Variability in host plant chemistry: behavioural responses and life-history parameters of the Colorado potato beetle (Leptinotarsa decemlineata)

Summary. Many studies investigating effects of plant chemicals on herbivore performance have reported contradictory results, perhaps because of possible interaction between different chemicals. Also, a herbivore’s performance is not necessarily consistent with its food or oviposition preference. Our aim was to investigate simultaneously antibiosis (larval growth and survival) and antixenosis (oviposition and feeding preferences) responses in herbivore to three plant chemicals, of which one is expected to have positive and two are expected to have negative effects. Antibiosis was measured by correlating the nitrogen and glycoalkaloid levels in host plants to the survival and adult size of Leptinotarsa decemlineata, by rearing larvae on whole plants of three potato varieties. Although host plants differed in their glycoalkaloid levels, survival rate and adult body size did not differ among beetles reared on different potato varieties. This suggests that beetles are quite robust for differences in both foliar α-chaconine and foliar α-solanine content. However, differences in antixenosis were found although they could not be directly predicted from the leaf chemistry. Females preferred to lay their eggs on the variety with high α-solanine content (Nevsky) towards which males showed a tendency to feeding preference. Overall, our results confirm that beetles are well adapted to the chemical defences of potato plants as potato varieties did not significantly affect beetle performance, but differences in oviposition preference may still result in major differences in the amount of damage inflicted on plants in the fields.     

Leaf surface compounds of the potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis>) and their influence on Colorado potato beetle (<Emphasis Type="Italic">Leptinotarsa decemlineata</Emphasis>) feeding

Summary.  The sampling behavior of the Colorado potato beetle Leptinotarsa decemlineata Say (CPB) involves examination of the surface of potato leaves. It has been suggested that leaf surface compounds (volatiles and cuticular waxes) may be involved in host-plant recognition, acceptance or discrimination. Here we report on the effect of leaf surface extracts of six Polish commercial potato varieties on CPB feeding. We tested the influence of potato leaf surface extracts on CPB adult and larval feeding, then separated the extracts with HPLC, and finally tested the effect of the HPLC-separated fractions on CPB feeding. The bioassays were performed using potato leaf discs deprived of their original surface compounds. Applied to test discs at concentrations ten times higher than natural (10 leaf area equivalent), the extracts deterred CPB adults and larvae from feeding. HPLC-separated fractions composed of alkanes, sesquiterpene hydrocarbons, wax esters, benzoic acid esters, fatty acid methyl, ethyl, isopropyl and phenylethyl esters, aldehydes, ketones, methyl ketones, fatty acids, primary alcohols, β-amyrin and sterols did not affect adult CPB feeding. Similarly, alkanes, sesquiterpene hydrocarbons, wax esters, methyl ketones, sesquiterpene alcohols and secondary alcohols had no effect on larval CPB feeding. The sterol fraction (cholesterol, β-sitosterol and stigmasterol) acted as a phagostimulant to CPB larvae. We isolated a fraction demonstrating a phagodeterrent effect on CPB adults and larvae. The qualitative composition of the deterrent fraction was quite similar in all potato extracts, but there were quantitative differences between the varieties. Much further work is needed to identify the compounds that can produce the deterrent effect.     

Herbivore species, infestation time, and herbivore density affect induced volatiles in tea plants

In response to insect herbivory, plants emit volatiles that are used by the herbivores’ natural enemies to locate their host or prey. Herbivore attack also enhances tea aroma. Herbivore-induced plant volatiles (HIPVs) vary both quantitatively and qualitatively with infestation duration and herbivore density. Thus, whether HIPVs can reliably communicate the identities of herbivores is of interest. Here, we studied the tea plant volatiles induced by the tea leafhopper (Empoasca vitis, a piercing–sucking insect), the tea geometrid (Ectropis oblique, a chewing insect), and methyl jasmonate (MeJA, a plant hormone). Geometrid feeding induced more complex volatile blends than did leafhopper infestation. The volatiles induced by both herbivores significantly increased in quality and quantity with time during the first 16 h of infestation, after which the profiles of induced volatile blends and the emission of induced compounds varied diurnally. (E)-β-Ocimene displayed a unique rhythm in which emission peaked at night. The amount of HIPVs significantly increased, while their profiles changed little, with herbivore density. Overall, the leafhoppers and geometrids induced significantly different volatiles from tea plants, while the HIPV profiles varied with a circadian rhythm and were similar at different herbivore densities. Our findings also suggest a new method of enhancing tea flavor using exogenously applied plant hormones, because the volatiles induced by leafhoppers and MeJA were similar in general composition.     

Latitudinal variation in top-down and bottom-up control of a salt marsh food web

The shrub Iva frutescens, which occupies the terrestrial border of U.S. Atlantic Coast salt marshes, supports a food web that varies strongly across latitude. We tested whether latitudinal variation in plant quality (higher at high latitudes), consumption by omnivores (a crab, present only at low latitudes), consumption by mesopredators (ladybugs, present at all latitudes), or the life history stage of an herbivorous beetle could explain continental-scale field patterns of herbivore density. In a mesocosm experiment, crabs exerted strong top-down control on herbivorous beetles, ladybugs exerted strong top-down control on aphids, and both predators benefited plants through trophic cascades. Latitude of plant origin had no effect on consumers. Herbivorous beetle density was greater if mesocosms were stocked with beetle adults rather than larvae, and aphid densities were reduced in the "adult beetle" treatment. Treatment combinations representing high and low latitudes produced patterns of herbivore density similar to those in the field. We conclude that latitudinal variation in plant quality is less important than latitudinal variation in top consumers and competition in mediating food web structure. Climate may also play a strong role in structuring high-latitude salt marshes by limiting the number of herbivore generations per growing season and causing high overwintering mortality.     

An Edge Effect Caused by Adult Corn-Rootworm Beetles on Sunflowers in Tallgrass Prairie Remnants

Abstract: The once-extensive tallgrass prairie community of North America has been reduced to small remnants, many of which are surrounded by intensive corn (   Zea mays ) agriculture. We investigated adult corn-rootworm beetles (Chrysomelidae : Diabrotica spp.), important pests of corn, on sunflowers (Asteraceae : Helianthus spp . ) in prairie remnants in southeast Minnesota. Large numbers of beetles invaded the prairie from surrounding corn fields in late summer. D. barberi and D. virgifera were captured on sticky traps in all locations in the prairie, but abundance was much greater near the edge adjacent to corn. We observed D. barberi ( but not D. virgifera ) feeding extensively on sunflower pollen and occasionally on other flower parts, such as petals. Sunflowers located nearer corn fields sustained more floral damage than those farther from corn. To determine the effect of beetle damage on seed set, we enclosed sunflower heads in bags with either zero, two, or four D. barberi adults. Seed set was reduced in heads enclosed with D. barberi . Thus, this agricultural pest may interfere with the successful reproduction of sunflowers and possibly other prairie composites that flower in late summer. Given the small size of most prairie remnants and the abundance of this flower-feeding beetle in landscapes dominated by corn agriculture, D. barberi may affect the sustainability of prairie plant populations.     

Faecal shield chemical defence is not important in larvae of the tortoise beetle Chelymorpha reimoseri (Chrysomelidae: Cassidinae: Stolaini)

The most frequently investigated defence mechanism among larvae of tortoise beetles is protection by faecal shields, which generally present chemicals that are directly sequestered and/or modified from larval host-plants. In the present work we investigate the tortoise beetle Chelymorpha reimoseri that feeds on the leaves of Ipomoea carnea fistulosa (Convolvulaceae), seeking for the importance of this mechanism to their larvae. We show that 4th instar larvae suffer low predation regardless of the presence of shields in field and laboratory bioassays with ants and chicks. Chloroform extract from larvae without shields provided high protection against predation as shown in bioassays in the field, as well as against chicks, suggesting that C. reimoseri does not rely on the shield for protection. The aqueous extract of the shields did not show activity in such bioassays. The compounds responsible for this protection have yet to be identified, and it remains an open question as to whether they are sequestered from the host plant or de novo biosynthesized. This is the first record of chemical defence in cassidine beetles without the need for faecal shields. These findings indicate that more attention should be paid to chemicals present in the tissues of larvae and/or adults of tortoise beetles; the protective compounds sequestered from host plants or de novo biosynthesized can provide an alternative or complementary strategy against predation in these insects.     

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.     

Attraction of the potential biocontrol agent <Emphasis Type="Italic">Galerucella placida</Emphasis> (Coleoptera: Chrysomelidae) to the volatiles of <Emphasis Type="Italic">Polygonum orientale</Emphasis> (Polygonaceae) weed leaves

Galerucella placida Baly (Coleoptera: Chrysomelidae) is a potential biocontrol agent of the rice-field weed Polygonum orientale L. (Polygonaceae). The volatile organic compound (VOC) profiles from undamaged and mechanically damaged plants, and from plants 12- and 36-h following continuous feeding of female G. placida adults and 2nd instar larvae were identified and quantified by GC–MS and GC-FID analyses. Twenty-four and 21 compounds were identified in volatiles of undamaged and insect feeding plants, respectively; whereas 22 compounds were detected in volatiles of mechanically damaged plants. Decanal and 1-dodecanol were unique to undamaged plants, and linalool was detected in volatiles of undamaged and mechanically damaged plants, but not in volatiles of insect damaged plants. However, the beetles are not attracted by none of these volatile components, when tested individually in Y-shaped glass tube olfactometer bioassays. In all plants, methyl jasmonate was predominant. 1-Undecanol was the least amount in undamaged plants, and plants 12-h after feeding by G. placida adults and larvae; whereas 1-tridecanol was the least abundant in plants 36-h after feeding by G. placida adults and larvae, and mechanically damaged plants. The beetles showed significant preference to the whole volatile blends from plants 12-h after feeding by larvae and plants 36-h after feeding by either larvae or adults compared to those of undamaged plants. Further, G. placida responded to individual synthetic compounds, 3-hexanol, 1-octen-3-ol, nonanal, and geraniol at 7, 1.38, 3.75 and 4.5 µg/25 µL CH₂Cl₂, respectively, and provide a basis for attraction of the potential biocontrol agent in the field.     

Does cannibalism in Spodoptera frugiperda (Lepidoptera: Noctuidae) reduce the risk of predation?

The incidence of cannibalism of larval Spodoptera frugiperda (Lepidoptera: Noctuidae) on maize under field conditions was investigated using field cages. Cannibalism was found to account for approximately 40% mortality when maize plants were infested with two or four fourth-instar larvae over a 3-day period. Field trials examined the effect of larval density on the prevalence of natural enemies of S. frugiperda. The abundance of predators (earwigs, staphylinids, other predatory beetles, and Chrysoperla spp.) was significantly greater on maize plants with higher levels of larval feeding damage, while the relationship between predator abundance and number of S. frugiperda larvae per plant was less clear. As larval damage is probably a more reliable indicator of previous larval density than numbers collected at an evaluation, this indicates that predation risk will be greater for larvae living in large groups. Parasitism accounted for 7.1% mortality of larvae in sorghum, and involved six species of Hymenoptera and Tachinidae. There was no effect of larval density or within-plant distribution on the probability of larval attack by parasitoids. The selective benefits of cannibalism, in relation to the risk of predation and parasitism, are discussed. Received: 23 March 2000 / Accepted: 24 June 2000     

Metabolism of neolignans from P. regnellii (Piperaceae) in the beetle Naupactus bipes (Coleoptera: Curculionidae)

Piper regnellii (Piperaceae) contains high levels neolignans with diverse biological activities, including insecticidal. Despite the insecticidal activity of these neolignans, Naupactus bipes (Coleoptera: Curculionidae) larvae and adults were found feeding on the leaves (adults) and roots (larvae) of P. regnellii. The present study investigated the metabolic pathway of neolignans from P. regnellii leaves in the beetle N. bipes, focusing on possible biotransformation or sequestration of these compounds by the beetle. Two of the four plant neolignans could be recovered in the feces of adults feeding on P. regnellii leaves. In addition, four degradation products were detected, including the neolignan 3-[(2R,3R)-2,3-dihydro-3-(methyl)-2-(4-hydroxyphenyl)-5-benzofuranyl]-(2E)-propenal (7), an oxidation product of conocarpan, the major plant neolignan. The adult beetle sequesters the neolignan conocarpan selectively from leaves as well as accumulates neolignan 7. Neolignan 7 was also detected in the larvae of lab-reared beetles.     

Quantitative analysis of pyrrolizidine alkaloids sequestered from diverse host plants in Longitarsus flea beetles (Coleoptera, Chrysomelidae)

Summary. Several species of the flea beetles genus Longitarsus are able to sequester pyrrolizidine alkaloids (PAs) from their host plants. In five Longitarsus species we compare the concentration of PAs present in their host plants belonging to the Asteraceae or Boraginaceae with those found in the beetles. To get an estimate of the intrapopulation variability, three samples of five beetles each and five individual plants were analyzed for each comparison. A strong intrapopulation variability could be detected both among plant and beetle samples. The total concentration found in the beetles varied strongly between species. The local host plant and its phenology influence the concentrations present in the beetles as evidenced in comparisons of a single beetle species from two different hosts and of one beetle species collected at the same site at different times of the year. In addition, different beetle species apparently vary in their capacity to sequester the alkaloids, at the lowest extreme the mean PA concentration in the beetles (0.034 μg PA/mg dry weight) was 1/30 of the mean concentration found in the plant leaves (L. aeruginosus from Eupatorium cannabinum), at the highest extreme (2.098 μg PA/mg dw) the concentration in the beetles was a 1000 fold higher than in the plant leaves (L. nasturtii from Symphytum officinale). The highest mean concentration found in the beetles was 3.446 μg/mg dw (L. exoletus from Cynoglossum officinale). The absolute concentrations found in the beetles are comparable to other insects which have been shown to be effectively defended against their potential predators. Received 22 June 1999; accepted 25 August 1999     

Different phagostimulants in potato foliage for Manduca sexta and Leptinotarsa decemlineata

Summary. In the past decades, several studies have focused on the identification of feeding stimulants for specialists that feed on solanaceous plants, especially potato (Solanum tuberosum). In the 1950's, a phagostimulant was isolated from potato and tomato for Manduca sexta and characterised as a glycoside. It was suggested that the phagostimulant for M. sexta and Leptinotarsa decemlineata is identical. We tested whether these insects indeed share the same recognition factor. Previous bioassays for L. decemlineata larvae and adults were found to be unsatisfactory, so a new assay system was developed, using starch and wheat flour-based wafers as a neutral substrate. An aqueous extract of potato foliage that was highly active as a stimulant was fractionated by reverse phase medium pressure liquid chromatography (MPLC). Both insects were stimulated by a MPLC fraction that eluted with 25% methanol in water. Further separation of this and the following fraction (35% methanol, stimulatory for the beetle only) by semi-preparative high performance liquid chromatography (HPLC), using gradients of water and acetonitrile, resulted in the isolation of two distinct fractions that stimulated feeding by L. decemlineata. None of the HPLC fractions was active for M. sexta. However, fractions of the flash chromatography with less polarity (45–75% methanol) had a stimulatory effect only on M. sexta. Thus, the two insects do not use a common feeding stimulant, and for both at least two compounds of different polarity are active. Received 3 July 2000; accepted 24 October 2000     

Plant-derived visual signals may protect beetle herbivores from bird predators

Insect herbivores often use chemical signals obtained from their food plants to deter enemies and/or attract sexual partners. Do plant-based visual signals act similarly, i.e., repel consumers' enemies and appeal to potential mates? We explored this question using the pollen-feeding beetle Pygopleurus israelitus (Glaphyridae), a specialized pollinator of Anemone coronaria's chemically defended red-morph flowers. We presented dead beetles, which had fed either on anemones or on cat food, to young domestic chicks on a red (anemone-like) or a green (leaf-like) background. We determined whether the beetles' background color and diet affected the chicks' feeding. Cuticle surface extracts from anemone-fed beetles, but not from cat food-fed beetles, contained a secondary metabolite characteristic of anemones. Latencies to the first picking up and consuming of beetles from green backgrounds were shorter than of beetles from red backgrounds. The picking up order of beetles also indicated that prey from the green background was preferred. The chicks retained this preference when re-tested, 3 days later. Handling times of anemone-fed beetles were longer than of cat food-fed beetles. A previous study showed that glaphyrids improve their mate-finding prospects by orienting to large red anemone flowers. Here, female beetles preferred cat food-fed to anemone-fed males in mate-choice assays, thus anemone-derived chemicals did not increase mating success. Instead, the combined results indicate that A. coronaria's red flowers provide a visual signal that may both deter its herbivore's predators and attract its mates. To our knowledge, this is the first experimental evidence for a potential protective role of plant-derived visual signals for insect herbivores/pollinators.     

Do birds and beetles show similar responses to urbanization?

To date, the vast majority of studies in urban areas have been carried out on birds, yet it is not known whether the responses of birds to urbanization are congruent with those of other taxa. In this paper, we compared the responses of breeding birds and carabid beetles to urbanization, specifically asking whether the emerging generalizations of the effects of extreme levels of urbanization on birds (declines in total species richness and the richness of specialist species, increases in total abundance and the abundances of native generalist and introduced species, and community simplification, including increasing similarity) could also be applied to ground beetles. We also directly tested for congruence between birds and ground beetles using correlations between variables describing bird and beetle community structure and correlations between bird and beetle distance matrices describing community dissimilarity between pairs of sampling locations. Breeding bird and carabid beetle community data were collected in Ottawa, Ontario, and Gatineau, Quebec, Canada, in two groups of sites: developed sites representing the predictor variable within-site housing density, and forested sites adjacent to development representing the predictor variable neighboring housing density (each site was 0.25 km2). Breeding birds and carabid beetles do not respond similarly to increasing within-site housing density but do exhibit some similar responses to increasing neighboring housing density. Birds displayed strong declines in diversity, compositional changes, and community simplification in response to increasing within-site housing density. Forest and introduced species of birds and beetles responded similarly to increasing housing density within a site, but responses of overall diversity and open-habitat species richness and patterns of community simplification differed between birds and beetles. Increasing neighboring housing density resulted in increases in the abundances of introduced birds and introduced beetles and similar patterns of community simplification in both taxa. To better understand and mitigate the effects of urbanization on biodiversity, we suggest that, in addition to the responses of birds, future research should focus on the responses of other taxa in the urban matrix.     

Mixed messages across multiple trophic levels: the ecology of bark beetle chemical communication systems

Summary. Chemical, physiological and behavioral components of pheromone communication have been described for a number of bark beetle species, yet our understanding of how these signals function under natural conditions remains relatively limited. Development of ecologically based models is complicated by the multiple functions and sources of variability inherent in bark beetle semiochemistry. This discussion addresses four ecological issues of chemical signaling in bark beetles: the effects of aggregation on individual fitness, the possibility of cheating, how plants can defend themselves against herbivores that employ aggregation pheromones, and the implications of variability in chemical communication systems to predator avoidance. An analysis of published data from thirteen scolytid – conifer systems indicates that the net benefit and optimal colonization density vary with host condition and beetle species. When beetles attack live trees, the benefit of cooperative host procurement exceeds losses due to competition for the limited substrate, at least up to moderate densities. When beetles colonize dead tissue, however, the effect of subsequently arriving beetles on initial colonizers is almost entirely negative. This suggests that aggregation originated as exploitation of senders, but evolved into manipulation of receivers. It is also proposed that the optimal colonization density which typifies each species or population may offer a more objective and less value–laden index of behavior than current labels such as “aggressiveness”. Beetles can maximize the relative benefits of group attack by incorporating instantaneous measures of host resistance into their colonization behavior, and by adjusting oviposition with colonization density. This system may provide opportunities for cheating. However a number of factors may select against a fixed strategy of cheating, including the linkage between tree allelochemistry and beetle semiochemistry, the reduced quality of substrate available to late arrivers, the short adult lifespans of most bark beetles, differential exposure to some predators, the difficulty of locating signalers during extensive endemic periods, and the low costs incurred during host assessment. However, the possibility that beetles employ flexible, density – dependent strategies deserves heightened attention. The ability of bark beetles to collectively exhaust host defenses poses a particular problem for plant defense. It is argued here that the ideal defense should include both direct resistance mechanisms against invading beetles, and indirect mechanisms that inhibit chemical communication. Evidence for the latter mechanism is explored. The ability of predators to efficiently exploit aggregation pheromones as kairomones in prey finding poses significant risk to bark beetles. It is proposed that minor alterations in pheromone components may provide colonizers with partial escape from such natural enemies while maintaining intraspecific functionality. Traditional interpretations emphasized the fidelity and consistency of pheromones, but under natural conditions chemical signals are modified by unpredictable features of the biotic and abiotic environment. Although we typically view variation in pheromonal signals as experimental noise or simple deviations from a population norm, such variation may reflect evolutionary dynamics. Complex ecological interactions may impose trade-offs between the clarity versus diversity of their signals. Received 3 July 2000; accepted 8 January 2001     

Influence of dung volatiles on the process of resource selection by coprophagous beetles

Summary. Most dung beetles colonize the faeces of several vertebrate species without much discrimination, and are thus often considered as polyphagous. Recent studies have provided evidence for clear feeding preferences in scarab beetles colonizing dung of herbivore species, but little is known about these insects’ abilities to discriminate among odours from faeces of various herbivores. In this study, trophic preferences were examined using blocks of pitfall traps baited with dung from four different herbivore species, i.e., sheep, cattle, horse, and red deer, in a mountainous area of south-central France. 4941 coprophagous scarabs, belonging to 27 species, were captured. Beetles were more attracted to dung of sheep (2257 individuals) than that of cattle (1294 individuals), followed by deer dung (768 individuals) and horse dung (622 individuals). Eleven of the 27 beetle species collected had significant feeding preferences for one of the four dung types. For each insect species, trophic habits did not vary between the two different sites of trapping, an open pasture and a wooded habitat. In laboratory olfactometer bioassays, scarab beetles orientated preferentially towards the dung volatiles from the dung type they preferred in the field. Trypocopris pyrenaeus, Anoplotrupes stercorosus, and Aphodius rufipes were more attracted to volatile compounds from sheep dung, Onthophagus fracticornis significantly preferred horse dung volatiles, and Aphodius haemorrhoidalis responded positively to deer dung odours. The role of dung olfactory cues in the process of resource selection by dung beetles is discussed.     

Ubiquitous eight-carbon volatiles of fungi are infochemicals for a specialist fungivore

An enormous variety of volatile substances are released in distinctive blends by fungal substrates that should be recognisable for fungivores. Certain compounds dominate in most of the fungal species. Fungal oxylipins as the eight-carbon volatiles are the most prominent. This raises the question whether such are specific enough to qualify as appropriate host cues for a fungivore. We could demonstrate differentiated responses of the fungivorous beetle Bolitophagus reticulatus to eight-carbon volatiles: Nine eight-carbon volatiles were identified with GC–MS from its host fungus Fomes fomentarius. 1-Octen-3-ol, 3-octanone and 3-octanol induced contrasting behaviour of beetles in olfactometer bioassays. Electroantennographic experiments investigating the beetle olfactory sense revealed distinguishable antennal responses. Moreover, their individual release from F. fomentarius fruiting bodies changes not only considerably, but also independently over successive stages of beetle colonisation. Concentrations of attractive and repellent eight-carbon volatiles correlate to frequency of beetles in the field and further substantiate their relevance as host cues. Our results show that a specialist fungivore is able to differentiate the most common eight-carbon volatiles of fungi to assess host quality. Key roles and marked similarities of fungal to plant oxylipins suggest a comparable importance of eight-carbon volatiles to fungivores as green leaf volatiles have to herbivores.     

Local consumers induce resistance differentially between Spartina populations in the field

Intraspecific variation in the strength of inducible plant defenses plays a central role in the interactions between plants and herbivores. Studies of this variation are typically conducted in the greenhouse or laboratory rather than the field. We simultaneously manipulated densities of local consumers in the field within Maine and South Carolina populations of the smooth cordgrass Spartina alterniflora. South Carolina, but not Maine, plants induced resistance when grazed by local consumers. South Carolina populations of Littoraria snails and planthoppers colonized control more than previously grazed South Carolina plants, and Littoraria snails consumed more control than previously grazed plants. The inducible feeding deterrents in South Carolina plants appear to be water soluble, but not phenolic based. In contrast, grazed and control plants from Maine populations did not differ in attractiveness or palatability to Maine consumers. Thus, inducible plant responses by South Carolina plants had a strong effect on the South Carolina consumer community, but no analogous effect occurred in Maine. Field experiments are a powerful approach to detecting the strength of inducible plant resistance and its impacts on local consumers, which in this case were shown to vary with location.     

Interplay between Senecio jacobaea and plant, soil, and aboveground insect community composition

To elucidate the factors that affect the performance of plants in their natural environment, it is essential to study interactions with other neighboring plants, as well as with above- and belowground higher trophic organisms. We used a long-term field experiment to study how local plant community diversity influenced colonization by the biennial composite Senecio jacobaea in its native range in The Netherlands in Europe. We tested the effect of sowing later-succession plant species (0, 4, or 15 species) on plant succession and S. jacobaea performance. Over a period of eight years, the percent cover of S. jacobaea was relatively low in communities sown with 15 or 4 later-succession plant species compared to plots that were not sown, but that were colonized naturally. However, after four years of high abundance, the density of S. jacobaea in unsown plots started to decline, and the size of the individual plants was smaller than in the plots sown with 15 or 4 plant species. In the unsown plots, densities of aboveground leaf-mining, flower-feeding, and stem-boring insects on S. jacobaea plants were lower than on plants in sown plots, and there was a strong positive relationship between plant size and levels of herbivory. In a greenhouse experiment, we grew S. jacobaea in sterilized soil inoculated with soil from the different sowing treatments of the field experiment. Biomass production was lower when S. jacobaea test plants were grown in soil from the unsown plots than in soil from the sown plots (4 or 15 species). Molecular analysis of the fungal and bacterial communities revealed that the composition of fungal communities in unsown plots differed significantly from those in sown plots, suggesting that soil fungi could have been involved in the relative growth reduction of S. jacobaea in the greenhouse bioassay. Our results show that, in its native habitat, the abundance of S. jacobaea depends on the initial composition of the plant community and that, on a scale of almost a decade, its interactions with plant and soil communities and aboveground invertebrates may influence the dynamics of this colonizing species.     

Preferences of six leaf beetle species among qualitatively different leaf age classes of three Salicaceous host species

Summary. As Salicaceous plants produce new leaves for a prolonged period of time, they expose a wide range of differentially aged leaves to herbivores during the growing season. In this work, I show that young leaves of three Salicaceous species, Populus tremula L., Salix phylicifolia L. and S. pentandra L., contain more nitrogen than conspecific old leaves. In P. tremula and S. pentandra young leaves also contained more low-molecular weight secondary compounds, phenolic glucosides. Leaves of S. phylicifolia did not contain phenolic glucosides in detectable amounts. Furthermore, in P. tremula and S. pentandra young leaves contained less polymeric digestability-reducing phenolics, condensed tannins, than old leaves. In S. phylicifolia, higher concentrations of condensed tannins were found in young leaves. In laboratory feeding trials with six leaf beetle species, young leaves of the studied plants were invariably preferred in all tested herbivore × host species combinations. In particular, it is remarkable that three leaf beetle species with known different overall relationships to phenolic glucosides equally preferred more glucoside-containing young S. pentandra leaves over conspecific old ones. Four beetle species were found to prefer young leaves of S. phylicifolia despite the higher content of condensed tannins in young leaves. These results indicate that the general preference of leaf beetles for young leaves of Salicaceous plants probably does not primarily result from variable distribution of secondary compounds. Apparently, the preference for young leaves is fundamentally due to variation in leaf nutritive traits, such as nitrogen content. Received 9 February 2001.