furanocoumarin chemistry. Because additive genetic variation exists in P. sativa for furanocoumarin chemical traits, these are subject to selection by webworms through herbivory. Third trophic level selective impacts on furanocoumarin traits may include selection for reduced production of those chemicals that affect parasitoid survivorship yet do not influence host plant choice by the herbivore. That such might be the case is suggested by patterns of furanocoumarin production in populations of P. sativa with different histories of infestation; in the Netherlands, where parasitism rates of webworms by C. sosares are high, plants produce lower levels of all linear furanocoumarins and proportionately less isopimpinellin than do midwestern U.S. populations of P. sativa, where natural enemies of the webworm are effectively absent.     

Chemical barriers to adaptation by a specialist herbivore

Summary Depressaria pastinacella, the parsnip webworm (Lepidoptera: Oecophoridae), feeds throughout eastern North America on Pastinaca sativa (wild parsnip) and few other species. The assumption that specialist herbivores such as the parsnip webworm are adapted to hostplant chemistry, and are therefore unaffected by chemical variation in hostplants, was tested. Flower buds from plants grown first in the greenhouse and then in the field were fed to ultimate instar webworms. Plant phenotype had a significant effect on virtually all webworm food utilization parameters. While nutritional factors (i.e., nitrogen content) were correlated with approximate digestibility, two constituents of the flowers — bergapten and xanthotoxin, both linear furanocoumarins — independently accounted for a significant amount of variation in food utilization indicies. The physiological effects of these furanocoumarins were confirmed in artificial diet experiments. Despite the fact that the two most important furanocoumarins in parsnip flowers relative to webworm feeding and growth are isomers, differing only in the positioning of a methoxy substituent, they have different physiological effects; while xanthotoxin in general has no effect on growth, bergapten decreases growth and digestibility of the diet. These results underscore the need in studies of plant-animal interactions to examine individual chemical components rather than classes of compounds.     

Parthenocarpic fruits in wild parsnip: Decoy defence against a specialist herbivore

Summary Parthenocarpy, the production of fruits without viable seeds, is a widespread phenomenon in plants. While failure to effect pollination or fertilization is often cited as the cause of parthenocarpy, this explanation alone is inadequate to explain why plants produce, maintain and further develop fruits. Wild parsnips (Pastinaca sativa) frequently produce parthenocarpic fruit. When parsnip webworms (Depressaria pastinacella), specialist feeders on wild parsnip, were given choices between normal fruit and parthenocarpic fruit, they exhibited a strong preference for parthenocarpic fruit. However, on parthenocarpic fruit, insects fed less efficiently and grew more slowly than insects fed normal fruit. Parthenocarpic fruits, then, may act as decoys that divert herbivores away from fruits that contain plant offspring.     

Compensatory reproduction in a biennial herb following insect defloration

Summary The ability of the biennial herb, Pastinaca sativa L. (wild parsnip), to respond to and compensate for destruction of primary umbel seeds by the larvae of Depressaria pastinacella (Lepidoptera: Oecophoridae) was analyzed by comparing umbel and seed production of damaged and undamaged plants collected from five populations. Plants with a basal stem diameter smaller than 8 mm suffer a reduction in seed set of about 50% when the primary umbel is destroyed but larger plants are able to compensate for loss of primary umbel seeds by increased seed set of tertiary umbels. Depending on plant size, this is due to either an increase in the number of tertiary umbels that reach maturity or an increase in the number of seeds per tertiary umbel. Although seeds of tertiary umbels are significantly smaller than those of primary or secondary umbels, their viability is equivalent to that of secondary seeds and may be greater than that of primary seeds. Characteristics of P. sativa's reproduction, such as the long flowering period and the initiation of more umbels than the plant is normally able to bring to maturity, are important to P. sativa's ability to compensate for the effects of herbivore damage.     

Optimal defence, kin conflict and the distribution of furanocoumarins among offspring of wild parsnip

The factors influencing the allocation of chemical defences to plant offspring have largely been unexplored, conceptually and experimentally. Because evolutionary interactions between maternal plants and their progeny can affect resource allocation patterns among sibling offspring, we suggest that kin conflict as well as herbivore–plant interaction theories need to be considered to predict chemical defence allocation patterns. Optimal defence theory predicts that maternal plants should defend more heavily those offspring in which resources have been disproportionately invested. In contrast, kin conflict theory predicts that natural selection will favour genotypes that can compete successfully for maternal defences irrespective of their quality, even at the expense of the fitness of siblings and the maternal plant. Evidence for these defence patterns were evaluated by examining the allocation of furanocoumarins to seeds of the wild parsnip (Pastinaca sativa, Apiaceae). Furanocoumarins are toxins that are localized within the oil tubes of the maternal tissues of seeds. We evaluated the role of offspring investment (endosperm mass) and seed genotype on furanocoumarin allocation by mating an array of pollen donors with pollen recipients. Furanocoumarins were found to be positively correlated with endosperm mass on one side of the seed, a result consistent with optimal defence theory; however, on the other side of the seed, furanocoumarin content was influenced by seed genotype and was unrelated to endosperm mass. These effects varied with maternal plant. Further experiments demonstrated that nearly 80% of furanocoumarin production occurs after pollination, when fertilization products are active. Although the amount of furanocoumarin influenced by the seed genotype is small relative to the total quantity in the seed, these furanocoumarins are the first line of defence against important predators, such as the parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae). We found that parsnip webworm larvae were able to discriminate among genotypes within an inflorescence. In line with previous studies, these results suggest that a genotype's ability to influence furanocoumarin defence may affect its probability of survival. We conclude that the distribution of defences among plant offspring in wild parsnip is probably influenced by competition among seed genotypes that conflicts with maternal optimal defence. This revised version was published online in July 2006 with corrections to the Cover Date.     

Impact of reassociation with a coevolved herbivore on oviposition deterrence in a hostplant

Although selection by herbivores for increased feeding deterrence in hostplants is well documented, selection for increased oviposition deterrence is rarely examined. We investigated chemical mediation of oviposition by the parsnip webworm (Depressaria pastinacella) on its principal hostplant Pastinaca sativa to determine whether ovipositing adults choose hostplants based on larval suitability and whether hostplants experience selection for increased oviposition deterrence. Webworms consume floral tissues and florivory selects for increased feeding deterrents; moths, however, oviposit on leaves of pre-bolting plants. Exclusive use of different plant parts for oviposition and larval feeding suggests oviposition should select for increased foliar deterrents. Recent webworm colonization of New Zealand (NZ) allowed us to assess phenotypic changes in foliar chemicals in response to webworm oviposition. In a common garden experiment, we compared NZ populations with and without a history of infestation from 2004 to 2006 for changes in leaf chemistry in response to oviposition. Three leaf volatiles, cis- and trans-ocimene, and β-farnesene, elicit strong responses in female moth antennae; these compounds were negatively associated with oviposition and are likely oviposition deterrents. Leaf β-farnesene was positively correlated with floral furanocoumarins that deter florivory; greater oviposition on plants with low floral furanocoumarins indicates that moths preferentially oviposit on parsnips most suitable for larval growth. Unlike florivory, high oviposition on leaves did not lower plant fitness, consistent with the fact that NZ parsnip foliar chemistry was unaffected by 3–6 years of webworm infestation. Thus, in this system, selection by ovipositing moths on foliar chemistry is weaker than selection by larvae on floral chemistry.     

A coumarin‐specific prenyltransferase catalyzes the crucial biosynthetic reaction for furanocoumarin formation in parsley

Furanocoumarins constitute a sub‐family of coumarin compounds with important defense properties against pathogens and insects, as well as allelopathic functions in plants. Furanocoumarins are divided into two sub‐groups according to the alignment of the furan ring with the lactone structure: linear psoralen and angular angelicin derivatives. Determination of furanocoumarin type is based on the prenylation position of the common precursor of all furanocoumarins, umbelliferone, at C6 or C8, which gives rise to the psoralen or angelicin derivatives, respectively. Here, we identified a membrane‐bound prenyltransferase PcPT from parsley (Petroselinum crispum), and characterized the properties of the gene product. PcPT expression in various parsley tissues is increased by UV irradiation, with a concomitant increase in furanocoumarin production. This enzyme has strict substrate specificity towards umbelliferone and dimethylallyl diphosphate, and a strong preference for the C6 position of the prenylated product (demethylsuberosin), leading to linear furanocoumarins. The C8‐prenylated derivative (osthenol) is also formed, but to a much lesser extent. The PcPT protein is targeted to the plastids in planta. Introduction of this PcPT into the coumarin‐producing plant Ruta graveolens showed increased consumption of endogenous umbelliferone. Expression of PcPT and a 4–coumaroyl CoA 2'–hydroxylase gene in Nicotiana benthamiana, which does not produce furanocoumarins, resulted in formation of demethylsuberosin, indicating that furanocoumarin production may be reconstructed by a metabolic engineering approach. The results demonstrate that a single prenyltransferase, such as PcPT, opens the pathway to linear furanocoumarins in parsley, but may also catalyze the synthesis of osthenol, the first intermediate committed to the angular furanocoumarin pathway, in other plants.     

Physiological price of an induced chemical defense: photosynthesis, respiration, biosynthesis, and growth

A recurring theme in defense allocation theories is that defenses are costly. Most studies that attempt to quantify a cost of defense seek to establish a trade-off between a component of plant fitness and the level of a constitutive defense. Such estimates are ambiguous because they cannot discount the cost of traits that are correlated with defense but are not themselves defensive. We examined the effects of damage-induced synthesis of furanocoumarins, known defense compounds, on the growth of wild parsnip. Plants that had 2% of their leaf area removed accumulated 8.6% less total biomass and 14% less root biomass than intact plants over a 4-week period. We also found that this small amount of leaf damage significantly reduced net photosynthetic rates 0.5 h after damage; the effect was temporary, as photosynthetic rates were no longer significantly different after 48 h. Lastly, we found that increases in respiration rates associated with damage coincided spatially and temporally with increases in furanocoumarin production, and that respiration increases were phenotypically correlated with furanocoumarin production. When damage-induced changes in furanocoumarin content and respiration rates were expressed in glucose equivalents and compared, the energetic cost of furanocoumarin production (12.6 μg glucose cm⁻²) accounted for all of the increase in respiration (12.0 μg glucose cm⁻²). A comparison of other secondary compounds in damaged and intact leaflets revealed that myristicin, a furanocoumarin synergist, is the only other compound aside from furanocoumarins that is inducible. The inducible defense system of wild parsnip thus appears to involve a small subset of secondary compounds. Synthesis of these compounds is tightly linked to damage-induced rates of respiration. Because the negative impact that damage had on the rate of net photosynthesis was short-lived, the impact of damage on growth observed in this study was likely due to the cost of furanocoumarin synthesis elicited by damage rather than the loss of photosynthetic tissue caused by damage. Received: 4 April 1996 / Accepted: 29 August 1996     

Reduced Aggressive Behavior: A Benefit of Silk-Spinning in the Parsnip Webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae)

The parsnip webworm, Depressaria pastinacella, spins a silken web within the umbels of its host plant, the wild parsnip Pastinaca sativa, and aggressively defends this web against conspecifics. We first established experimentally that the number of aggressive interactions between caterpillars with their webs removed was significantly higher than for webworms with intact webs. In order to determine whether web-spinning acts to divide food resources and reduce aggressive interactions, we measured relative weight gain and total silk production of parsnip webworms isolated from one another, grouped together with webbing undisturbed, and grouped together with webbing removed daily. Parsnip webworms isolated from one another and therefore unable to engage in aggressive interactions attained the highest pupal weights and spun the smallest amount of silk; caterpillars with webs removed daily and therefore with frequent aggressive interactions until territories were reestablished had the lowest pupal weights and spun the greatest quantity of silk. Our findings indicate that, for the parsnip webworm, constructing a silken web reduces aggressive encounters among conspecifics.     

Phenotype matching in wild parsnip and parsnip webworms: causes and consequences

According to the geographic mosaic theory of coevolution, selection intensity in interactions varies across a landscape, forming a selection mosaic; interaction traits match at coevolutionary hotspots where selection is reciprocal and mismatch at coldspots where reciprocity is not a factor. Chemical traits play an important role in the interaction between wild parsnip (Pastinaca sativa) and the parsnip webworm (Depressaria pastinacella). Furanocoumarins, produced as plant defenses, are detoxified by the webworms by cytochrome P450 monooxygenases; significant additive genetic variation exists for both furanocoumarin production in the plant and detoxification in the insect, making these traits available for selection. To test the hypothesis that differences in selection intensity affect the distribution of coevolutionary hotspots and coldspots in this interaction, we examined 20 populations of webworms and wild parsnips in Illinois and Wisconsin that varied in size, extent of infestation, proximity to woods (and potential vertebrate predators), and proximity to a chemically distinct alternate host plant, Heracleum lanatum (cow parsnip). Twelve of 20 populations displayed phenotype matching between plant defense and insect detoxification profiles. Of the eight mismatched populations, a logistic regression model related matching probability to two predictors: the presence of the alternate host and average content of xanthotoxin (one of the five furanocoumarins produced by P. sativa). The odds of mismatching were significantly increased by the presence of the alternate host (odds ratio = 15.4) and by increased xanthotoxin content (odds ratio = 6.053). Parsnips growing near cow parsnip displayed chemical phenotypes that were chemically intermediate between cow parsnip and parsnips growing in isolation. Rapid phenotype matching in this system is likely due in part to differential mortality every season; larvae transferred to a plant 30 m or more from the plant on which they developed tended to experience increased mortality over larvae transferred to another umbel on the same plant on which they had developed, and plant populations that mismatched in 2001 displayed a change in chemical phenotype distribution from the previous year. Trait mixing through gene flow is also a likely factor in determining mismatch frequency. Populations from which webworms were eradicated the previous year were all recolonized; in three of seven of these populations, infestation rates exceeded 90%. Our findings, consistent with the geographic mosaic theory, suggest that the presence of a chemically distinct alternate host plant can affect selection intensity in such a way as to reduce the likelihood of reciprocity in the coevolutionary interaction between wild parsnip and the parsnip webworm.     

Furanocoumarin metabolism in Papilio polyxenes: biochemistry,genetic variability,and ecological significance

The ubiquitous occurrence of series of biosynthetically related plant secondary compounds within individual species has given rise to the suggestion that such multiplicity is adaptive; one possible mechanism that would serve to maintain such within-plant diversity is analog synergism. In a series of experiments, we provide evidence that synergism may account for the presence of multiple structurally related furanocoumarins in apiaceous plants. The black swallowtail, Papilio polyxenes, feeds exclusively on plant species containing furanocoumarins. Growth of larvae fed parsley leaves treated with both xanthotoxin and angelicin, two furanocoumarins that co-occur widely in swallowtail hostplants, was significantly slower than that of larvae fed leaves with an equimolar concentration of either xanthotoxin or angelicin. A multivariate combination of growth, food consumption and frass excretion differed significantly between larvae fed leaves treated with both xanthotoxin and angelicin and larvae fed leaves treated with angelicin alone. In addition, we measured rates of in vitro cytochrome P450-mediated metabolism of three furanocoumarins — bergapten, xanthotoxin, and angelicin. While bergapten and xanthotoxin, both linear furanocoumarins, were metabolized at similar rates (8.07 and 9.86 nmoles/min/g fw caterpillar, respectively), angelicin, an angular furanocoumarin, was metabolized more slowly (2.76 nmoles/min/g fw caterpillar). When all three furanocoumarins were assayed together, overall rates of metabolism were significantly reduced, suggesting substrate inhibition. Thus, the pattern of growth of larvae is consistent with the pattern of in vitro metabolism and is evidence in support of analog synergism. In a separate experiment, metabolism of xanthotoxin and angelicin individually and together were compared in six maternal families. Again, angelicin was metabolized more slowly than xanthotoxin and each furanocoumarin inhibited metabolism of the other. That significant family effects were found for rates of metabolism and for the ratio of moles of angelicin metabolized for each mole of xanthotoxin metabolized raises the possibility that genetic variation exists for the rate and specificity of metabolism and suggests that insect herbivores may be able to adapt to analog synergism.     

Furanocoumarins in seeds of wild and cultivated parsnip

The furanocoumarin content of ripe seeds of seven cultivars of Pastinaca sativa was compared to that of ripe seeds from a naturalized population in central Illinois. Five furanocoumarins were identified and quantified in the intact seeds. While seeds of wild and cultivated plants contain furanocoumarin components in identical proportions, seeds from wild plants contain almost three times the quantity of furanocoumarins on a dry weight basis as do seeds of cultivars.     

Community‐wide impact of an exotic aphid on introduced tall goldenrod

1. The aphid Uroleucon nigrotuberculatum Olive, which is specialised to the tall goldenrod, Solidago altissima L., in its native range, has become a dominant species on the introduced tall goldenrod in Japan. How this exotic aphid influenced arthropod communities on the introduced tall goldenrod in aphid‐present (spring) and aphid‐absent (autumn) seasons was examined, using an aphid removal experiment. 2. In spring, aphid presence increased ant abundance because aphid honeydew attracted foraging ant workers. A significant negative correlation was found between the numbers of ants and herbivorous insects other than aphids on the aphid‐exposed plants, but no significant correlation was detected on the aphid‐free plants. Thus, the aphid presence was likely to decrease the abundance of co‐occurring herbivorous insects through removal behaviour of the aphid‐tending ants. There were no significant differences in plant traits between the aphid‐exposed and aphid‐free plants. 3. In autumn, the numbers of lateral shoots and leaves, and the leaf nitrogen content were increased in response to the aphid infestation in spring. Because of the improvement of plant traits by aphid feeding, the abundance of leaf chewers increased on aphid‐exposed plants. In contrast, the abundance of sap feeders decreased on the aphid‐exposed plants. In particular, the dominant scale insect among sap feeders, Parasaissetia nigra Nietner, decreased, followed by a decrease in the abundance of ants attending P. nigra. Thus, aphid feeding may have attenuated the negative impacts of the tending ants on leaf chewers. 4. Aphid presence did not change herbivore species richness but changed the relative density of dominant herbivores, resulting in community‐wide effects on co‐occurring herbivores through ant‐mediated indirect effects, and on temporally separated herbivores through plant‐ and ant‐mediated indirect effects. The aphid also altered predator community composition by increasing and decreasing the relative abundance of aphid‐tending ants in the spring and autumn, respectively.     

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

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

Generalist and specialist host–parasitoid associations respond differently to wild parsnip (Pastinaca sativa) defensive chemistry

1. Plant defensive chemistry is predicted to have a more negative effect on generalist herbivores and their parasitoids than on specialist herbivores and their parasitoids. 2. This prediction was examined by comparing the effects of the wild parsnip (Pastinaca sativa L.) toxin, xanthotoxin, on a generalist herbivore–parasitoid association [the cabbage looper, Trichoplusia ni Hübner, and its polyembryonic parasitoid, Copidosoma floridanum (Ashmead)] and a specialist herbivore–parasitoid association [the parsnip webworm, Depressaria pastinacella (Duponchel), and its polyembryonic parasitoid, Copidosoma sosares (Walker)]. 3. Copidosoma floridanum brood sizes were smaller and experienced lower survivorship when reared in a host feeding on an artificial diet containing a low concentration of xanthotoxin. No T. ni hosts, parasitised or unparasitised, survived on a diet high in xanthotoxin. In contrast, C. sosares brood size and survivorship were unaffected by the presence of low levels of xanthotoxin in the host diet. Copidosoma sosares experienced reduced brood size and survivorship only when its host consumed a diet containing 15 times the level of xanthotoxin as the diet adversely affecting its congener. 4. The differences in response to xanthotoxin exhibited by C. floridanum and C. sosares are explained partly by a differential reduction in host quality and partly by differential exposure to xanthotoxin in host haemolymph. Unlike D. pastinacella, T. ni experienced reduced pupal weight and survivorship and prolonged developmental time on a low‐xanthotoxin diet. More xanthotoxin passed unmetabolised into the haemolymph of T. ni than into the haemolymph of D. pastinacella.     

Metabolism of linear and angular furanocoumarins by Papilio polyxenes CYP6B1 co-expressed with NADPH cytochrome P450 reductase

One challenge in the heterologous expression of microsomal cytochrome P450 monooxygenases (P450s) is fulfilling their obligatory requirement for electrons transferred from NADPH P450 reductase. We have established co-expression parameters for Papilio polyxenes CYP6B1 and house fly P450 reductase in baculovirus-infected Sf9 cells that allow for efficient expression of both components and significantly enhance metabolic turnover of this insect P450's substrates. These expression conditions have allowed us to reexamine the turnover capacities of CYP6B1 toward linear and angular furanocoumarins present in the host plants for the specialist caterpillar P. polyxenes. Coexpression of CYP6B1 and P450 reductase at equivalent viral concentrations [MOI (multiplicity of infection) ratio of 1] results in turnover rates for the linear furanocoumarins xanthotoxin and psoralen, which are increased 32-33 fold over the turnover rates obtained with CYP6B1 expressed alone. The turnover rate for the angular furanocoumarin angelicin is also significantly increased to 4.76 nmol/min/nmol P450 compared to its barely detectable level obtained with CYP6B1 expressed alone. Substrate binding analyses indicate that all three of these compounds elicit typical type I binding spectra but with varying magnitudes and affinities that are indicative of each substrate's effectiveness at coordinating with the heme iron. The relative proportions of high spin state generated with these substrates are consistent with CYP6B1 metabolizing these furanocoumarins in the rank order xanthotoxin>psoralen>angelicin. These differential activities for CYP6B1 suggest that it may have been an ancient participant in the coevolutionary arms race between papilionid butterflies and their apiaceous host plants. Due to its ability to handle a range of furanocoumarin structures, CYP6B1 may have contributed to P. polyxenes' early colonization of linear furanocoumarin-containing plants and to its subsequent colonization of angular furanocoumarin-containing plants.     

Effects of florivory on floral volatile emissions and pollination success in the wild parsnip

In addition to reducing fitness by consuming reproductive structures, florivores may also reduce plant fitness by altering interactions with pollinators. To date, the effects of florivore activity on the volatile profile of flowers and subsequent attractiveness to pollinators have not been extensively investigated. In this study, we had three specific objectives: to determine the impact of florivory by the parsnip webworm Depressaria pastinacella on the floral volatile profile of the wild parsnip Pastinaca sativa, to ascertain the mechanisms by which florivory changes the volatile profile, and to estimate the consequences of florivory on visitation by pollinators and eventual seed set. An overall indirect effect of webworms on seed set, that is, the effect of infestation on pollination success, was not detected. However, this overall lack of indirect effect masks the heterogeneity among individual plants. For seven of 14 plants examined, pollination success was altered by webworms, and in four of these plants the alteration in pollination success was consistent with webworm-altered visitation. Webworms significantly altered floral fragrance, in particular causing disproportionate increases in the emissions of octyl esters. Additionally, volatiles from webworm frass, which contains large amounts of the octyl ester metabolite n-octanol, may alter the floral fragrance in ways that change attractiveness of flowers to pollinators. This study suggests that the effects of florivores on plant fitness are not limited to the removal of floral units but may also involve alterations in floral volatile composition, through damage-induced release and detoxification of particular constituents, that affect visitation and pollination success. Handling Editor: Steve Johnson. An erratum to this article can be found at     

Plant biomass and nutrient levels of a tropical macrophyte (Cyperus papyrus L.) receiving domestic wastewater

The aboveground biomass and nutrient content ofCyperus papyrus were determined in a small tropical swamp receiving domestic wastewater. It was found that the biomass (4,955 g.m^–2 dry weight) was the highest ever reported for papyrus. The levels of both nitrogen and phosphorus in the plant organs were very high. The nitrogen concentrations of the various plant organs were 4.8% roots, 8.4% rhizomes, 4.5% scales, 4.8% culms, and 6.2% umbels on dry weight basis. As to phosphorus the concentrations were 0.09% roots, 0.11% rhizomes, 0.09% scales, 0.10% culms, and 0.13% umbels. The high biomass and nutrient contents of the plants may have been caused by the high nutrient levels in the surrounding water. Comparison of the nitrogen to phosphorus ratios in the plants to those in the surrounding water showed that the plants stored very high amounts of nitrogen.