A trophic cascade in a macrophyte-based food web at the land–water ecotone

Trophic cascades may purportedly be more common in aquatic than terrestrial food webs, but herbivory on freshwater vascular plants has historically been considered low. Water lilies are an exception, suffering severe grazing damage by leaf beetles. To test whether a central prediction of cascade models—that predator effects propagate downwards to plants—operates in a macrophyte-based food web, we experimentally manipulated predation pressure on a key herbivore of water lilies in the littoral zone of a lake in Michigan, USA. Field experiments comprised combinations of caging treatments to alter the number of predators (larvae of the ladybird beetle Coleomegilla maculata) that hunt the grazers of the macrophytes (larvae of the leaf beetles Galerucella nymphaeae) on the leaves of the water lily Nuphar advena. Predatory larvae of the ladybird beetles significantly reduced grazing damage to water-lily leaves by 35–43%. The predators reduced plant damage chiefly via density-mediated effects, when lower densities of grazers translated to significant declines in plant damage. Plant damage caused by the surviving herbivores was less than predicted from individual grazing rates under predator-free conditions. This suggests that trait-mediated effects may possibly also operate in this cascade. The observed strong effect of predators on a non-adjacent trophic level concurs with an essential component of the trophic cascade model, and the cascade occurred at the ecotone between aquatic and terrestrial habitats: Nuphar is an aquatic macrophyte with emergent and floating leaves, whereas both beetle species are semi-terrestrial and use the dry, emergent and floating leaves of the water lily as habitat. Also, the cascade is underpinned by freshwater macrophytes—a group for which trophic processes have often been underappreciated in the past.     

Galerucella nymphaeae (Col., Chrysomelidae) grazing increases Nuphar leaf production and affects carbon and nitrogen dynamics in ponds

Summary The grazing effects of the waterlily beetle Galerucella nymphaeae on Nuphar lutea stands were studied in three ponds in Central Finland. Production of floating leaves of N. lutea and growth in the G. nymphaeae population were investigated in the ponds and bioenergetics of the beetle larvae in the laboratory. Combination of field and laboratory data enabled estimation of the effect of the beetle on the production of floating leaves of N. lutea and the consequences of grazing for the input of detritus from Nuphar into the ponds. Adults and larvae of G. nymphaeae consumed 3.0–6.1% of the net annual floating leaf production during the growing period. In addition to consumption losses, feeding accelerated the degradation rate of the leaves. This was associated with an increased flow of detrital material of Nuphar origin, and also with increased production of floating leaves in the ponds. These increments were estimated to be up to 3 times greater in the presence of grazing than without it. Grazing by G. nymphaeae releases substantial amounts of carbon and nitrogen bound in Nuphar, particularly in ponds with a dense Nuphar vegetation. It is hypothesized that feeding by this beetle may markedly affect the structure and functioning of such small aquatic systems.     

Leaf beetle grazing does not induce willow trichome defence in the coppicing willow Salix viminalis

Abstract 1 Willows are frequently attacked and defoliated by adult leaf beetles (Phratora vulgatissima L.) early in the season and the plants are then attacked again when new larvae emerge. The native willow Salix cinerea has previously been shown to respond to adult grazing by producing new leaves with an increased trichome density. Subsequent larval feeding was reduced on new leaves. This type of induced plant response may reduce insect damage and could potentially be utilized for plant protection in agricultural systems. 2 Here, we investigated if the willow species most commonly used for biomass production in short rotation coppice, Salix viminalis, also responds to adult beetle grazing by increasing trichome density. Larval performance and feeding behaviour on plants previously exposed to adult beetles was compared with that on undefoliated control plants in a greenhouse. 3 We found an overall decrease in trichome density within all the plants (i.e. trichome density was lower on new leaves compared to that for older basal leaves on S. viminalis). However, leaves of beetle defoliated plants had a higher trichome density compared to control plants. Larval growth and feeding was not affected by this difference between treatments. Larvae appeared to remove trichomes when feeding on S. viminalis, a behaviour that might explain the lack of difference between treatments.     

Responses of grassland specialist and generalist beetles to management and landscape complexity

We tested the influence of grazing intensity and effect of landscape complexity on grassland specialist and generalist beetles of three beetle families, i.e. Carabidae, Chrysomelidae, and Curculionidae, on extensively and intensively grazed cattle pastures in three regions of the Hungarian Great Plain. In every region we investigated seven pairs of grazed grasslands. On each field, samples were taken along two 95-m-long transects; one transect at the edge and the other one 50 m away from the edge in the grassland interior (altogether 84 transects). Carabids (Carabidae) were sampled using funnel traps for three 2-week sampling periods during spring and early summer. Leaf-beetles (Chrysomelidae) and weevils (Curculionidae) were surveyed by sweep netting in May and June 2003. Analysing the grazing intensity and landscape complexity effects on generalist and specialist beetles with linear mixed models, grazing effect was detected only on specialist leaf-beetle species richness with more species in the extensively grazed sites. Landscape complexity had contrasting effects on specialist and generalist species. Habitat generalists were more and negatively affected by increasing grassland coverage (reduced heterogeneity) than specialists. At species level analyses on four species out of 21, landscape effects were shown, which suggested that landscape composition might have strong effects on the species composition of the beetle assemblages. Our results suggest that conservation of biodiversity in agricultural systems (such as in managed Central European grasslands) requires a landscape perspective besides investigating management effects.     

Comparative feeding selectivity of herbivorous insects on water lilies: aquatic vs. semi‐terrestrial insects and submersed vs. floating leaves

1. The rate of grazing damage experienced by submersed and floating leaves of water lilies (Nuphar variegata and Nymphaea odorata) was monitored in lakes in the Upper Peninsula of Michigan, U.S.A. Herbivores damaged 0.2–1.7% of the leaf surface of water lilies per day. These grazing rates differed between plant species, between submersed and floating leaves, and between lakes. Some leaves had more than 60% of their surface damaged and an overall mean of 16% damage occurred during the 2–3 week monitoring period of this study. 2. Snapshot measurements of grazing damage on randomly collected submersed and floating leaves of Nuphar showed that submersed leaves were more damaged (11.0 ± 1.6%, n = 84) than floating leaves (3.8 ± 0.6%, n = 92). Overall, these 176 Nuphar leaves had 7.2% of their area damaged. 3. Five species of herbivorous insects were commonly found on water lilies (Nymphaeacea). One primarily aquatic insect (sensu 1 ), a caddisfly larva (Trichoptera: Limniphilidae), had a generalized diet of water lilies, other macrophytes, algae, and detritus. Four of the five insects were from primarily terrestrial insect groups (Coleoptera and Diptera;‘secondary invaders’, sensu 1 ) and consumed only water lilies in food preference experiments. 4. The feeding preferences of the generalist trichopteran were altered when the macrophytes were freeze-dried, ground into a powder, and reconstituted in an alginate gel. This suggests that plant structure may be an important feeding determinant for this insect. In contrast, a specialist weevil preferred its host plant in choice assays, regardless of whether fresh tissue or reconstituted macrophytes were used, suggesting this insect cued on a unique, non-structural property of its host plant. 5. These results suggest that herbivory on freshwater macrophytes is of a similar magnitude to that on terrestrial plants. The findings of this study are consistent with the hypothesis that herbivorous insects of primarily terrestrial groups have a narrower diet breadth than insects of primarily aquatic groups.     

Impact of reindeer grazing on ground-dwelling Carabidae and Curculionidae assemblages in Lapland

Reindeer Rangifer tarandus L. grazing shapes forest vegetation, microclimate, and soil respiration in Lapland, especially due to grazing on lichens (Cladina). We studied how these changes and their magnitude affect ground‐dwelling species of beetle families Carabidae (predators) and Curculionidae (herbivores), by using pitfall traps to collect invertebrates from pairs of grazed and ungrazed study plots over a wide range of site types. Changes in abundance, composition, richness and diversity of beetle assemblage were tested in relation to magnitude of the impacts on vegetation. The species compositions of Carabidae and Curculionidae differed between grazed and ungrazed plots in all sites. The relative difference between grazed and ungrazed plots in the number of individuals increased linearly with the impact of reindeer on vegetation cover. Carabid beetles, as a family, were more common in grazed plots in all sites. Curculionid beetles were more common in ungrazed plots in the birch dominated sites. This difference was mainly due to the species that feeds on deciduous leaves. In the pine dominated sites with high Cladina cover and more changes in ground vegetation, the number of curculionids feeding on conifers was higher in grazed plots. Species richness and diversity (H’) of both families were higher in grazed plots. Of the total 27 species, 11 were found only in grazed plots, while not a single species was found only in ungrazed plots. The relative difference between plots in diversity and evennes (H’/H'max) had humped response to the difference in Cladina cover. The diversity values were greater in grazed plots at the intermediate levels of grazing impact, and only in sites with very low or extremely high Cladina cover difference was the diversity higher in ungrazed plots. The response of beetle diversity resembled the hypotheses suggested for the relationship between grazing and vegetation diversity: greatest positive effect at intermediate grazing intensity and negative effects at unproductive sites.     

Mate finding in the forest cockchafer, Melolontha hippocastani, mediated by volatiles from plants and females

The response of the forest cockchafer, Melolontha hippocastani F. (Coleoptera, Scarabaeidae), towards volatiles emitted by different host plants and conspecifics was tested in field experiments during the flight period at dusk. Funnel traps containing artificially damaged leaves from the host plants Carpinus betulus L. and Quercus rubra L., as well as from the non‐host plant Prunus serotina Ehrh. caught significantly more beetles than empty control traps. On the other hand, traps baited with undamaged leaves from Q. rubra did not catch significantly more beetles than empty controls. Leaves from C. betulus damaged by beetle feeding did not attract more beetles than artificially damaged leaves. By use of gas chromatography coupled with electroantennographic detection (GC‐EAD) electrophysiological responses of males and females were shown for 18 typical plant volatiles. A synthetic mixture of selected typical green plant volatiles was also highly attractive in the field. A total of 9982 beetles was caught during the field experiments, among them only 33 females. This suggests that attraction to damaged foliage during flight period at dusk is male‐specific. Field experiments testing the attractiveness of female M. hippocastani towards conspecific males by employing caged beetles and beetle extracts indicated that males of M. hippocastani use a female‐derived sex pheromone for mate location. On wired cages containing either unmated feeding females, or unmated females without access to foliage, or feeding males in combination with extracts from unmated females, significantly more males landed during the flight period than on comparable control cages containing feeding males or male extracts. A possible scenario of mate location in M. hippocastani involving feeding‐induced plant volatiles and a female‐derived sex pheromone is discussed.     

A fungal root symbiont modifies plant resistance to an insect herbivore

Vesicular-arbuscular mycorrhizal (VAM) fungi are common root-colonizing symbionts that affect nutrient uptake by plants and can alter plant susceptibility to herbivores. I conducted a factorial experiment to test the hypotheses that colonization by VAM fungi (1) improves soybean (Glycine max) tolerance to grazing by folivorous Mexican bean beetle (Epilachna varivestis), and (2) indirectly affects herbivores by increasing host resistance. Soybean seedlings were inoculated with the VAM fungus Glomus etunicatum or VAM-free filtrate and fertilized with high-[P] or low-[P] fertilizer. After plants had grown for 7 weeks first-instar beetle larvae were placed on bagged leaves. Growth of soybean was little affected by grazing larvae, and no effects of treatments on tolerance of soybeans to herbivores were evident. Colonization by VAM fungus doubled the size of phosphorus-stressed plants but these plants were still half the size of plants given adequate phosphorus. High-[P] fertilizer increased levels of phosphorus and soluble carbohydrates, and decreased levels of soluble proteins in leaves of grazed plants. Colonization of grazed plants by VAM fungus had no significant effect on plant soluble carbohydrates, but increased concentration of phosphorus and decreased levels of proteins in phosphorus-stressed plants to concentrations similar to those of plants given adequate phosphorus. Mexican bean beetle mass at pupation, pupation rate, and survival to eclosion were greatest for beetles reared on phosphorus-stressed, VAM-colonized plants, refuting the hypothesis that VAM colonization improves host plant resistance. VAM colonization indirectly affected performance of Mexician bean beetle larvae by improving growth and nutrition of the host plant. Received: 28 February 1997 / Accepted: 23 June 1997     

The effect of modifying the glucosinolate content of leaves of oilseed rape (Brassica napus ssp. oleifera) on its interaction with specialist and generalist pests

Twenty eight Brassica napus lines were developed which had contrasting leaf glucosinolate profiles to those found in commercial oilseed rape cultivars. The lines varied both in the total amount of aliphatic glucosinolates and in the ratio of different side chain structures. The lines were used in field experiments to assess the manner by which glucosinolates mediate the interactions between Brasssica and specialist pests (Psylliodes chrysocephala and Pieris rapae) and generalist pests (pigeons and slugs). Increases in the level of glucosinolates resulted in greater damage by adult flea beetles (P. chrysocephala) and a greater incidence of Pieris rapae larvae, but reduced the extent of grazing by pigeons and slugs. Decreasing the side chain length of aliphatic glucosinolates and reducing the extent of hydroxylation of butenyl glucosinolates increased the extent of adult flea beetle feeding. The implications of modifying the glucosinolate content of the leaves of oilseed rape and the role of these secondary metabolites in plant/herbivore interactions are discussed.     

Effects of leaf and sap feeding insects on photosynthetic rates of goldenrod

Summary Herbivory can alter the balance between sources and sinks within a plant, and changes in the source-sink ratio often lead to changes in plant photosynthetic rates. We investigated how feeding by three insect herbivores affected photosynthetic rates and growth of goldenrod (Solidago altissima). One, a phloem-sap feeding aphid (Uroleucon caligatum), creates an additional sink, and the other two, a leaf-chewing beetle (Trirhabda sp.) and a xylem-sap feeding spittlebug (Philaenus spumarius) both reduce source supply by decreasing leaf area. Plants were grown outside in large pots and insects were placed on them at predetermined densities, with undamaged plants included as controls. All insects were removed after a 12-day feeding period. We measured photosynthetic rates both of damaged leaves and of undamaged leaves that were produced after insect removal. Photosynthetic rates per unit area of damaged leaves were reduced by spittlebug feeding, but not by beetle or aphid feeding. Conductance of spittlebugdamaged leaves did not differ from controls, but internal carbon dioxide concentrations were increased. These results indicate that spittlebug feeding does not cause stomatal closure, but impairs fixation within the leaf. Effects of spittlebug feeding on photosynthetic rates persisted after the insects were removed from the plants. Photosynthetic rates per unit area of leaves produced after insect removal on spittlegug-damaged plants were lower than control levels, even though the measurements were taken 12 days after insect removal. The measurement leaf on spittlebugdamaged plants was reduced in area by 27% relative to the controls, but specific leaf area (leaf area/leaf weight) was increased by 18%. Because of the shift in specific leaf area, photosynthetic rates were also examined per unit leaf weight, and when this was done there were no significant differences between control and spittlebug-damaged plants. Beetle and aphid feeding had no effects on the photosynthetic rate of the leaves produced after insect removal. Plant relative growth rates (in terms of height) were reduced by spittlebugs during the period that the insects were feeding on the plants. Relative growth rates of spittlebug-damaged plants were increased above control levels after insect removal, but these plants were still shorter than controls 17 days after insect removal. Beetles and aphids did not affect plant relative growth rates or plant height. Feeding by both spittlebugs and beetles reduced leaf area, and the effect of the spittlebug was more severe than that of the beetle. These results show that effects of herbivory on photosynthetic rates cannot be predicted simply by considering changes in the source-sink ratio, and that spittlebug feeding is more damaging to the host plant than beetle or aphid feeding.     

Role of olfaction in host plant selection and local adaptation of a polyphagous herbivore,Eucolaspis Sharp

Host plant cues are known to shape insect–host plant association in many insect groups. More pronounced associations are generally manifested in specialist herbivores, but little is known in generalist herbivores. We used a polyphagous native beetle from New Zealand, bronze beetle, Eucolaspis sp. ‘Hawkes Bay’ (Chrysomelidae: Eumolpinae) to explore the role of olfaction in locating host plants and local adaptation. We also tested the role of other cues in the degree of acceptance or rejection of hosts. Adult Eucolaspis beetles were attracted to fresh leaf volatiles from apple and blackberry (Rosaceae). Male and female beetles responded similarly to olfactory cues of host plants. An indication of evolutionary affiliation was observed in olfactory preferences of geographically isolated conspecific populations. We found that geographically isolated populations of the beetles differ in their olfactory responses and exhibit some degree of local adaptation. However, irrespective of geographical and ecological associations, blackberry was preferred over apple as a feeding plant, and another novel plant, bush lawyer (Rubus australis), was readily accepted by 53.25% of the tested beetles. We show that plant volatiles play an important role in host location by Eucolaspis, but the acceptance or rejection of a particular host could also involve visual and contact cues.     

Consumptive and non‐consumptive effects of wolf spiders on cucumber beetles and cucumber plant damage

Biological control research often focuses on the ability of predators to reduce pest densities and protect crops through consumption. Less studied is their ability to protect crops by altering pest behaviour (non‐consumptive effects). Lab experiments were conducted to test predation rates of striped cucumber beetles (Acalymma vittatum; Coleoptera: Chrysomelidae) and spotted cucumber beetles (Diabrotica undecimpunctata howardi; Coleoptera: Chrysomelidae) by large (>10 mm) wolf spiders (Araneae: Lycosidae). Field experiments were conducted to examine how the physical presence and/or cues of spiders impact the behaviour and mortality of A. vittatum (specialist) and D. undecimpunctata (generalist) cucumber beetles as well as growth and damage of cucumber plants (Cucumis sativus; Cucurbitaceae). A. vittatum and D. undecimpunctata adults were added to caged cucumber plants without a spider, with spider cues only (spider removed before beetle inclusion), with spider only (spider introduced to plants immediately before beetle inclusion), and with spiders and their cues present (spiders introduced 24 hr in advance of beetle inclusion). A. vittatum responded to spider cues primarily by emigrating from plants. Contrarily, D. undecimpunctata did not display obvious responses, such as reduced feeding or increased emigration, to spider foraging and/or cues. Actively foraging lycosids increased A. vittatum mortality and reduced densities of D. undecimpunctata in the field when cucumber plants were flowering. This study highlights how non‐consumptive and consumptive effects can play a role in modifying pest populations, and how these effects can vary across species and plant growth stages.     

Low rates of iridoid glycoside hydrolysis in two Longitarsus leaf beetles with different feeding specialization confer tolerance to iridoid glycoside containing host plants

Iridoid glycosides are plant defence compounds that are deterrent and/or toxic for unadapted herbivores but are readily sequestered by dietary specialists of different insect orders. Hydrolysis of iridoid glycosides by β‐glucosidase leads to protein denaturation. Insect digestive β‐glucosidases thus have the potential to mediate plant–insect interactions. In the present study, mechanisms associated with iridoid glycoside tolerance are investigated in two closely‐related leaf beetle species (Coleoptera: Chrysomelidae) that feed on iridoid glycoside containing host plants. The polyphagous Longitarsus luridus Scopoli does not sequester iridoid glycosides, whereas the specialist Longitarsus tabidus Fabricius sequesters these compounds from its host plants. To study whether the biochemical properties of their β‐glucosidases correspond to the differences in feeding specialization, the number of β‐glucosidase isoforms and their kinetic properties are compared between the two beetle species. To examine the impact of iridoid glycosides on the β‐glucosidase activity of the generalist, L. luridus beetles are kept on host plants with or without iridoid glycosides. Furthermore, β‐glucosidase activities of both species are examined using an artificial β‐glucosidase substrate and the iridoid glycoside aucubin present in their host plants. Both species have one or two β‐glucosidases with different substrate affinities. Interestingly, host plant use does not influence the specific β‐glucosidase activities of the generalist. Both species hydrolyse aucubin with a much lower affinity than the standard substrate. The neutral pH reduces the β‐glucosidase activity of the specialist beetles by approximately 60% relative to its pH optimum. These low rates of aucubin hydrolysis suggest that the ability to sequester iridoid glycosides has evolved as a key to potentially preventing iridoid glycoside hydrolysis by plant‐derived β‐glucosidases.     

Temporal dynamics of herbivore‐induced responses in Brassica juncea and their effect on generalist and specialist herbivores

Herbivore feeding may induce an array of responses in plants, and each response may have its own temporal dynamics. Precise timing of these plant responses is vital for them to have optimal effect on the herbivores feeding on the plant. This study measured the temporal dynamics of various systemically induced responses occurring in Brassica juncea (L.) Czern. (Brassicaceae) leaves after insect herbivory in India and The Netherlands. Morphological (trichomes, leaf size) and chemical (glucosinolates, amino acids, sugars) responses were analysed. The effects of systemic responses were assessed using a specialist [Plutella xylostella L. (Lepidoptera: Plutellidae)] and a generalist [Spodoptera litura Fabricius (Lepidoptera: Noctuidae)] herbivore. We tested the hypotheses that morphological responses were slower than chemical responses and that generalist herbivores would be more affected by induced responses than specialists. Glucosinolates and trichomes were found to increase systemically as quickly as 4 and 7 days after herbivore damage, respectively. Amino acids, sugars, and leaf size remained unaffected during this period. The generalist S. litura showed a significant feeding preference for undamaged leaves, whereas the specialist herbivore P. xylostella preferred leaves that were damaged 9 days before. Performance bioassays on generalist S. litura revealed that larvae gained half the weight on leaves from damaged plants as compared to larvae feeding on leaves from undamaged plants. These studies show that although morphological responses are somewhat slower than chemical responses, they also contribute to induced plant resistance in a relatively short time span. We argue that before considering induced responses as resistance factors, their effect should be assessed at various points in time with both generalist and specialist herbivores.     

Differences in morphology and reproductive traits of Galerucella nymphaeae from four host plant species

The water lily beetle Galerucella nymphaeae L. (Coleoptera: Chrysomelidae) exploits different hosts, including Nuphar lutea Sm. and Nymphaea alba L. (both Nymphaeaceae), as well as Polygonum amphibium L. and Rumex hydrolapathum Hudson (both Polygonaceae). The present study investigates whether within-species differences in morphological and reproductive traits are associated with differences in host species exploitation. A total of 1103 adult beetles were collected from 11 localities in The Netherlands, one of which contained all four hosts and three other localities contained hosts from both families (sympatric localities). Adults originating from Nuphar and Nymphaea were on average darker in colour and larger in size and had disproportionally bigger mandibles than beetles originating from Polygonum and Rumex across the 11 localities. Head capsules of first instar larvae from Nymphaeaceae hosts were between 17% and 28% larger than those of larvae from Polygonaceae hosts. Furthermore, beetles from Nuphar and Nymphaea laid larger sized eggs, but fewer eggs per clutch than beetles originating from Polygonum and Rumex. Although host related variation was less pronounced at the sympatric localities than in the allopatric localities, differences in larval and adult size were still highly significant at the sympatric localities. It is not clear whether the observed differences are genetically based, as opposed to host induced. However, leaf toughness varied among species in a way suggesting that leaf toughness may be partly responsible for host related differences in G. nymphaeae.     

Effects of metaldehyde and methiocarb on feeding preferences and survival of a slug predator (Pterostichus melanarius (F.): Carabidae,Pterostichini)

The potential for reducing slug populations in crops through predation by generalist carabid beetles is well documented. However, few studies have considered interactions between biological and chemical control agents of slugs. Laboratory assays supported previous findings that the consumption of metaldehyde by slugs (Deroceras reticulatum) leads to increased duration of feeding bouts by carabid beetles (such as Pterostichini) on sub‐lethally affected individuals. However, a similar effect was not found for Pterostichus melanarius exposed to slugs fed on the other widely applied pelleted molluscicide formulation (methiocarb). Examination of beetle survival after consumption of slugs containing molluscicides demonstrated the strong biocidal properties of methiocarb, whereas metaldehyde consumption (ingested through slug predation) did not differ from control slugs killed by freezing. Beetle avoidance of slugs containing a more toxic molluscicide compound and the interaction between slug mucus production and beetle attack rates are discussed.     

The effects of wilt symptom development and peroxidase induction on interactions between vascular wilt bacteria and cucumber beetles

Infection of cucumber (Cucumis sativus L.) with the bacterial pathogen Erwinia tracheiphila E. F. Smith causes vascular wilt disease in leaves, which may alter the suitability of the host plant for insects and other pathogens. In this study, densities of spotted (Diabrotica undecimpunctata howardi Barber) and striped (Acalymma vittata (Fab.) cucumber beetles (Coleoptera: Chrysomelidae) were higher on wilted leaves of mature and seedling field plants inoculated with E. tracheiphila. Bacterial infection or feeding by D. undecimpunctata howardii beetles increased total peroxidase enzyme activity in inoculated or infested leaves of greenhouse seedlings, but only beetle feeding induced higher activities in untreated systemic leaves on the same plants. Neither bacterial infection nor beetle infestation led to the development of systemic acquired resistance (SAR) to the fungal pathogen Colletotrichum orbiculare (Berk & Mont.) Arx. Susceptibility to this fungus was greater on E. tracheiphila-infected plants than on controls. The positive association between leaf wilt symptom development and beetle occurrence thus occurs in the presence of an oxidative but not anti-pathogenic response induced by both the insect and the pathogen.     

Host location in Oomyzus gallerucae (Hymenoptera: Eulophidae), an egg parasitoid of the elm leaf beetle Xanthogaleruca luteola (Coleoptera: Chrysomelidae)

Eggs of the elm leaf beetle Xanthogaleruca luteola are often heavily attacked by the chalcidoid wasp Oomyzus gallerucae. We studied the chemical signals mediating interactions between the egg parasitoid, its host, and the plant Ulmus campestris. Olfactometer bioassays with O. gallerucae showed that volatiles of the host-plant complex attract the parasitoid. In order to determine the source of attractive volatiles within this host-plant-complex, we tested separately the effect of odours of eggs, gravid elm leaf beetle females, faeces of the beetles and elm twigs (with undamaged leaves and leaves damaged either mechanically or by feeding of the beetles). Odours of faeces of the elm leaf beetle were attractive, whereas neither volatiles from eggs nor from gravid females acted as attractants. Volatiles from undamaged or damaged plants did not elicit a positive reaction in O. gallerucae, whereas volatiles from feeding-damaged plants onto which host eggs had been deposited were attractive. This latter result suggests that it is not feeding but deposition of host eggs onto elm leaves that induces the production of plant volatiles attractive to the egg parasitoid. Investigations of the search patterns of O. gallerucae within the habitat by laboratory bioassays revealed that the egg parasitoid encounters host eggs by chance. Contact kairomones from faeces were demonstrated to be important in microhabitat acceptance, while contact kairomones isolated from the host eggs are relevant for host recognition. Received: 12 February 1997 / Accepted: 29 April 1997     

Herbivory limits the yellow water lily in an overgrown lake and in flowing water

Aquatic macrophytes with floating leaves are often key ecological species that affect entire aquatic ecosystems. Here we describe an investigation of the importance of insect herbivory for population growth and leaf senescence in the yellow water lily (Nuphar lutea). In order to gain a general picture of the importance of herbivory under different conditions, we experimentally manipulated herbivory in a large lily population in natural still water and observed the natural development of 32 smaller populations in flowing water. Herbivory drastically increased leaf senescence, reducing leaf density. In the still water, over one summer, leaf density increased by a factor of 1.23 in the presence of water lily leaf beetles and 1.61 when herbivory was eliminated. In flowing water, population growth was restricted mainly by leaf crowdedness, which limited large dense populations. Herbivory by water lily leaf beetles also had a limiting effect on yellow water lily, again mainly in large dense populations. Small populations supported a lower density of beetles. Previous studies have not addressed population-level responses of vascular plants with floating leaves. Our results suggest that herbivory can result in greater light penetration into the water and reduce “enemy-free space” for aquatic species that find such space in water lily stands. We suggest that the water lily leaf beetle should be considered an “ecological engineer.”     

Comparing macrophyte herbivory by introduced Louisiana crayfish (Procambarus clarkii) (Crustacea: Cambaridae) and native Dytiscid beetles (Cybister tripunctatus) (Coleoptera: Dytiscidae), in Kenya

The omnivorous Louisiana crayfish, Procambarus clarkii, has caused significant changes to macrophyte communities worldwide and may have similar negative effects in Kenya if used as a biological control agent for snails harbouring human schistosomes. Here we examine how herbivory by the introduced Louisiana crayfish might change macrophyte abundance and species composition relative to herbivory by a prevalent native beetle Cybister tripunctatus. Our observations are thought to be the first on the consumption of live macrophytes by adult dytiscid beetles, which are commonly known as predaceous diving beetles. In a laboratory experiment, we compare herbivory rates and macrophyte species-selectivity between the beetle and the crayfish. Combining the herbivory rates with population estimates of crayfish and beetles in a local pond, we estimate that herbivory in the natural environment is of the same order of magnitude. Preferences among macrophyte species are also similar between herbivores, except that crayfish are likely to have a larger impact than beetles on cultivated rice (Oryza sativa). Results from a laboratory toxicity experiment suggest that the crayfish is not currently a rice pest in Kenya because it is killed by Furadan^® 5G, the commonly applied pesticide.