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.     

Dietary and developmental influences on induced detoxification in an oligophage

Many plant secondary compounds induce detoxification activity in herbivorous insects. Although inducibility may be advantageous as a means of reducing costs associated with maintenance of metabolism, another benefit of inducibility is that it may allow insects to tailor their detoxification profiles to multiple substrate toxins in their diets. The parsnip webworm, Depressaria pastinacella, must contend with many types of furanocoumarins, toxins present in abundance in all of its host plants. Previous studies have documented that cytochrome P-450s are responsible for metabolism of furanocoumarins in this species and that this overall activity is inducible. In this study, we examined the effects of ingestion of single furanocoumarins on metabolism of multiple furanocoumarins and the ability of webworms to adjust their metabolism profiles to match artificial diets with furanocoumarin content differing qualitatively and quantitatively from the average content found in their principal host. That detoxification rates of newly molted sixth instars prior to feeding did not differ from those of actively feeding fifth or sixth instars suggests that constitutive activities of furanocoumarin-metabolizing enzymes are maintained in the absence of substrates. All of the induction assays in this study were performed with ultimate instars. Each of the furanocoumarins assayed was found to induce metabolism of five different furanocoumarin substrates; however, the induction profile was independent of the inducing agent. Consistent with this finding, webworms were incapable of matching their detoxification profiles to diets with different furanocoumarin compositions. Thus, the profile of detoxification within individuals of this species appears to be genetically fixed, although there is considerable variation in profiles among individuals.     

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.     

Characterization of furanocoumarin metabolites in parsnip webworm,Depressaria pastinacella

Although metabolites of furanocoumarins have been characterized in a wide range of organisms, to date they have been identified in only a single insect species, Papilio polyxenes. Depressaria pastinacella, the parsnip webworm, like P. polyxenes a specialist on Apiaceae, routinely consumes plant tissues higher in furanocoumarin content than does P. polyxenes and is capable of faster cytochrome P-450-mediated detoxification of these compounds. In this study, we characterized metabolites of xanthotoxin, a linear furanocoumarin, and sphondin, an angular furanocoumarin, in midguts and frass of parsnip webworms. Two metabolites were isolated and identified from webworms fed artificial diet containing xanthotoxin. LC-ESI-MS analysis resulted in the determination of a MW of 266 for the compound in the frass and one of the compounds in the midgut; ¹H NMR confirmed its structure as 6-(7-hydroxy-8-methoxycoumaryl)-hydroxyacetic acid (HCHA). The second compound from the midgut had a MW of 252 and was identified by ¹H NMR and ¹³C NMR analysis as 6-(7-hydroxy-8-methoxycoumaryl)-hydroxyethanol) (HMCH). Whereas HCHA has been found in frass of Papilio polyxenes fed xanthotoxin, HMCH has not been reported previously in insects. Although the first step of metabolism of xanthotoxin in webworms as well as P. polyxenes is likely the formation of an epoxide on the furan ring, angular furanocoumarin metabolism in webworms appears to differ. The principal metabolite of sphondin was identified as demethylated sphondin (6-hydroxy-2H-furo[2,3-h]-1-benzopyran-2-one) by LC-ESI-MS and confirmed by ¹H NMR and ¹³C NMR analyses. That webworms produce metabolites of xanthotoxin in common not only with other Lepidoptera (e.g., HCHA) but with other vertebrates (e.g., HMCH) suggests a remarkable conservatism in the metabolic capabilities of cytochrome P-450s and raises the possibility that insects may share other detoxification reactions with vertebrates with respect to toxins in foodplants.     

Defense of parsnip webworm against phototoxic furanocoumarins: Role of antioxidant enzymes

The parsnip webworm,Depressaria pastinacella (Lepidoptera: Oecophoridae), feeds on plants rich in furanocoumarins, phototoxic allomones. Final-instar larvae possess high levels of activities of antioxidant enzymes (Superoxide dismutase, catalase, glutathione reductase), which detoxify oxygen radicals generated from the furanocoumarins of their host plants. When added to an artificial diet, three linear furanocoumarins (xanthotoxin, bergapten, imperatorin) do not increase levels of the antioxidant enzymes. However, on diets containing both xanthotoxin and piperonyl butoxide, a cytochrome P-450 inhibitor, food utilization indices of the insect are reduced and superoxide dismutase activity is enhanced. These data suggest that cytochrome P-450s act as a primary detoxification system of ingested furanocoumarin, and antioxidant enzymes as a backup system to detoxify oxygen radicals generated by unmetabolized furanocoumarins.     

Metabolism and excretion of the furanocoumarin xanthotoxin by parsnip webworm,Depressaria pastinacella

The parsnip webworm,Depressaria pastinacella, feeds on plants containing high concentrations of furanocoumarins. compounds toxic to many organisms. Parsnip webworm larvae were fed radiolabeled xanthotoxin to quantify the detoxification of this furanocoumarin. They metabolized approximately 95% of the ingested xanthotoxin, indicating that metabolic detoxification is important in their tolerance to this allelochemical. Excretion of xanthotoxin and its metabolites was not restricted to the frass but also occurred by means of the silk glands. The silk glands contained half as much of the tritiated compounds as the rest of the body. Because of the feeding habits of this insect, such an excretory pathway may have implications for interactions with predators and pathogens.     

Decreased sensitivity of mixed-function oxidases frompapilio polyxenes to inhibitors in host plants

Caterpillars ofPapilio polyxenes, the black swallowtail, feed on umbellifers that contain both toxic furanocoumarins and methylenedioxyphenyl compounds such as myristicin and safrole. These phytosynergists enhance the toxicity of furanocoumarins by inhibiting mixed-function oxidases (MFOs), the detoxification enzymes responsible for metabolizing furanocoumarins. In model substrate assays, MFOs fromP. polyxenes are twice as active as MFOs fromHeliothis zea, a generalist herbivore not adapted to feeding on either furanocoumarins or furanocoumarin/phytosynergist combinations.P. polyxenes MFOs are 10 and 46 times less sensitive to inhibition by myristicin and safrole, respectively, thanH. zea MFOs and eight times less sensitive to inhibition by safrole than MFOs fromPapilio troilus, a closely related species that does not encounter furanocoumarin/phytosynergist combinations in its diet. Higher MFO activity and decreased sensitivity to MFO inhibitors are important adaptations that allow black swallowtail caterpillars to feed on many umbelliferous plants.     

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.     

Tritrophic Effects of Xanthotoxin on the Polyembryonic Parasitoid <Emphasis Type="Italic">Copidosoma sosares</Emphasis> (Hymenoptera: Encyrtidae)

Plant chemistry can have deleterious effects on insect parasitoids, which include the reduction in body size, increased development time, and increased mortality. We examined the effects of xanthotoxin, a linear furanocoumarin, on the polyembryonic encyrtid wasp Copidosoma sosares, a specialist parasitoid that attacks the parsnip webworm, Depressaria pastinacella, itself a specialist on furanocoumarin-producing plants. Furanocoumarins, allelochemicals abundant in the Apiaceae and Rutaceae, are toxic to a wide range of herbivores. In this study, we reared parasitized webworms on artificial diets containing no xanthotoxin (control) or low or high concentrations of xanthotoxin. Clutch sizes of both male and female C. sosares broods were more than 20% smaller when they developed in hosts fed the diet containing high concentrations of xanthotoxin. Xanthotoxin concentration in the artificial diet had no effect on the development time of C. sosares, nor did it have an effect on the body size (length of hind tibia) of individual adult male and female C. sosares in single-sex broods. Webworms fed artificial diets containing low or high concentrations of xanthotoxin were not significantly smaller, and their development time was similar to that of webworms fed a xanthotoxin-free diet. Mortality of webworms was not affected by xanthotoxin in their artificial diet. Therefore, dietary xanthotoxin did not appear to affect C. sosares via impairment of host health. However, unmetabolized xanthotoxin was found in D. pastinacella hemolymph where C. sosares embryos develop. Hemolymph concentrations were fourfold greater in webworms fed the high-xanthotoxin-containing diet than in webworms fed the low-xanthotoxin-containing diet. We failed to detect any xanthotoxin metabolism by either C. sosares embryos or precocious larvae. Therefore, the observed tritrophic effects of xanthotoxin are likely to be due to the effects of xanthotoxin after direct contact in the hemolymph rather than to the effects of compromised host quality.     

Response to walnut olfactory and visual cues by the parasitic wasp Diachasmimorpha juglandis

Diachasmimorpha juglandis is a specialist parasitoid attacking fly larvae in the genus Rhagoletis that feed exclusively on walnut fruit husks. In a free-foraging assay comparing response to uninfested, infested, and mechanically damaged fruits, we first determined that D. juglandis use host feeding damage on the fruit as a cue for host presence. In another free-foraging assay that used artificial walnut models and wind tunnel experiments, D. juglandis distinguished infested from uninfested fruits by using either olfactory or visual cues separately. However, the response rate of wasps in the wind tunnel was raised considerably when visual cues were also available. We analyzed the volatile compounds emitted by cohorts of uninfested, mechanically damaged, and infested fruits 1, 3, 5, 8, and 10 days after oviposition by flies into the infested fruits. Total volatile emissions did not differ significantly among treatments, but quantitative changes in volatiles distinguished infested fruits from uninfested and damaged fruits. The fact that parasitoids did not distinguish between infested and damaged fruits in assays where damage was visible indicates that they rely on visual cues when those are available.     

Systemic release of herbivore-induced plant volatiles by turnips infested by concealed root-feeding larvae Delia radicum L

When attacked by herbivorous insects, many plants emit volatile compounds that are used as cues by predators and parasitoids foraging for prey or hosts. While such interactions have been demonstrated in several host–plant complexes, in most studies, the herbivores involved are leaf-feeding arthropods. We studied the long-range plant volatiles involved in host location in a system based on a very different interaction since the herbivore is a fly whose larvae feed on the roots of cole plants in the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). The parasitoid studied is Trybliographa rapae Westwood (Hymenoptera: Figitidae), a specialist larval endoparasitoid of D. radicum. Using a four-arm olfactometer, the attraction of naive T. rapae females toward uninfested and infested turnip plants was investigated. T. rapae females were not attracted to volatiles emanating from uninfested plants, whether presented as whole plants, roots, or leaves. In contrast, they were highly attracted to volatiles emitted by roots infested with D. radicum larvae, by undamaged parts of infested roots, and by undamaged leaves of infested plants. The production of parasitoid-attracting volatiles appeared to be systemic in this particular tritrophic system. The possible factors triggering this volatile emission were also investigated. Volatiles from leaves of water-stressed plants and artificially damaged plants were not attractive to T. rapae females, while volatiles emitted by leaves of artificially damaged plants treated with crushed D. radicum larvae were highly attractive. However, T. rapae females were not attracted to volatiles emitted by artificially damaged plants treated only with crushed salivary glands from D. radicum larvae. These results demonstrate the systemic production of herbivore-induced volatiles in this host-plant complex. Although the emission of parasitoid attracting volatiles is induced by factors present in the herbivorous host, their exact origin remains unclear. The probable nature of the volatiles involved and the possible origin of the elicitor of volatiles release are discussed.     

Volatiles from whitefly-infested plants elicit a host-locating response in the parasitoid,Encarsia formosa

The blend of volatile compounds emitted by bean plants (Phaseolus vulgaris) infested with greenhouse whitefly (Trialeurodes vaporariorum) has been studied comparatively with undamaged plants and whiteflies themselves. Collection of the volatiles and analysis by gas chromatography revealed more than 20 compounds produced by plants infested with whitefly. Of these, 4 compounds, (Z)-3-hexen-1-ol, 4,8-dimethyl-1,3,7-nonatriene, 3-octanone, and one unidentified compound were emitted at higher levels than from the undamaged control plants. Synthetic (Z)-3-hexen-1-ol, 4,8-dimethyl-1,3,7-nonatriene, or 3-octanone all elicited a significant increase in oriented flight and landing on the source by the parasitoid, Encarsia formosa, in wind tunnel bioassays. Two-component mixtures of the compounds and the three-component mixture all elicited a similar or, in most cases, a better response by the parasitoid, the most effective being a mixture of (Z)-3-hexen-1-ol and 3-octanone. These results demonstrate that E. formosa uses volatiles from the plant-host complex as olfactory cues for host location.     

Phototoxicity ofCitrus jambhiri to fungi under enhanced UV-B radiation: Role of furanocoumarins

Extracts ofCitrus jambhiri foliage exposed to and shielded from UV-B radiation were assayed for phytochemical changes and phototoxicity against four fungal pathogens, two of which (Fusarium solani andF. oxysporum) are causative agents of root rots and two of which (Penicillium italicum andP. digitatum) are associated with fruit rots. Conidial pigment mutants of these four fungal species were assayed to determine whether pigments play a role in protecting fungi against plant photosensitizers. Exposure to 10.2 kJ/ day UV-B radiation for 95 days significantly reduced phototoxicity of leaf extracts to fungi. Although furanocoumarin levels were reduced by UV-B, analysis of covariance revealed that variation in phototoxicity of the extracts cannot be attributed entirely to variation in furanocoumarin content; thus, the possibility exists that nonfuranocoumarin phototoxic constituents, as yet unidentified, respond to UV-B exposure and contribute to overall phototoxic defense ofC. jambhiri against pathogens. Root rot fungi were substantially more sensitive to furanocoumarin phototoxicity than were fruit rot fungi, a pattern consistent with the amount of light exposure normally experienced by these fungi when associated with phototoxic plants. Although pigmented strains of all four species displayed greater resistance to phototoxicity of pure furanocoumarins, no strain differences were detected in assays of foliar extracts; this finding also suggests that nonfuranocoumarin constituents may be involved in the phototoxic defense ofC. jambhiri against pathogens.Deceased.     

Effect of the Presence of a Nonhost Herbivore on the Response of the Aphid Parasitoid <Emphasis Type="Italic">Diaeretiella rapae</Emphasis> to Host-infested Cabbage Plants

The vast majority of studies of plant indirect defense strategies have considered simple tritrophic systems that involve plant responses to attack by a single herbivore species. However, responses by predators and parasitoids to specific, herbivore-induced, volatile blends could be compromised when two or more different herbivores are feeding on the same plant. In Y-tube olfactometer studies, we investigated the responses of an aphid parasitoid, Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae), to odors from cabbage plants infested with the peach-potato aphid Myzus persicae (Sulzer) (Homoptera: Aphididae), in both the presence and absence of a lepidopteran caterpillar, Plutella xylostella L. (Lepidoptera: Plutellidae). Female parasitoids chose aphid-infested plants over uninfested plants but did not distinguish between caterpillar-infested and uninfested plants. When given a choice between odors from an aphid-infested plant and those from a plant infested with diamondback moth larvae, they significantly chose the former. Furthermore, the parasitoids responded equally to odors from a plant infested with aphids only and those from a plant infested with both aphids and caterpillars. The results support the hypothesis that the aphid and the caterpillar induce different changes in the volatile profile of cabbage plants and that D. rapae females readily distinguish between the two. Furthermore, the changes to the plant volatile profile induced by the caterpillar damage did not hinder the responses of the parasitoid to aphid-induced signals.     

Attraction of the Parasitoid <Emphasis Type="Italic">Anagrus nilaparvatae</Emphasis> to Rice Volatiles Induced by the Rice Brown Planthopper <Emphasis Type="Italic">Nilaparvata lugens</Emphasis>

Anagrus nilaparvatae, an egg parasitoid of the rice brown planthopper Nilaparvata lugens, was attracted to volatiles released from N. lugens-infested plants, whereas there was no attraction to volatiles from undamaged plants, artificially damaged plants, or volatiles from N. lugens nymphs, female adults, eggs, honeydew, and exuvia. There was no difference in attractiveness between plants infested by N. lugens nymphs or those infested by gravid females. Attraction was correlated with time after infestation and host density; attraction was only evident between 6 and 24 hr after infestation by 10 adult females per plant, but not before or after. Similarly, after 24 hr of infestation, wasps were attracted to plants with 10 to 20 female planthoppers, but not to plants with lower or higher numbers of female planthoppers. The attractive time periods and densities may be correlated with the survival chances of the wasps' offspring, which do not survive if the plants die before the wasps emerge. Wasps were also attracted to undamaged mature leaves of a rice plant when one of the other mature leaves had been infested by 10 N. lugens for 1 d, implying that the volatile cues involved in host location by the parasitoid are systemically released. Collection and analyses of volatiles revealed that 1 d of N. lugens infestation did not result in the emission of new compounds or an increase in the total amount of volatiles, but rather the proportions among the compounds in the blend were altered. The total amounts and proportions of the chemicals were also affected by infestation duration. These changes in volatile profiles might provide the wasps with specific information on host habitat quality and thus could explain the observed behavioral responses of the parasitoid.     

Artificial defloration and furanocoumarin induction inPastinaca sativa (Umbelliferae)

Damage simulating herbivory was tested as an inducer of furanocoumarins in the floral parts of the wild parsnip,Pastinaca sativa (Umbelliferae). In one experiment, primary umbels ofP. sativa were partially deflorated over the course of nine days, and higher-order umbels as well as the remaining primary umbel floral parts were sampled. Total furanocoumarin concentration was not significantly affected by defloration, but one furanocoumarin, isopimpinellin, increased in one floral stage of the secondary umbel in damaged plants. In a second experiment, primary umbels were completely deflorated and the higher-order umbels allowed to set seed. No significant effect of defloration on furanocoumarin content was found in the seeds of the higher-order umbels.     

Aliphatic Esters as Targets of Esterase Activity in the Parsnip Webworm (Depressaria pastinacella)

As a specialist on the reproductive structures of Pastinaca sativa and species in the related genus Heracleum, the parsnip webworm (Depressaria pastinacella) routinely encounters a distinctive suite of phytochemicals in hostplant tissues. Little is known, however, about the detoxification mechanisms upon which this species relies to metabolize these compounds. In this study, larval guts containing hostplant tissues were homogenized, and metabolism was determined by incubating reactions with and without NADPH and analyzing for substrate disappearance and product appearance by gas chromatography–mass spectrometry. Using this approach, we found indications of carboxylesterase activity, in the form of appropriate alcohol metabolites for three aliphatic esters in hostplant tissues—octyl acetate, octyl butyrate, and hexyl butyrate. Involvement of webworm esterases in hostplant detoxification subsequently was confirmed with metabolism assays with pure compounds. This study is the first to implicate esterases in lepidopteran larval midgut metabolism of aliphatic esters, ubiquitous constituents of flowers and fruits. In addition, this method confirmed that webworms detoxify furanocoumarins and myristicin in their hostplants via cytochrome P450-mediated metabolism, and demonstrated that these enzymes also metabolize the coumarin osthol and the fatty acid derivative palmitolactone.     

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.     

Effects of molasses grass,Melinis minutiflora volatiles on the foraging behavior of the cereal stemborer parasitoid,Cotesia sesamiae

Olfactory responses of the cereal stemborer parasitoid Cotesia sesamiae to volatiles emitted by gramineous host and nonhost plants of the stemborers were studied in a Y-tube olfactometer. The host plants were maize (Zea mays) and sorghum ( Sorghum bicolor), while the nonhost plant was molasses grass (Melinis minutiflora). In single-choice tests, females of C. sesamiae chose volatiles from infested and uninfested host plants and molasses grass over volatiles from the control (soil). In dual-choice tests, the wasp preferred volatiles from infested host plants to those from uninfested host plants. There was no discrimination between molasses grass volatiles and those of uninfested maize, uninfested sorghum, or infested maize. The wasp preferred sorghum volatiles over maize. Combining uninfested maize or sorghum with molasses grass did not make volatiles from the combination more attractive as compared to only uninfested host plants. Infested maize alone was as attractive as when combined with molasses grass. Infested sorghum was preferred over its combination with molasses grass. Local growth conditions of the molasses grasses influenced attractiveness to the parasitoids. Volatiles from Thika molasses grass were attractive, while those from Mbita molasses grass were not. Growing the Thika molasses grass in Mbita rendered it unattractive and vice versa with the Mbita molasses grass. This is a case of the same genotype expressing different phenotypes due to environmental factors.     

Psoralens in senescing leaves ofRuta graveolens

Concentrations of three furanocoumarins, psoralen, xanthotoxin, and bergapten, were measured on the surface and within mature whole leaves of two groups ofRuta graveolens L. late autumn plants, 2 and 6 years old, which contained green, yellow, and dry yellow leaves. Upper green leaves contained higher concentrations of these coumarins than lower green leaves, green leaves contained several times as much as yellow leaves, and dry leaves contained even smaller amounts than yellow ones. The dry yellow leaves contained only a very small percentage of furanocoumarins on the surface, suggesting that extrusion to the surface of yellow leaves was slower or had stopped, while loss from the surface continued. The loss of psoralen was the most dramatic in and on the dry leaves. Bergapten's ratio to the other cournarins increased during senescence. Xanthotoxin was always the predominant furanocoumarin in this species.