Ranking of individual mountain birch trees in terms of leaf chemistry: seasonal and annual variation

Summary. The quality of tree leaves as food for herbivores changes rapidly especially during the spring and early summer. However, whether the quality of an individual tree in relation to other trees in the population changes during the growing season and between years is less clear. We studied the seasonal and annual stability of chemical and physical traits affecting leaf quality for herbivores. Rankings of trees in terms of the contents of two major groups of phenolics in their leaves, hydrolyzable tannins and proanthocyanidins (condensed tannins), were very stable from the early spring to the end of the growing season. There were also strong positive within-season correlations in the levels of some other groups of phenolics in the leaves (kaempferol glycosides, myricetin glycosides and p-coumaroylquinic acid derivatives). The contents of individual sugars and the sum content of protein-bound amino acids showed patterns of seasonal consistency in mature leaves, but not in young developing leaves. The seasonal correlations in leaf water content and toughness were also strongest in mature leaves. The correlations between two years at corresponding times of the growing season were strongly positive for the major groups of phenolics throughout the season, but were more variable for the contents of proteins and some sugars. Leaf toughness and water content showed strong positive correlations in mature leaves. Despite the consistency of tree ranking in terms of leaf phenolics, the relative resistance status of trees may, however, change during a growing season because there was a negative correlation between the content of hydrolyzable tannins (early-season resistance compounds) in leaves early in the season and the content of proanthocyanidins (late-season resistance compounds) late in the season, and vice versa. Thus, assuming that phenolics affect herbivore preference and performance, different plants may suffer damage at different times of the growing season, and the overall variation between trees in the fitness consequences may be low. In addition, the adaptation of herbivorous insects to mountain birch foliage in general, as well as to specific tree individuals, may be constrained by variation in the relative resistance status of the trees.     

Sequestration of pyrrolizidine alkaloids by larvae ofTellervo zoilus (Lepidoptera: Ithomiinae) and their role in the chemical protection of adults against the spiderNephila maculata (Araneidae)

Summary Life stages of the primitive Australian ithomiine butterflyTellervo zoilus and its larval hostplant, the apocynaceous vineParsonsia straminea, were quantitatively assayed for pyrrolizidine alkaloids (PAs). PAs were found in all stages, mainly as N-oxides, being most concentrated in larvae and freshly-emerged adults. Although adults feed at various confirmed PA sources this probably does not compensate for losses, as wild-caught adults had considerably lower concentrations of PAs. The main alkaloid present in both freshly-emerged adults and in leaves of the host-plant was lycopsamine (1b), stored by butterflies in the N-oxide form. Its presence in higher proportion, in relation to intermedine (1a), in larvae, pupae and adults ofTellervo in relation to the host-plants suggests the inversion of intermedine to lycopsamine by the insects. No 14-member ring macrocyclic PAs were detected in either food-plant or butterflies. Several other PAs were found in wild-caught adults reflecting visits to other PA sources. PAs were also found in high concentrations in freshly-emerged individuals of the danaineEuploea core bred onParsonsia straminea. Wild-caughtDanaus affinis had high PA levels acquired from adult feeding. Freshly emergedEuploea raised onIschnocarpus frutescens andDanaus raised onIschnostemma carnosum (both PA-free) were preyed on by the orb weaving spiderNephila maculata, and showed no PAs. In all cases where PAs were present, most butterflies were liberated, usually cut out of the web unharmed, byNephila. The spider's response was not closely linked to PA concentration, however, and may also depend on hunger levels and previous experience with PA-containing butterflies. All control and other non-PA containing butterflies were consumed although rejection of some body parts of freshly-emergedDanaus affinis suggests that compounds other than PAs may be involved.     

Sequestration of prenylated benzoic acid and chromenes by Naupactus bipes (Coleoptera: Curculionidae) feeding on Piper gaudichaudianum (Piperaceae)

The curculionid beetle Naupactus bipes (Germar, 1824) (Coleoptera: Curculionidae: Brachycerinae) has shown feeding preference for leaves of Piper gaudichaudianum, demonstrating an unexpected specificity for an insect considered to be a generalist. The leaves of P. gaudichaudianum contain the prenylated chromenes gaudichaudianic acid (4, major compound) and its methyl ester (5) in addition to a chromene (3) lacking one prenyl residue. In addition to 4, roots contain the chromone methyl ester (1) and methyl taboganate (2, major compound). Feeding on roots, larvae of N. bipes sequester exclusively the root-specific compounds 1 and 2. Adult beetles sequester the leaf-specific chromenes 3 and 4, but were found to also contain compounds 1 and 2 that are absent in leaves. Therefore, it is suggested that 1 and 2 are sequestered by larvae and can be found in the body of adult insects after long-term storage. In addition, 3 and 4, the major compounds in leaves were found to be associated with the eggs.     

Host-plant green-leaf volatiles synergize the synthetic sex pheromones of the corn earworm and codling moth (Lepidoptera)

Summary The capture of adult male moths in female sex pheromone traps of two key agricultural pests, the corn earworm (Helicoverpa zea) and the codling moth (Cydia pomonella), is enhanced or synergized by a certain group of host-plant volatiles, the green-leaf volatiles (GLVs). Since female adults of both species call and release their sex pheromones while perched upon the leaves of their host-plants, the volatile constituents from the leaves of a number of host-plants were compared. Sex pheromone traps containing one of the prominent leaf volatiles of certainH. zea hosts, (Z)-3-hexenyl acetate, not only significantly increased the capture ofH. zea males but were preferred over traps baited only with sex pheromone. Similarly, traps baited with synthetic sex pheromome ofC. pomonella plus a blend of GLVs captured significantly more males than traps baited only with sex pheromone. Since male moths are not captured in traps baited only with these GLVs, it appears that these GLVs act as pheromone synergists which increase or enhance the attraction or arrestment of male moths in pheromone traps.     

Characterization of the benthic vegetation in the Farwà Lagoon (Libya)

This study forms part of the Action Plan for the Conservation of Marine Vegetation in the Mediterranean SEa (United Nation Environmental Program). It was carried out in June 2000 in the Farwà Lagoon, Libya. The mapping of the main benthic vegetation was achieved by compiling the field observations (transect method), and remote sensing of SPOT satellite images. The phytobenthos in the Farwà lagoon covers an area of 1820 ha (65%). Three benthic macrophyte species dominate, namely the marine phanerogamsCymodocea nodosa andPosidonia oceanica, and the algaCaulerpa prolifera. DeadPosidonia oceanica leaves (litter) form veritable mounds in the vicinity of the openings leading to open sea. These leaves, which come from the coastal sea, are brought into the lagoon by currents and tides; their decomposition will lead to high oxygen consumption and the release of hydrogen sulphide. The phenological data ofPosidonia oceanica shoots sampled in the lagoon are similar to those from other stations in the Mediterranean. Conversely, the lepidochronological parameters of shoots sampled in the central part of the lagoon exhibit values that are substantially higher than those generally recorded in the Mediterranean. The mean number of leaves produced annually is 9.9 (mean value for the Mediterranean: 7.5) and the rhizome growth rate is of 35.7 mm.yr⁻¹ (mean value for the Mediterranean: 7.5 mm.yr⁻¹). This hypersaline environment would seem to provide optimum growth conditions for the speciesPosidonia oceanica.     

Secondary metabolites of the leaf surface affected by sulphur fertilisation and perceived by the cabbage root fly

Summary. Surrogate leaves treated with methanolic leaf surface extracts of Brassica napus L. (cv Express) plants that received three different sulphur fertilisation treatments showed even more marked differences by the oviposition choice of Delia radicum L. than the potted plants. This confirms that the oviposition preference of D. radicum is mediated by chemical compounds on the leaf surface and that the quality of host-plants in terms of their nutrition status can be perceived by the female insect.The oviposition data were positively correlated with the content of fractionated surface extracts containing either CIF (cabbage identification factor; 1,2-dehydro-3-thia- 4,10,10b-triaza-cyclopenta[.a.]fluorine-1-carboxylic acid) or glucosinolates. Electrophysiological recordings from the tarsal chemoreceptor sensilla C₅ and D_3,4 showed that receptor neurons react to glucosinolate- and CIF-fractions. We found that the chemosensory activity of specific glucosinolate- and CIF-receptor neurons corresponded with the respective behavioural activity in the oviposition choice assays. The responses of D. radicum to glucosinolates in the electrophysiological recordings studies corresponded to the observed oviposition preference on plants or artificial leaves characterised with an higher amount of glucosinolates on leave surfaces. The presented data suggested that CIF and glucosinolates are involved in host-plant preference of D. radicum and are perceived by tarsal chemoreceptors.     

Integrating phenological, chemical and biotic defences in ant-plant protection mutualisms: a case study of two myrmecophyte lineages

Summary. We examined the role of plant phenology in the evolution of anti-herbivore defence in symbiotic ant-plant protection mutualisms. Phenology of the host-plant affects traits of its herbivores, including size, growth rate, development time, and gregariousness. Traits of herbivores in turn determine what traits ants must have to protect their host. Diversity in plant phenological traits could thus help explain the great ecological diversity of coevolved ant-plant mutualisms. We explored the postulated causal chain linking phenology of the plant, herbivore adaptations to phenology, and ant adaptations for protection, by comparing two myrmecophytes presenting strong contrasts in phenology. In Leonardoxa africana, a slow-growing understory tree, growth at each twig terminal is intermittent, the rapid flushing of a single leaf-bearing internode being followed by a pause of several months. In contrast, axes of Barteria nigritana, a tree of open areas, grow continuously. Analysis of the phenology (kinetics of expansion) and chemistry of leaf development (contents of chlorophylls, lignin, and nitrogen during leaf growth) showed that these two species exhibit strongly contrasting strategies. Leonardoxa exhibited a delayed greening strategy, with rapid expansion of leaves during a short period, followed by synthesis of chlorophylls and lignins only after final leaf size has been reached. In contrast, leaves of Barteria expanded more slowly, with chlorophylls and lignin gradually synthesised throughout development. Differences in the phenology of leaf development are reflected in differences in the duration of larval development, and thereby in size, of the principal lepidopteran herbivores observed on these two plants. This difference may in turn have led to different requirements for effective defence by ants. The strategy of phenological defence may thus affect the evolution of biotic defence.     

Survival of first instar larvae ofDanaus plexippus (Lepidoptera: Danainae) in relation to cardiac glycoside and latex content ofAsclepias humistrata (Asclepiadaceae)

Summary Our paper addresses field survivorship of first instar monarch butterfly larvae (Danaus plexippus L., Lep.: Danainae) in relation to the dual cardenolide and latex chemical defenses of the sand hill milkweed plant,Asclepias humistrata (Asclepiadaceae) growing naturally in north central Florida. Survival of first instar larvae in the field was 11.5% in the first experiment (15–20 April 1990), and dropped to 3.4% in the second experiment (20–30 April). About 30% of the larvae were found glued to the leaf surface by the milkweed latex. Predator exclusion of non-flying inverte-brates by applying tanglefoot to the plant stems suggested that the balance of the mortality was due to volant inverte-brates, or to falling and/or moving off the plants. Regression analyses to isolate some of the other variables affecting survivorship indicated that first instar mortality was correlated with (1) increasing cardiac glycoside concentration of the leaves, (2) increasing age of the plants, and (3) the temporal increase in concentration of cardiac glycosides in the leaves. The study also provided confirmatory data of previous studies that wild monarch females tend to oviposit onA. humistrata plants containing intermediate concentrations of cardiac glycosides. Cardiac glycoside concentration in the leaves was not correlated with that in the latex. The concentration of cardenolide in the latex is extremely high, constituting an average of 1.2 and 9.5% of the mass of the wet and dry latex, respectively. The data suggest that an increase in water content of the latex is compensated for by an influx of cardenolide with the result that the cardenolide concentration remains constant in the latex systems of plants that are growing naturally. We also observed first instar larvae taking their first bite of milkweed leaves in the field. In addition to confirming other workers findings that monarch larvae possess elaborate sabotaging behaviour of the milkweed's latex system, we discovered that several larvae on their first bite involuntarily imbided a small globule of latex and instantly became cataleptic. This catalepsis, lasting up to 10 min, may have been in response to the high concentration of cardenolide present in the latex ofA. humistrata, more than 10 times that in the leaves. The results of the present study suggest that more attention should be directed to plant chemical defenses upon initial attack by first instar insect larvae, rather than attempting correlations of plant chemistry with older larvae that have already passed the early instar gauntlet. The first bite of neonate insects may be the most critical moment for coping with the chemical defenses of many plants and may play a much more important role in the evolution of insect herbivory than has previously been recognized.     

The iridoid glycoside,catalpol, as a deterrent to the predatorCamponotus floridanus (Formicidae)

Summary We investigated the role of the iridoid glycoside, catalpol, as a deterrent to the predator,Camponotus floridanus. Four laboratory colonies of this ant were offered buckeye caterpillars (Junonia coenia: Nymphalidae) raised on diets with and without catalpol. The same colonies were offered sugar-water solutions containing varying concentrations of catalpol, in both no-choice and choice tests. Regardless of diet, buckeye caterpillars appeared to be morphologically protected from predation by the ants, possibly because of their large spines or tough cuticle. However, buckeyes raised on diets with catalpol had high concentrations of catalpol in their hemolymph; extracts of this high-catalpol hemolymph proved to be an effective deterrent to the ants. When starved ants were not given the choice of food items, they were more likely to consume sucrose solutions that contained 5 mg catalpol/ml or 10 mg catalpol/ml than they were to consume solutions with 20 mg catalpol/ml. When they were given a choice of sugar solution or a sugar solution containing catalpol, the ants avoided solutions with catalpol at any of these concentrations. Ant colony responses to catalpol in sucrose solutions varied considerably over time and among colonies.     

Chemoecological studies of the exocrine glandular larval secretions of two chrysomelid species (Coleoptera):Phaedon cochleariae andChrysomela lapponica

Summary The exocrine glandular secretions of larvae of the subfamily Chrysomelinae are known to repel conspecific adults, other competitive phytophagous insects and natural enemies. InPhaedon cochleariae, the intraspecific activity of tlc fractions of the larval secretion was tested in order to examine the ecological significance of two fractions containing minor components and a fraction containing the major compound, the cyclopentanoid monoterpene (epi)chrysomelidial. InChrysomela lapponica, the defensive activity of the larval secretion against ants is known from specimens feeding upon willow or birch. The feeding preferences of larvae and adults ofC. lapponica from a Finnish and a Czech population were tested. The Finnish individuals significantly preferred feeding uponSalix borealis, whereas they hardly fed upon birch. The Czech specimens clearly preferred birch (Betula pubescens) to willow species. Application of salicin onto leaves of a willow species free of this phenolglycoside revealed that the Finnish individuals preferred feeding upon leaves with salicin. On the other hand, the Czech individuals avoided feeding upon leaves ofB. pubescens treated with salicin. The chemical composition of the glandular secretion of the Finnish larvae differed from the one of the Czech larvae. GC-MS-analyses of the secretions revealed that salicylaldehyde was the only major component of the secretion of Finnish larvae feeding upon the salicin-containing willowS. borealis. The glandular secretion of the Czech larvae feeding upon birch contained numerous esters of isobutyric acid and 2-methylbutyric acid. When Czech larvae had fed upon a salicin-containing willow (S. fragilis), the major compounds of their secretion were benzoic acid, salicylalcohol and benzoic acid esters; salicylaldehyde was only detected in traces. Thus,C. lapponica individuals from the Finland population adapted so closely to a salicincontaining willow that they clearly prefer this plant for food and that they obviously derive their main larval defensive compound (salicylaldehyde) from their host-plant.     

Why are larger convict cichlid (Cichlasoma nigrofasciatum) fry sometimes adopted into broods of smaller fry?

Females of Elasmucha grisea defend their eggs and small nymphs against invertebrate predators. Females sometimes guard their clutches side by side on the same birch leaf. We studied benefits of this joint guarding both in the field and in the laboratory. We found that adjacent females had significantly larger clutches than solitary females. In the laboratory, we studied the effectiveness of joint versus single defence against ant (Formica uralensis) predators. We established female pairs from initially singly guarding females by cutting off pieces of leaves with egg clutches and pasting them beside another female guarding her clutch. In the control group the females with their clutches were similarly cut off but these clutches were placed on another leaf without any female. The birch twigs where females guarded their clutches were placed in cages in close proximity to laboratory ant nests. In the experimental treatment, two females guarded their clutches together and at the same nest there was another birch twig without a female. In the control treatment two twigs with one female on each were placed close to another ant nest. Two females defended their clutches significantly more successfully, losing fewer eggs than did the single females. This primitive form of female sociality in parent bugs resembles colonial nesting in birds, where communal defence is also important. However, to our knowledge this is the first experiment where the benefit of joint guarding has been tested directly by manipulating the size of the breeding group rather than by measuring the risk of predation in groups of different size.     

Diet-induced chemical phytomimesis by twig-like caterpillars of Biston robustum Butler (Lepidoptera: Geometridae)

Summary. Polyphagous caterpillars of the giant geometer Biston robustum resemble the twigs of their respective food sources in color and shape. Common predatory ants, including Lasius and Formica, were often observed to freely prowl directly on caterpillars bodies, even after antennal contact. This suggests that the cuticular chemicals of the caterpillars resemble those of the twigs of the foodplants, so we analyzed both by GC and GC-MS. The chemical compositions differed among caterpillars fed on a cherry, Prunus yedoensis, a chinquapin Castanopsis cuspidata, and a camellia Camellia japonica. The cuticular chemicals of the caterpillars resembled those of their corresponding food sources. When the caterpillar diets were switched from the cherry to camellia or chinquapin at the 4^th instars, the caterpillars cuticular chemicals changed after molting to resemble those of their respective foods. Caterpillars also changed their cuticular chemicals when they perched on cherry twigs and fed on camellia or chinquapin leaves, but not when they perched on camellia or chinquapin twigs and fed on cherry leaves. The chemical similarities between the caterpillars and the twigs were due to the digestion of host leaves, which indicates that this is a diet-induced adaptation.     

Responses of Metaphycussp. nr. flavus to semiochemicals released from a scale host, Coccus hesperidum

Summary. Metaphycus sp. nr. flavus (Encyrtidae: Hymenoptera) is a parasitoid species collected from the Mediterranean region which lays its eggs in the immature stages of several economically important soft scale insects (Hemiptera: Coccidae), including brown soft scale, Coccus hesperidum L. (= host insect). Preliminary tests suggested that the parasitoid is most successful in producing offspring when it oviposits in the younger stages of brown soft scale. In Y-olfactometer bioassays measuring wasp choices and residence times, naïve parasitoids were significantly more attracted to yucca leaves infested with 26, 27, or 28 d-old scale than to uninfested leaves, whereas leaves with older (29-30 d-old) scale were no more attractive than uninfested leaves. Parasitoids also spent significantly more time in the arm with yucca leaves infested with 26 d-old scale than in the arm with uninfested leaves. These results are consistent with observations of the parasitoids reproductive success on scale of different ages, whereby older scale are more likely to encapsulate the developing eggs of M. sp. nr. flavusfemales than are younger scale. Further bioassays determined that yucca leaves that had been infested with 26 d-old scale but from which the scale had been removed were as attractive as infested leaves. In contrast, infested yucca leaves from which scale had been removed and the leaves subsequently washed with distilled water were less attractive than infested leaves. Furthermore, the wash water containing scale residues was attractive to female wasps. In total, these results suggest that Metaphycussp. nr. flavus females utilize volatile, water soluble compounds produced by brown soft scale as cues to locate suitable hosts.     

Insecticidal and growth-reducing activity of foliar extracts from Meliaceae

Summary Thirty-one species in twenty genera of the plant family Meliaceae were assayed for the production of growth-inhibiting phytochemicals, using the generalist herbivorePeridroma saucia. Most species were inhibitory when methanolic extracts were incorporated into artificial diets at concentrations at or below those occurring naturally. In general members of the subfamily Melioideae were more inhibitory than members of the Swietenioideae. Extracts of deciduous species with short leaf lifetimes were significantly more inhibitory than those of evergreen species with longer leaf lifetimes. In a smaller sample of species, evergreen species showed a trend towards having tougher leaves than deciduous species. These results support the resource availability hypothesis of Coleyet al. (1985), and suggest that life history attributes may be of some value in selecting plants for phytochemical prospecting.     

Host-plant leaf surface compounds influencing oviposition in Delia antiqua

Summary. Delia antiqua (Diptera: Anthomyiidae) females lay eggs between the leaves of onion plants or in the soil around the base of the plants, then the maggots feed on the onion bulb and roots causing rapid secondary infection by fungi and bacteria. It is well known that the first sensory modality used by the onion fly is vision, therefore the shape (vertical narrow cylinders) and colour (yellow) of the plant play a crucial role in the recognition of a potential host plant. In the past it has been shown that n-dipropyl disulfide (Pr₂S₂), a typical component of onion volatiles, is an important chemical host plant cue. We extracted host leaf surface to verify if Pr₂S₂ is the major chemical oviposition stimulant and to determine if other as yet unknown substances may play a role in host-plant selection. We confirmed that the females laid more eggs around onion plants with leaves than when only the onion bulb was present and that the odour of chopped onion stimulates oviposition. Extraction of the surface of onion leaves revealed that only the apolar fraction contained substances that stimulate egg-laying in D. antiqua. GC-EAD analysis indicated that a minor constituent, Pr₂S₂, is perceived by the olfactory receptor on the antennae of the onion fly females. This confirmed the importance of Pr₂S₂ as oviposition stimulant. Contact with the polar fraction did not stimulate egg-laying behaviour in this Delia species. We discuss the oviposition strategy of D. antiqua in comparison with its closely related species, D. radicum, in which the oviposition behaviour is stimulated mainly through contact with the cabbage leaf surface and only partially by the host volatiles.     

Temporal and spatial variation in mountain birch foliar enzyme activities during the larval period of <Emphasis Type="Italic">Epirrita autumnata</Emphasis>

Summary. Mountain birches are deciduous trees consisting of several clonal and partly autonomous ramets. Autumnal moth, Epirrita autumnata, is an extremely harmful pest of mountain birches that during outbreak years may cause the death of trees over large areas. During the larval development of E. autumnata, leaf characters and nutritional suitability vary considerably both temporally and spatially among and within mountain birches. Compared to variation between trees, however, the extent of within-tree variation is known for only a limited number of potentially defensive traits. Plant oxidases, polyphenoloxidases (PPOs) and peroxidases (PODs) may play an important part in dictating the suitability of a tree for plant-eating animals, including E. autumnata. In this study, we observed changes in the activities of oxidative and antioxidative enzymes along leaf development during the larval period of E. autumnata. We also estimated the relative amounts of spatial variation among and within trees. Our results show that POD activities were remarkably high during the rapid elongation growth phase of leaves but decreased with leaf maturation. Thus PODs are suggested to take part in leaf elongation growth. Time and within tree variation accounted for the most of variation in POD activities whereas there was no among tree variation. In contrast, the activities of antioxidative CATs, which act as antagonists to PODs, were low in young leaves and increased towards leaf maturation, reflecting an increase in the oxidative status of source leaves. Within tree variation accounted for the most of the variation in CAT activities. The absolute PPO activities decreased along leaf growth due to the dilution effect whereas the specific activity of PPOs, which has been shown to be defensive against E. autumnata larvae, did not vary temporally that might reflect the importance of these enzymes in the defence of birches. Acidic PPOs showed marked within- and among-tree variation, which may impact herbivores performance on certain genotypes and increase larval mobility within the tree canopy.     

Leaf volatiles and polyphenols in pear trees infested byPsylla pyricola. Evidence of simultaneously induced responses

Summary Feeding by the homopteranPsylla pyricola on leaves of pear trees induces the production of volatile compounds, such as (E,E)--farnesene and methyl-salicylate, as well as the production of polyphenols. The inference on induction is based on GC-MS and HPLC chromatograms from the same samples ofPsylla infested leaves, leaves from the same pear tree beforePsylla infestation and uninfested leaves from other pear trees.Psylla infestation greatly enhanced the production of volatiles ((E,E)--farnesene, methyl-salicylate and others) and triggered the production of new polyphenols, characterized by much longer retention times.However, the responses to infestation depend critically on leaf age (defined by leaf distance to apex). With respect to the leaf volatiles it appears that infested, old leaves produce fewer compounds and lower amounts of the volatiles than infested, young leaves. Moreover, there seem to be differences in pattern. Relative to (E,E)--farnesene, methyl-salicylate was found in much lower amounts in heavily infested, old leaves. With respect to polyphenols it was found that infested old leaves collected in August have polyphenols with the same retention times, but more or less equal amounts as uninfested young leaves collected in May. This shows thatPsylla infestation causes the induced response mostly in young leaves.The induced leaf volatiles may act as synomones to heteropteran bugs. As shown elsewhere,Anthocoris nemoralis responds significantly to (E,E)--farnesene and methyl-salicylate when offered in pure form against clean air in a Y-tube olfactometer. The effect of polyphenols on the performance ofP. pyricola is not yet known. Hence, a role in direct defence is still to be investigated.     

加拿大一枝黄花和小飞蓬叶浸提液对大豆幼苗生长的复合化感效应

加拿大一枝黄花(Solidago canadensis)和小飞蓬(Conyza canadensis)常共同入侵至同一农田生态系统。探究了两者对大豆幼苗生长的复合化感作用。加拿大一枝黄花叶浸提液显著抑制大豆幼苗地上生物量。而小飞蓬叶浸提液也明显抑制大豆幼苗地上生物量(未达到显著水平)。与对照处理相比,两者复合叶浸提液处理未显著影响大豆幼苗地上生物量。两者复合叶浸提液处理下大豆幼苗地上生物量显著大于加拿大一枝黄花叶浸提液单一处理,同时也大于小飞蓬叶浸提液单一处理(未达到显著水平)。两者叶浸提液单一处理均明显降低大豆幼苗株高(未达到显著水平)。与对照处理相比,两者复合叶浸提液处理未显著影响大豆幼苗株高。两者复合叶浸提液处理下大豆幼苗株高明显大于两者叶浸提液单一处理(未达到显著水平)。因此,两者叶浸提液对大豆幼苗的生长均具有一定程度的抑制效应,且加拿大一枝黄花叶浸提液对大豆幼苗产生的化感作用(尤其是地上生物量)明显大于小飞蓬。两者复合化感作用明显低于两者单一化感作用,尤其是两者对大豆幼苗地上生物量的复合化感作用显著低于加拿大一枝黄花的单一化感作用。所以,化感效应可能在两者共同入侵(即两者复合作用)进程中所起的贡献低于在两者单一入侵进程中所起的作用。     

Multiple feeding stimulants in <Emphasis Type="Italic">Sinapis alba</Emphasis> for the oligophagous leaf beetle <Emphasis Type="Italic">Phaedon cochleariae</Emphasis>

Summary. In earlier investigations on host plant discrimination of leaf beetles glucosinolates were described as feeding stimulants for the Brassicaceae specialist Phaedon cochleariae F. (Coleoptera: Chrysomelidae). However, since these findings could not be confirmed in later studies offering 2-propenylglucosinolate in concentrations corresponding to those detected in host plant leaf material, the identification of feeding stimulants of this leaf beetle species remained unclear. In order to investigate which compounds of the host plant Sinapis alba (Brassicaceae) are involved in feeding stimulation, leaf extracts of different polarities were tested in bioassays with adults of P. cochleariae. Number of feeding beetles and net consumption rates were highest on pea leaves painted with methanol extracts of S. alba, whereas weak feeding responses were also detectable for hexane extracts. In subsequent bioassay-guided fractionations of methanol extracts with semi-preparative high performance liquid chromatography, two distinct fractions, one containing glucosinolates and another containing flavonoids, were found to stimulate beetles to feed to variable degrees. Other collected fractions had zero activity. The combination of both active fractions evoked significantly higher consumption rates and stimulated more beetles to feed than fractions tested individually. At least one compound of each fraction, among these the main glucosinolate of S. alba, 4-hydroxybenzylglucosinolate, act additively. Effects of two different naturally-occurring ratios of glucosinolates and flavonoids on the strength of feeding responses were investigated by use of extracts of two sets of host plants differently exposed to radiation. One set was outdoors-exposed, whereas the second set was kept in the greenhouse. However, the feeding behaviour of P. cochleariae was not affected by the significantly different relative compositions of both compound classes in the host material. In conclusion, mustard leaf beetles need a combination of distinct plant metabolites acting in concert for feeding stimulation, whereby the mere presence of these stimulants, but probably not the ratio of involved compounds, determines their feeding response.     

Population and leaf-level variation of iridoid glycosides in the invasive weed Verbascum thapsus L. (common mullein): implications for herbivory by generalist insects

Plant–insect interactions, which are strongly mediated by chemical defenses, have the potential to shape invasion dynamics. Despite this, few studies have quantified natural variation in key defensive compounds of invasive plant populations, or how those defenses relate to levels of herbivory. Here, we evaluated variation in the iridoid glycosides aucubin and catalpol in rosette plants of naturally occurring, introduced populations of the North American invader, Verbascum thapsus L. (common mullein; Scrophulariaceae). We examined two scales that are likely to structure interactions with insect herbivores—among populations and within plant tissues (i.e., between young and old leaves). We additionally estimated the severity of damage incurred at these scales due to insect chewing herbivores (predominantly grasshoppers and caterpillars), and evaluated the relationship between iridoid glycoside content and leaf damage. We found significant variation in iridoid glycoside concentrations among populations and between young and old leaves, with levels of herbivory strongly tracking leaf-level investment in defense. Specifically, across populations, young leaves were highly defended by iridoids (averaging 6.5× the concentration present in old leaves, and containing higher proportions of the potentially more toxic iridoid, catalpol) and suffered only minimal damage from generalist herbivores. In contrast, old leaves were significantly less defended and accordingly more substantially utilized. These findings reveal that quantitative variation in iridoid glycosides is a key feature explaining patterns of herbivory in an introduced plant. In particular, these data support the hypothesis that defenses limit the ability of generalists to feed on mullein’s well-defended young leaves, resulting in minimal losses of high-quality tissue, and increasing performance of this introduced species.