Wild Parsnip Research Articles

PARSNIP WEBWORMS AND HOST PLANTS AT HOME AND ABROAD: TROPHIC COMPLEXITY IN A GEOGRAPHIC MOSAIC

Due to differences in the structure of communities in which interactions are embedded, the intensity of interactions between species may vary with location; thus, what results from differences in outcomes and in degree of specialization is a geographic mosaic, which provides the raw material for divergent coevolutionary trajectories. Where selection intensity is great, reciprocal responses are likely in so-called "hotspots"; in contrast, where selection pressures are relaxed, reciprocal responses in "coldspots" are far less likely to occur. There are few if any studies examining how a gradient of increasing trophic complexity might influence the probability of phenotype matching and, correspondingly, the "temperature" of the coevolutionary interaction. This study was conducted to compare outcomes of the interaction between wild parsnip (Pastinaca sativa) and parsnip webworm (Depressaria pastinacella) in its indigenous area, Europe, to its area of introduction, the midwestern United States. Specifically, we tested the hypothesis that increasing trophic complexity, represented by alternate host plants or the presence of natural enemies, reduces the selective impact of parsnip webworms and hence diminishes linkage between host plant chemistry and webworms that would be expected in coevolutionary hotspots. This comparison of a two-species interaction in its area of introduction and its area of indigeneity revealed common patterns that are more reflective of interaction temperature than of continental origin. Where webworms are rare, parsnips produce lower levels of xanthotoxin and bergapten in both the midwestern United States and Netherlands populations. However, the most striking result from this intercontinental comparison is that what is a ubiquitous two-species interaction in North America is in fact exceptional in Europe; webworms could more reliably be found infesting H. sphondylium even where P. sativa was available as well. This preference for H. sphondylium exists despite the comparatively high probability of parasitism associated with this host plant and may reflect the overall lower furanocoumarin content of H. sphondylium. The interaction of parsnip webworms and wild parsnips at home and away demonstrates clearly the potential for rapid contemporary evolution of chemical traits upon re-association with a coevolved enemy, a potential evolutionary outcome that should be considered in the design and implementation of future weed biological control programs.

Furanocoumarins and their detoxification in a tri-trophic interaction

The parsnip webworm, Depressaria pastinacella, specializes on wild parsnip, Pastinaca sativa, and several species of Heracleum, hostplants rich in toxic furanocoumarins. Rates of furanocoumarin metabolism in this species are among the highest known for any insect. Within its native range in Europe, webworms are heavily parasitized by the polyembryonic parasitoid wasp Copidosoma sosares. In this study, we determined whether these parasitoids are exposed to furanocoumarins in host hemolymph, whether they can metabolize furanocoumarins, and whether parasitism influences the ability of webworms to detoxify furanocoumarins. Hemolymph of webworms fed artificial diet containing 0.3 % fresh weight xanthotoxin, a furanocoumarin prevalent in wild parsnip hosts, contained trace amounts of this toxin; as well, hemolymph of webworms consuming P. sativa flowers and fruits contained trace amounts of six of seven furanocoumarins present in the hostplant. Thus, parasitoids likely encounter furanocoumarins in host hemolymph. Assays of xanthotoxin metabolism in C. sosares failed to show any ability to metabolize this compound. Parasitized webworms, collected from populations of Heracleum sphondylium in the Netherlands in 2004, were on average 55 % larger by weight than unparasitized individuals. This weight is inclusive of host and parasitoid masses. Absolute rates of detoxification (nmoles min−1) of five different furanocoumarins were indistinguishable between parasitized and unparasitized ultimate instars, suggesting that the intrinsic rates of metabolism are fixed. Thus, although parasitized larvae are larger, detoxification rates are not commensurate with size; rates in parasitized larvae expressed per gram of larval mass were 25 % lower than in unparasitized larvae.

Genetic Variation in Primary Metabolites of Pastinaca sativa; Can Herbivores Act as Selective Agents?

Although insect herbivory has been shown to act as a selective agent on plant secondary metabolism, whether primary metabolites contribute to resistance and can respond to selection by herbivores remains untested. In the wild parsnip (Pastinaca sativa), its principal herbivore, Depressaria pastinacella, acts as a selective agent on furanocoumarin resistance factors. In this study, we determined whether webworms can, by causing differential reductions in fitness, act as selective agents on parsnip primary metabolites. Estimates of narrow-sense heritabilities were significantly different from zero for C18 fatty acids in buds and developing fruits, fructose and sorbitol in buds, fructose, myo-inositol, bergapten, and psoralen in fruits. Wild parsnips protected from webworms by insecticide produced 2.5 times as much seed biomass as unsprayed plants; that webworms accounted for this difference in plant fitness was indicated by a significant negative relationship between reproductive effort and an index of webworm damage. Only a handful of metabolites influenced resistance to webworms; these included osthol, sorbitol, and protein in developing fruits as well as previously documented furanocoumarins. Osthol, a coumarinic compound, enhanced resistance, as did protein content, while sorbitol lowered resistance. Other primary metabolites may affect resistance to webworms, but their effect was context-dependent, that is, their effect depended on concentrations of other metabolites (epistasis). Susceptible plant phenotypes were found to have average chemical compositions. Although there was genetic variation in some of the primary metabolites in parsnips, the epistatic nature of their involvement in resistance and the lack of genetic variation in some suggest that selection on them from webworms will be either inconsistent or ineffective.

Microwave-facilitated extraction of furanocoumarins onto paper substrates: an imaging technique to analyse spatial distribution and abundance in leaves.

An impediment to studying the implications of fine-scale variation in the distribution of secondary compounds within plants has been an inability to map their distribution. Conventional approaches require fine-scale dissections, followed by extraction and analysis, a strategy which is often limited by the large number of extractions required and by the difficulty in quantifying compounds in very small samples. A method has been developed which allows the microwave-facilitated transfer of furanocoumarins from fresh leaves to two-dimensional substrates (phase separation paper). The fluorescence of these compounds under UV light is enhanced by treatment with base, which probably opens the lactone ring of the compounds. The developed prints are then photographed under UV light and the images analysed with commonly available software programs. Images so obtained for furanocoumarins in wild parsnip foliage revealed that the total fluorescence from extracted furanocoumarins is correlated with the amount of furanocoumarins remaining in the leaf. Thus, the images provide information not only on spatial distribution but also quantitative data.

Fathers, fruits and photosynthesis: pollen donor effects on fruit photosynthesis in wild parsnip

AbstractChlorophyll is frequently present in plant reproductive tissues and indicates that photosynthesis is occurring in these parts. Photosynthesis by a reproductive organ can contribute as much as 65% to its own growth. Given the advantages that increased photosynthetic rates might have on development of individual seeds competing for resources, selection can be expected to favour the ability of offspring to influence photosynthetic rates of the tissues surrounding them. We report in this study the first evidence that the pollen genotype can influence the rate of photosynthesis in the fruit tissues surrounding the developing offspring. Using a novel chlorophyll fluorescence imaging instrument to quantify quantum efficiency of photosystem II, we found significant differences in photosynthetic rates among fruits in wild parsnip, Pastinaca sativa L, associated with different pollen genotypes.

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.

Fly on the Wall

Although I’m often asked to participate in panel discussions for various groups promoting women in science, I tend to be a disappointment to the organizers. By great good fortune, or else total obliviousness on my part, I’ve never felt discriminated against in any professional context based on the fact that I possess two X chromosomes. Accordingly, I have no horror stories to share and can’t really offer any hard-earned insights on overcoming barriers, which I evidently either avoided completely by happenstance or stumbled over without realizing they were there. (In the interests of complete disclosure, though, there was an awkward moment in graduate school when, at a dinner party hosted by Paul Feeny, my major professor, his guest, Dennis Leston, an authority on the ants of West Africa and the Congo Basin, boozily leaned over and whispered to me that I was “attractive if a bit thick in the middle.” At the time my feelings were hurt, but in retrospect I suppose I should have expected a myrmecologist to prefer waspwaisted women.) This is not to say that I haven’t missed out on some things by virtue of being female. One example came to light recently. This past summer my colleague and longtime collaborator Arthur Zangerl took a 7-week trip to Europe in search of wild parsnips, Pastinaca sativa, and parsnip webworms, Depressaria pastinacella (Duponchel). Both species are native to Europe, so, after studying the interaction throughout the midwestern United States for more than 20 years (Berenbaum and Zangerl 2003) (well, mostly in Urbana, Charleston, and Peotone, Ill.), we decided to expand our horizons and investigate the interaction where it originated. Art brought back hundreds of samples, dozens of digital images, Fly on the Wall

Paradoxical host shift by Depressaria pastinacella in North America: is enemy‐free space involved?

Herbivorous insects often preferentially oviposit and develop on hostplants that support suboptimal growth relative to alternative hostplants available in the same habitat. In some cases, this preference results from reduced predation risk on the nutritionally suboptimal hostplant. Thus, the availability of enemy‐free space has been postulated to affect hostplant choice for herbivorous insects. In the midwestern United States, populations of the parsnip webworm, Depressaria pastinacella Duponchel, introduced from Europe over 150 years ago, feed not only on wild parsnip, Pastinaca sativa L., one of its hosts in Europe, but also on cow parsnip, Heracleum lanatum Michx., a native North American plant. Survival of larvae in the laboratory on fruits of H. lanatum, however, is lower than on fruits of P. sativa, suggesting nutritional or chemical unsuitability. In view of the persistent use of this novel hostplant, we compared susceptibility to predation on these two hostplants in the field. Over a four‐year period, contrary to expectations, webworms pupating in cow parsnip stems consistently experienced intense predation, whereas pupae in parsnip stems escaped predation. The pattern persisted when stems of both hostplants containing pupae were placed in pots at a single location. The predator appears to be avian, as only exclosures that prevented access by both birds and mice eliminated predation, whereas exclosures preventing access by mice alone did not reduce predation. The apparent existence of enemy‐free space in wild parsnip may be due to differences in stem characteristics; parsnip stems contain significantly greater amounts of the highly photoactive furanocoumarin xanthotoxin and are also significantly tougher compared to stems of cow parsnip. The persistent use of cow parsnips by parsnip webworms in the face of high levels of predation and without clear benefits in terms of growth and development suggests that webworms may have lost genetic variation for host discrimination as a consequence of introduction.

Impact of folivory on photosynthesis is greater than the sum of its holes.

The effects of herbivores on plant production and fitness may not relate directly to the quantity of biomass removed because folivory may alter photosynthetic rates at a considerable distance from the damaged tissue [Welter, S. C. (1989) in Insect-Plant Interactions, ed. Bernays, E. A. (CRC, Boca Raton), pp. 135-151.]. An impediment to understanding the effects of leaf damage on photosynthesis has been an inability to map photosynthetic function within a single leaf. We developed an instrument for imaging chlorophyll fluorescence and used it to map the effects of caterpillar feeding on whole-leaf photosynthesis in wild parsnip. The adverse effects of caterpillar feeding on photosynthesis were found to extend well beyond the areas of the leaflet in which caterpillars removed tissue. These "indirectly" affected areas remained impaired for at least 3 days after the caterpillars were removed and were six times as large as the area directly damaged by the caterpillars. Although photosynthesis in indirectly affected areas was reduced and not eliminated, these areas accounted for three times as much of the overall reduction in photosynthesis as the area removed by the caterpillars. The size of the indirect effects was positively correlated with defense-related synthesis of furanocoumarins, suggesting that costs of chemical defense may be one factor that accounts for the indirect effects of herbivory on plants.

Effects of furanocoumarins on feeding behavior of parsnip webworms Depressaria pastinacella.

The parsnip webworm, Depressaria pastinacella, exhibits limited physiological resistance to furanocoumarin toxins in its principal host, the wild parsnip, Pastinaca sativa. These insects are typically found attacking individual plants low in furanocoumarins, relative to others within populations. They also feed preferentially on parthenocarpic fruits, which are lower in furanocoumarin content than are normal fruits. However, in a previous study with artificial diets, they did not appear to discriminate between high and low concentrations of furanocoumarins. In this study, the ability of webworms to distinguish between diets differing in furanocoumarin content was examined with an artificial diet containing wild parsnip and in green parsnip fruits with and without supplemental furanocoumarins. Larvae showed no preference for high or low furanocoumarin diets containing equal amounts of freeze-dried parsnip fruit powder. When given a choice between otherwise similar wild parsnip fruits, webworms strongly preferred fruits that were not augmented with furanocoumarins in one plant but showed no preference or only a weak preference for nonaugmented fruits in four other plants. In order to identify chemical constituents other than furanocoumarins that might determine feeding preferences, we compared the chemical profile of parthenocarpic fruits (which are preferred) to that of normal fruit. Octyl butyrate, a known deterrent to webworms, is highly correlated with furanocoumarin content, occurs in all plants, and differs significantly among normal and parthenocarpic fruit, suggesting that webworms may be able to avoid furanocoumarins by virtue of their behavioral response to octyl butyrate.

Behavioral responses of the parsnip webworm to host plant volatiles.

Sixth instars of the parsnip webworm, Depressaria pastinacella, orient by olfaction to bud, male flowers, and female flowers of their primary host plant, Pastinaca sativa. Because octyl acetate and octyl butyrate are characteristic of tissues consumed by the sixth instar, we investigated the influence of these esters on webworm feeding behavior and chemo-orientation. Although octyl acetate and octyl butyrate are feeding deterrents, and octyl butyrate is an olfactory repellent, octyl acetate serves as an olfactory attractant. In olfactometers, webworms do not show a preference when given a choice between octyl acetate and host plant tissues. These findings suggest that octyl acetate is a sufficient cue for olfactory orientation. Such behavior may explain differences in the relative abundance of these esters observed among populations of wild parsnip under differential selection pressure from these insects.

Heritability estimates for octyl acetate and octyl butyrate in the mature fruit of the wild parsnip.

The aliphatic esters octyl acetate and octyl butyrate occur as major components of essential oils in the vittae, or oil tubes, of the wild parsnip (Pastinaca sativa). We determined phenotypic variation and narrow-sense heritabilities of these octyl esters in wild parsnip fruits from 30 maternal families. The mean octyl acetate content was 1.56 microg/mg dry fruit (0.08-5.51 microg/mg dry fruit) and the mean octyl butyrate content was 4.28 microg/mg dry fruit (1.28-14.22 microg/ mg dry fruit). Narrow-sense heritabilities for each ester's content were calculated by analysis of half-sib families (HS) and parent-offspring regression (OP). Heritabilities were 0.389 (HS) and 0.654 (OP) for octyl acetate and 0.670 (HS) and 0.626 (OP) for octyl butyrate. The amounts of the esters were phenotypically correlated with each other and with the linear furanocoumarins bergapten and xanthotoxin, phototoxic compounds that co-occur in the vittae with the esters. Ester amounts were not genetically correlated, indicating that these compounds could respond independently to selection pressures. These octyl esters may serve as carrier solvents that enhance penetration of these furanocoumarins into herbivore integuments and gut walls.

Damage-inducibility of primary and secondary metabolites in the wild parsnip ( Pastinaca sativa )

Induction of secondary metabolites to herbivore damage is a widespread phenomenon among plants and serves to enhance resistance by reducing suitability or increasing toxicity of foliage. Post-damage responses of primary metabolites are less well known; reductions in primary metabolites may increase resistance by decreasing palatability or nutritional suitability for herbivores or by potentiating toxicity of secondary metabolites. In this study, we examined response to simulated herbivory in Pastinaca sativa, the wild parsnip, in both primary and secondary metabolites. We found that induction of secondary metabolites in response to damage is largely restricted to a single class of compounds, the furanocoumarins. These prooxidant compounds are known to be toxic to a wide variety of insect herbivores. The only primary metabolite that responded to damage was total soluble protein, which increased significantly 24 h after damage. After 24 h, the correlation between total furanocoumarins and total sugars was significant and negative (r = − 0.697). This correlation likely reflects an independent response of sugar to damage, rather than a diversion of resources into furanocoumarin production, inasmuch as this correlation at 3 h, after furanocoumarin induction had taken place, was not significant. In other secondary metabolite pathways, damage produced a significant decline in farnesene and a C-16 unsaturated fatty acid, 7,10,13-hexadecatrienoic acid, each of which may potentiate the furanocoumarin defense response. Farnesene may enhance photooxidative activation of the furanocoumarins and 7,10,13-hexadecatrienoic acid may serve as a precursor to jasmonic acid, a key hormone in regulating induction responses. With few key exceptions, quantities of both primary and secondary metabolites in wild parsnip foliage are unaffected by damage. Those that are affected may well play a role in resistance of parsnips to herbivores.

Patterns of defensive chemical production in wild parsnip seedlings (Apiaceae: Pastinaca sativa L.)

To ascertain patterns of allocation between growth and defense in seedlings, we measured plant biomass and the amount of six furanocoumarin defensive chemicals in wild parsnip roots and shoots (Apiaceae: Pastinaca sativa L.) in sequential harvests for sixty days following emergence and compared them to previous studies on mature plants. Furanocoumarins were present from the first day of emergence and were actively synthesized from the onset of seedling growth. Although initial amounts of each furanocoumarin species differed, they were produced at the same rate. Furanocoumarin production was not commensurate with biomass accretion, and fluctuations in concentration varied dramatically between roots and shoots. Concentrations of furanocoumarins in seedlings are far lower than in adult plants, perhaps because the selective regime of seedlings differs from that of mature plants, and inter-seedling competition favors investment in growth rather than defense.

Chemical phenotype matching between a plant and its insect herbivore.

Two potential outcomes of a coevolutionary interaction are an escalating arms race and stable cycling. The general expectation has been that arms races predominate in cases of polygenic inheritance of resistance traits and permanent cycling predominates in cases in which resistance is controlled by major genes. In the interaction between Depressaria pastinacella, the parsnip webworm, and Pastinaca sativa, the wild parsnip, traits for plant resistance to insect herbivory (production of defensive furanocoumarins) as well as traits for herbivore "virulence" (ability to metabolize furanocoumarins) are characterized by continuous heritable variation. Furanocoumarin production in plants and rates of metabolism in insects were compared among four midwestern populations; these traits then were classified into four clusters describing multitrait phenotypes occurring in all or most of the populations. When the frequency of plant phenotypes belonging to each of the clusters is compared with the frequency of the insect phenotypes in each of the clusters across populations, a remarkable degree of frequency matching is revealed in three of the populations. That frequencies of phenotypes vary among populations is consistent with the fact that spatial variation occurs in the temporal cycling of phenotypes; such processes contribute in generating a geographic mosaic in this coevolutionary interaction on the landscape scale. Comparisons of contemporary plant phenotype distributions with phenotypes of herbarium specimens collected 9-125 years ago from across a similar latitudinal gradient, however, suggest that for at least one resistance trait-sphondin concentration-interactions with webworms have led to escalatory change.

Tyramine from the leaves of wild parsnip: a stimulant and synergist for oviposition by the black swallowtail butterfly

Tyramine from the leaves of wild parsnip: a stimulant and synergist for oviposition by the black swallowtail butterfly

Tyramine from the leaves of wild parsnip: a stimulant and synergist for oviposition by the black swallowtail butterfly

Abstract.Oviposition stimulants from the foliage of wild parsnip, Pastinaca sativa (Apiaceae), were isolated by column chromatography and HPLC and tested in bioassay experiments with hand‐held female black swallowtail butterflies, Papilio polyxenes (Lepidoptera: Papilionidae). Two of the stimulants were identified as tyramine and trans‐chlorogenic acid. A combination of tyramine, trans‐chlorogenic acid and an active neutral fraction was needed to elicit a significant oviposition response. These results are discussed in the context of previous research on the oviposition stimulants of swallowtail butterflies and on the significance of tyramine as a neuromodulator of physiological processes in invertebrates.

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.

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.

Behavioral Effects of Carotenoid Sequestration by the Parsnip WebwormDepressaria pastinacella

The parsnip webwormDepressaria pastinacella, acquires a distinct yellow stripe when it consumes the yellow flowers of its principal host plantPastinaca sativa, the wild parsnip. Caterpillars raised on artificial diet lacking host-plant material lack this yellow coloration. By chemical characterization and comparison of caterpillars raised on parsnip flowers and on artificial diet, we were able to determine that lutein, along with smaller amounts of other xanthophylls from the host plant, is selectively sequestered in the fat body. In bioassays designed to measure avoidance of ultraviolet light, caterpillars raised on parsnip flowers or on artificial diet supplemented with lutein were less likely to avoid exposure to ultraviolet light than caterpillars raised on unaugmented artificial diet and thus lacking sequestered carotenoids. The ability to sequester xanthophylls, which are highly effective antioxidants, may confer a selective advantage on these caterpillars, whose apiaceous host plants produce large quantities of furanocoumarins, natural products that are photoactivated by light wavelengths in the ultraviolet region; such sequestered pigments may reduce not only the oxidative stress associated with ultraviolet light and diurnal foraging but also the photooxidative stress associated with ingestion of photoactive furanocoumarins.