Is palatability of a root-hemiparasitic plant influenced by its host species?

Palatability of parasitic plants may be influenced by their host species, because the parasites take up nutrients and secondary compounds from the hosts. If parasitic plants acquired the full spectrum of secondary compounds from their host, one would expect a correlation between host and parasite palatability. We examined the palatability of leaves of the root-hemiparasite Melampyrum arvense grown with different host plants and the palatability of these host plants for two generalist herbivores, the caterpillar of Spodoptera littoralis and the slug Arion lusitanicus. We used 19 species of host plants from 11 families that are known to contain a wide spectrum of anti-herbivore compounds. Growth of M. arvense was strongly influenced by the host species. The palatability of the individual host species for the two herbivores differed strongly. Both A. lusitanicus and S. littoralis discriminated also between hemiparasites grown with different host plants. There was no correlation between the palatability of a host species and that of the parasites grown on that host, i.e., hemiparasites grown on palatable host species were not more palatable than those grown on unpalatable hosts. We suggest an interacting pattern of specific effects of chemical anti-herbivore defences and indirect effects of the hosts on herbivores through effects on growth and tissue quality of the parasites.     

Grasshoppers (Orthoptera: Acrididae) select vegetation patches in local‐scale responses to foliar nitrogen but not phosphorus in native grassland

Abstract Key elements such as nitrogen (N) and phosphorus (P) are often limiting relative to the nutritional needs of herbivores that feed on them. While N often limits insect herbivores in natural terrestrial ecosystems, the effect of P is poorly studied in the field, even though compelling hypotheses from the ecological stoichiometry literature predict its importance. We evaluated small‐scale spatial distributions of, and herbivory by, grasshoppers among neighboring plots that vary in foliar‐N and ‐P in tallgrass prairie. Grasshopper densities were 67% greater in N‐fertilized plots but detected no effect to grasshopper densities from P‐fertilizer. Leaf damage to the dominant grass Andropogon gerardii was 32% greater in N‐fertilized plots, but no response to foliar‐P was detected. Herbivore damage to a common forb, goldenrod (Solidago missouriensis), was not strongly linked by fertilizer treatments, although there was increased leaf damage in N‐fertilizer treatments when no P was applied (a significant N × P interaction). Under field conditions at local scales, we conclude that spatially heterogeneous distributions of grasshoppers are primarily affected by foliar‐N in host plants with little evidence that P‐levels contribute to the spatial patterns.     

The effect of nitrogen fertilizer applied to Euphorbia pulcherrima on the parasitization of Bemisia argentifolii by the parasitoid Encarsia formosa

More wasps of Encarsia formosa Gahan (Hymenoptera: Aphelinidae) were found on fertilized poinsettias, Euphorbia pulcherrima (Willd.) (Euphorbiaceae), than on non-fertilized plants. Parasitization of Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) by E. formosa was higher on plants treated with calcium nitrate than with ammonium nitrate or on control plants. In a no-choice test, host feeding by E. formosa was higher when hosts were on fertilized plants than when hosts were on control plants. The nitrogen content of whitefly pupae reared on plants treated with ammonium nitrate was higher than those on calcium nitrate-treated plants.Variability in the parasitization of B. argentifolii by E. formosa appears to be due to host plant-mediated differences in the whiteflies. E. formosa may be influenced by the nutritional suitability of the host, which influences whether wasps continue to oviposit, feed, or disperse.     

Influence of feeding guild on insect response to host plant fertilization

Abstract.

• 1 The densities of insect herbivores in fertilized and unfertilized field plots of goldenrods, Solidago altissima (Compositae), were monitored over a period of 4 years.
• 2 A total of seventeen insect taxa occurred on the plots over the course of the study, including sap feeders, leaf chewers, leaf miners, leaf gallers and stem gallers with multiple representatives in each of these feeding guilds.
• 3 Nine of the seventeen taxa significantly increased in density on fertilized plots in at least one year of the study, two taxa showed marginally significant increases on fertilized plots, two significantly decreased in density on fertilized plots in at least one year, and the remaining taxa were unaffected by the fertilizer treatment.
• 4 The effects of fertilization on the insects were not strongly related to feeding guild; the group of insects that increased on fertilized plots was functionally diverse, and for the most part members of the same guild did not respond to the fertilizer treatment in consistent ways.
• 5 Differences between fertilized and unfertilized plots were greatest in the fourth year. The insects that showed delayed responses to fertilizer treatment may have been affected by changes in microclimate that developed slowly over the course of the study, suggesting that long-term studies may be necessary to detect effects of host plant stress on insect herbivores.

     

Quality or quantity: the direct and indirect effects of host plants on herbivores and their natural enemies

Resource quality (plant nitrogen) and resource quantity (plant density) have often been argued to be among the most important factors influencing herbivore densities. A difficulty inherent in the studies that manipulate resource quality, by changing nutrient levels, is that resource quantity can be influenced simultaneously, i.e. fertilized plants grow more. In this study we disentangled the potentially confounding effects of plant quality and quantity on herbivore trophic dynamics by separately manipulating nutrients and plant density, while simultaneously reducing pressure from natural enemies (parasitoids) in a fully factorial design. Plant quality of the sea oxeye daisy, Borrichia frutescens, a common coastal species in Florida, was manipulated by adding nitrogen fertilizer to increase and sugar to decrease available nitrogen. Plant density was manipulated by pulling by hand 25 or 50% of Borrichia stems on each plot. Because our main focal herbivore was a gall making fly, Asphondylia borrichiae, which attacks only the apical meristems of plants, manipulating plant nitrogen levels was a convenient and reliable way to change plant quality without impacting quantity because fertilizer and sugar altered plant nitrogen content but not plant density. Our other focal herbivore was a sap-sucker, Pissonotus quadripustulatus, which taps the main veins of leaves. Parasitism of both herbivores was reduced via yellow sticky traps that caught hymenopteran parasitoids. Plant quality significantly affected the per stem density of both herbivores, with fertilization increasing, and sugar decreasing the densities of the two species but stem density manipulations had no significant effects. Parasitoid removal significantly increased the densities of both herbivores. Top-down manipulations resulted in a trophic cascade, as the density of Borrichia stems decreased significantly on parasitoid removal plots. This is because reduced parasitism increases gall density and galls can kill plant stems. In this system, plant quality and natural enemies impact per stem herbivore population densities but plant density does not.     

Effects of nonhost and host plants on insect herbivory covarying with plant size in the cruciferous plant <Emphasis Type="Italic">Turritis glabra</Emphasis>

Correlation between plant size and reproductive output may be modified by herbivory in accordance with host plant density and the presence of nonhost plants. To elucidate the effects of nonhost plant density and host plant density on the intensity of herbivory and reproductive output of the host plant in relation to plant size under natural conditions, we investigated the abundance of three lepidopteran insects, Plutella maculipennis, Anthocharis scolymus, and Pieris rapae the intensity of herbivory, and fruit set of their host plant, Turritis glabra (Cruciferae). To elucidate the effects of nonhost and host plant density, we selected four categories of plots under natural conditions: low density of nonhost and high density of host plants; low density of both nonhost and host plants; high density of both nonhost and host plants; and high density of nonhost and low density of host plants. The plant size indicated by stem diameter was a good predictor of the abundance of all herbivorous species. The effects of density of nonhost and host plants on the abundance of insects varied among species and stages of insects. As the abundance of insects affected the intensity of herbivory, herbivory was more apparent on larger host plants in plots with low density of both nonhost and host plants. Consequently, the correlation between plant size and the number of fruits disappeared in low plots with density of both nonhost and host plants. In this T. glabra– herbivorous insect system, the density of nonhost plants and host plants plays an important role in modifying the relationship between plants and herbivores under natural conditions. Received: July 19, 1999 / Accepted: June 15, 2000     

Foraging behavior of specialist and generalist caterpillars on plantain (Plantago lanceolata) altered by predatory stinkbugs

Summary To examine the effects of predators and plant genotype on the behavior, patterns of herbivory, growth and survivorship of caterpillars, we used an experimental garden in which we contrasted two hostplant genotypes of plantain (Plantago lanceolata), two kinds of herbivores (specialist Junonia coenia vs. generalist Pyrrharctia isabella) and two levels of caterpillar predation (with and without Podisus maculiventris stinkbugs). Each of the replicate plots per treatment contained two plants of the same genotype. The stinkbugs reduced the survivorship of the specialist caterpillars but not that of the generalists, which reflects the differences in predatoravoidance behaviors of these species. Nonetheless, the stinkbugs influenced the behavior of both caterpillar species. When stinkbugs were present, both specialist and generalist caterpillars were less likely to be found on the plant upon which they were initially placed (=initial plant), and they were more likely to be off both plants within the plot than larvae in the absence of predators. Consequently in the presence of the stinkbug predators, the proportion of the initial plants consumed was less than in the absence of the predators. Plant genotype influenced plant size and the proportion of individual plants eaten, but it did not affect larval location on the plots. Neither presence of predators nor plant genotype had an effect on relative growth rate of the caterpillars.     

Variation in the natural abundance of 15N in the halophyte,Salicornia virginica,associated with groundwater subsidies of nitrogen in a southern California salt-marsh

To provide insight into the importance of the salt-marsh ecotone as a sink for inorganic nitrogen in perched groundwater, measurements were made of the natural abundance of ¹⁵N in dissolved NO₃-N and NH₄-N and in the salt-marsh halophyte, Salicornia virginica, along an environmental gradient from agricultural land into a salt-marsh. The increase in the natural abundance of ¹⁵N (expressed by convention as ¹⁵N) of NO₃-N, accompanied by the decrease in NO₃-N (and total dissolved inorganic N, DIN) concentration along the gradient, suggested that the salt-marsh ecotone is a site of transformation, most likely through denitrification, of inorganic nitrogen in groundwater. ¹⁵N enrichment in S. virginica (and the parasitic herb, Cuscuta salina), along the tidal marsh boundary, relative to high and middle marsh locations, indicated the retention of groundwater nitrogen as vegetative biomass. The correlation between ¹⁵N _Salicornia and ¹⁵N_NH4 suggested a preference for NH₄-N over NO₃-N during uptake by this plant. Groundwater inputs enhanced the standing crop, above-ground productivity, and nitrogen content of S. virginica but the ralative effects of pore water salinity and DIN concentration on these parameters were not determined. ¹⁵N enrichment of marsh plants by groundwater DIN inputs could prove useful in tracing the fate of these inputs in the marsh food web.     

Within species variation in host plant quality and movement behavior of Lygus rugulipennis nymphs

We studied movement behavior of the polyphagous herbivore Lygus rugulipennis Poppius (Heteroptera: Miridae) on wheat (Triticum aestivum) that had been cultivated in pots using three different levels of nitrogen fertilization. The probability of moving for fourth instar nymphs, and the time spent moving by mobile fourth and fifth instar nymphs increased with the nitrogen fertilization level of the pot. The nitrogen fertilization level of the pot did not appear to influence the probability of moving for fifth instar nymphs. The difference in movement behavior of nymphs on the pots of wheat fertilized with the lowest and the highest nitrogen levels seemed to be as great as the difference in movement behavior reported earlier on two different host plant species, viz., wheat and Tripleurospermum inodorum Schultz. The magnitude of the difference in movement behavior on the extreme nitrogen levels illustrates the importance of recognizing within species variation in plant quality when relating movement patterns of insect herbivores with host plant species.     

Consequences of nitrogen and phosphorus limitation for the performance of two planthoppers with divergent life-history strategies

Phytophagous insects have a much higher nitrogen and phosphorus content than their host plants, an elemental mismatch that places inherent constraints on meeting nutritional requirements. Although nitrogen limitation is well documented in insect herbivores, phosphorus limitation is poorly studied. Using factorial experiments in the laboratory and field, in which levels of soil nitrogen and phosphorus were manipulated, we studied the relative consequences of macronutrient limitation for two herbivores, namely the phloem-feeding planthoppers Prokelisia dolus and P. marginata. These planthoppers inhabit the salt marshes of North America where large stands of their Spartina host plant are found. Notably, these congeners differ in their dispersal abilities; P. marginata is dispersive whereas P. dolus is sedentary. Both nitrogen and phosphorus subsidies enhanced the nitrogen and phosphorus content of Spartina. When P. dolus and P. marginata were raised on plants with an enriched nitrogen signature, they exhibited greater survival, grew to a larger size, developed more rapidly, and achieved higher densities than on nitrogen-deficient plants. However, P. marginata experienced greater fitness penalties than P. dolus on nitrogen-deficient plants. Phosphorus limitation and associated fitness penalties were not as severe as nitrogen limitation for P. marginata, and were not detected in P. dolus. The tempered response of P. dolus to N- and P-deficient Spartina is probably due to its greater investment in feeding musculature and hence ability to compensate for nutrient deficiencies with increased ingestion. To cope with deteriorating plant quality, P. dolus employs compensatory feeding, whereas P. marginata disperses to higher quality Spartina. When its option of dispersal is eliminated and P. marginata is confined on nutrient-deficient plants, its performance is drastically reduced compared with P. dolus. This research highlights the importance of interfacing herbivore life-history strategies with ecological stoichiometry in order to interpret the consequences of macronutrient limitation on herbivore performance and population dynamics.     

Life history plasticity and fitness in a caddisfly in response to proximate cues of pond-drying

The majority of plants are involved in symbioses with arbuscular mycorrhizal fungi (AMF), and these associations are known to have a strong influence on the performance of both plants and insect herbivores. Little is known about the impact of AMF on complex trophic chains, although such effects are conceivable. In a greenhouse study we examined the effects of two AMF species, Glomus intraradices and G. mosseae on trophic interactions between the grass Phleum pratense, the aphid Rhopalosiphum padi, and the parasitic wasp Aphidius rhopalosiphi. Inoculation with AMF in our study system generally enhanced plant biomass (+5.2%) and decreased aphid population growth (−47%), but there were no fungal species-specific effects. When plants were infested with G. intraradices, the rate of parasitism in aphids increased by 140% relative to the G. mosseae and control treatment. When plants were associated with AMF, the developmental time of the parasitoids decreased by 4.3% and weight at eclosion increased by 23.8%. There were no clear effects of AMF on the concentration of nitrogen and phosphorus in plant foliage. Our study demonstrates that the effects of AMF go beyond a simple amelioration of the plants’ nutritional status and involve rather more complex species-specific cascading effects of AMF in the food chain that have a strong impact not only on the performance of plants but also on higher trophic levels, such as herbivores and parasitoids.     

Is host-plant quality responsible for the populational pulses of salt-marsh planthoppers (Homoptera: Delphacidae) in northwestern Florida?

Abstract.

• 1 Rare populational pulses of the sibling species of plant-hoppers Prokelisia dolus Wilson and P.marginata (Van Duzee) occur along the Gulf Coast of Florida. Densities at least three standard deviations ahove the average number of adult planthoppers per stem host plant are exhibited during these pulses.
• 2 This context of rare local irruptions provides a basis for testing a component of T. C. R. White's hypothesis, which states that the trigger for insect outbreaks is an increase in the nutritional quality, in terms of available nitrogen, of host-plant tissue.
• 3 In the field, over approximately five generations of these hoppers (9 months), we elevated foliar nitrogen in host plants substantially by fertilizing, six times, salt-marsh plots of 10 m² with ammonium nitrate. No densities approaching a populational pulse occurred in either fertilized or unfertilized plots during the course of this study or in the year following. Fertilization did increase adult densities in the spring but this increase deteriorated over the summer months and was lost in the autumn. Host plants in fertilized plots retained high levels of nitrogen for at least the second year.
• 4 Several very dense pulses, of more than thirty adult hoppers per stem, occurred at other sites during the course of this study. Host-plant nitrogen in a pulse site did not differ from that found in host plants immediately adjacent to the pulse. We conclude that increased foliar nitrogen is not the proximate cause of the populational pulse behaviour exhibited by these planthoppers.
• 5 We speculate that these pulses result from behavioural aggregation of adult hoppers.

     

Effects of different leaf traits on growth rates of insect herbivores on willows

We examined relative effects of traits of leaf quality of ten willow species (Salix: Salicaceae) on growth rates of five species of insect herbivores found in interior Alaska (a willow sawfly, Nematus calais; the tiger swallowtail butterfly, Papilio canadensis; and three species of chrysomelid beetles, Gonioctena occidentalis, Calligrapha verrucosa, and Chrysomela falsa). Leaf traits examined were water content, toughness, total nitrogen contnet, pubescence, and presence or absence of phenolic glycosides. Of ten Salix species, four species contain phenolic glycosides in their leaves. We examined relative effects of water content, toughness, and nitrogen content of the Salix leaves on larval growth rates at three different levels, i.e., on a single host species, between different host species, and between herbivore species. The within-host analyses showed that effects of water content, toughness and/or nitrogen content on herbivore growth rates were generally significant in early-season herbivores but not in late-season herbivores. For each herbivore species, differences in growth rates between hosts were not explained by differences in water content, toughness, or nitrogen content. The between-herbivore analysis showed that the interspecific difference in larval growth rates were related to difference in water and nitrogen content of the hosts. Pubescence of Salix leaves had little effects on herbivore growth rates. Presence of phenolic glycosides had a positive effects on growth rates of a specialist, N. calais, but no effect on the other specialist, Ch. falsa. Presence of phenolic glycosides had, in general, negative effects on growth rates of nonspecialists, G. occidentalis, C. verrucosa, and P. canadensis.     

Diet mixing in a parasitic plant: adaptation or constraint?

Some herbivores deliberately consume a mixed diet, either to obtain a superior mix of nutrients or to avoid consuming too much of any one toxin. Few studies have examined diet mixing in parasitic plants, which typically have very broad host ranges. We offered the parasitic plant Cuscuta indecora (dodder), a range of mixtures of two hosts (Iva frutescens and Borrichia frutescens) in the greenhouse, and observed correlations between the host community and Cuscuta infection in the field. In the greenhouse, Cuscuta performed better on mixtures with a higher relative abundance of Iva. Cuscuta selectively foraged on whichever host was more abundant (diet switching), the exact opposite of the behavior that would be expected if diet mixing was advantageous. In the field, the intensity of Cuscuta infections was decreased by the presence of non-hosts (grasses), not strongly affected by the presence of intermediate hosts, and increased by the presence of Borrichia. We conclude that Cuscuta does not obtain nutritional benefits from a broad diet, but instead is constrained by its relative lack of mobility to attack hosts of intermediate value. In general, the lack of mobility of parasitic plants compared to herbivores probably selects for broad host ranges in parasitic plants.     

Effect of experimental phosphorus enrichment on oligotrophic tropical marshes in Belize,Central America

Sedges from genus Eleocharisdominate extensive wetlands in the sugar cane growing areas of the Caribbean. Correlative data suggest that macrophytes in these wetlands are phosphorus limited. To determine effects of increased P input that can be expected, e.g. from agricultural runoff, a common sugar cane fertilizer was applied to representative plots in four marl-based and four peat-based marshes. The plots were located in the proximity of patches of Typha domingensis, which has been reported to be able to outcompete Eleocharis under nutrient rich conditions. Responses to the fertilizer treatment were documented as changes in: Plant height, density, biomass, net primary production, nutrient resorption, decomposition, plant and soil nutrient concentrations, percent cover of cyanobacterial mats, and potential colonization by Typha. Additions of phosphorus significantly increased plant density and height and, consequently, the aboveground net primary production. Phosphorus resorption efficiency following senescence was independent of fertilizer addition in Eleocharis but decreased in Typha from the fertilized plots. Phosphorus resorption proficiency was lower in fertilized plots for both Typha and Eleocharis. Decomposition of litter and cellulose assays was significantly faster in fertilized plots. No spontaneous establishment of Typha occurred in the fertilized plots, but survival of transplanted Typha was higher in fertilized plots than in controls. Increased plant density in fertilized plots led to elimination of a key component of these ecosystems, the nitrogen fixing cyanobacterial mats.     

Direct and ecological costs of resistance and tolerance in the stinging nettle

Plant resistance and tolerance to herbivores, parasites, pathogens, and abiotic factors may involve two types of costs. First, resistance and tolerance may be costly in terms of plant fitness. Second, resistance and tolerance to multiple enemies may involve ecological trade-offs. Our study species, the stinging nettle ( Urtica dioica L.) has significant variation among seed families in resistance and tolerance as well as costs of resistance and tolerance to the holoparasitic plant Cuscuta europaea L. Here we report on variation among seed families (i.e. genetic) in tolerance to nutrient limitation and in resistance to both mammalian herbivores (i.e. number of stinging trichomes) and an invertebrate herbivore (i.e. inverse of the performance of a generalist snail, Arianta arbustorum). Our results indicate direct fitness costs of snail resistance in terms of host reproduction whereas we did not detect fitness costs of mammalian resistance or tolerance to nutrient limitation. We further tested for ecological trade-offs among tolerance or resistance to the parasitic plant, herbivore resistance, and tolerance to nutrient limitation in the stinging nettle. Tolerance of nettles to nutrient limitation and resistance to mammalian herbivores tended to correlate negatively. However, there were no significant correlations among resistance and tolerance to the different natural enemies (i.e. parasitic plants, snails, and mammals). The results of this greenhouse study thus suggest that resistance and tolerance of nettles to diverse enemies are free to evolve independently of each other but not completely without direct costs in terms of plant fitness.     

Ant species confer different partner benefits on two neotropical myrmecophytes

The dynamics of mutualistic interactions involving more than a single pair of species depend on the relative costs and benefits of interaction among alternative partners. The neotropical myrmecophytes Cordia nodosa and Duroia hirsuta associate with several species of obligately symbiotic ants. I compared the ant partners of Cordia and Duroia with respect to two benefits known to be important in ant-myrmecophyte interactions: protection against herbivores provided by ants, and protection against encroaching vegetation provided by ants. Azteca spp., Myrmelachista schumanni, and Allomerus octoarticulatus demerarae ants all provide the leaves of Cordia and Duroia some protection against herbivores. However, Azteca and Allomerus provide more protection than does Myrmelachista to the leaves of their host plants. Although Allomerus protects the leaves of its hosts, plants occupied by Allomerus suffer more attacks by herbivores to their stems than do plants occupied by other ants. Relative to Azteca or Allomerus, Myrmelachista ants provide better protection against encroaching vegetation, increasing canopy openness over their host plants. These differences in benefits among the ant partners of Cordia and Duroia are reflected in the effect of each ant species on host plant size, growth rate, and reproduction. The results of this study show how mutualistic ant partners can differ with respect to both the magnitude and type of benefits they provide to the same species of myrmecophytic host.     

The nocturnal larvae of a specialist folivore perform better on Chromolaena odorata leaves from a shaded environment

Increasing evidence suggests that individuals of the same plant species occurring in different microhabitats often show a degree of phenotypic and phytochemical variation. Consequently, insect herbivores associated with such plant species must deal with environment‐mediated changes or variability in the traits of their host plants. In this study, we examined the effects of habitat condition (shaded vs. full‐sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) King & Robinson (Asteraceae). In addition, the performance was evaluated in two generations of a specialist folivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae), on leaves obtained from both shaded and full‐sun habitats. The study was done in an area where the insect was introduced as a biological control agent. Leaves growing in shade were less tough, had higher water and nitrogen content, and lower total non‐structural carbohydrate, compared with leaves growing in full sun. Plants growing in shade had longer leaves and were taller, but above‐ground biomass was significantly reduced compared with plants growing in full sun. In both generations (parents and offspring), P. insulata developed faster and had larger pupal mass, increased growth rate, and higher fecundity when reared on shaded foliage compared with full‐sun foliage. Although immature survival and adult longevity did not differ between habitats, Maw's host suitability index indicated that shaded leaves were more suitable for the growth and reproduction of P. insulata. We suggest that the benefits obtained by P. insulata feeding on shaded foliage are associated with reduced toughness and enhanced nitrogen and water content of leaves. These results demonstrate that light‐mediated changes in plant traits and leaf characteristics can affect insect folivore performance.     

Effects of environmental context on the susceptibility of Atriplex patula to attack by herbivorous beetles

The susceptibility of plants to attack by insect herbivores often depends on local environmental conditions. This study documents variation in herbivore damage by the chrysomelid beetle Erynephala maritima to the annual forb Atriplex patula in two microhabitats within New England salt marshes: bare patches and dense matrix vegetation. Environmental conditions within bare patches differ from those within matrix vegetation in a number of ways. Bare patches are characterized by the absence of perennial grasses and rushes (matrix vegetation) and greater levels of physical stress, and are rapidly colonized by the fugitive annual, Salicornia europaea, a second host plant of these beetles. Surveys of herbivore damage across three marshes revealed that A. patula in bare patches had a greater proportion of leaves damaged by beetles than those within matrix vegetation. Presence or absence of matrix vegetation and presence or absence of S. europaea were experimentally manipulated to determine the proximate cause of this pattern. The presence of S. europaea significantly increased the susceptibility of A. patula to herbivory in experimental plots. Both the extent of herbivore damage to plants and the proportion of plants damaged through time were greater in treatments with S. europaea than in controls, regardless of the presence or absence of matrix vegetation. Plants in S. europaea addition treatments were also less likely to survive to reproduction. Decreased survival appears to result from increased herbivory, suggesting that the negative effect of S. europaea on A. patula is mediated indirectly through shared insect herbivores. These results support the hypothesis that indirect interactions between alternative host plants, mediated by insect herbivores, can be important in natural communities. Received: 9 January 1999 / Accepted: 29 April 1999     

Integrated nitrogen,carbon, and water relations of a xylem-tapping mistletoe following nitrogen fertilization of the host

Xylem-tapping mistletoes transpire large volumes of water (E) while conducting photosynthesis (A) at low rates, thus maintaining low instantaneous wateruse efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitrogen dissolved at low concentration in host xylem water; however, low A/E also results in substantial heterotrophic carbon gain. In this study, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to determine whether mistletoe A/E would approach that of the host if mistletoes were supplied with abundant nitrogen. Fertilization significantly increased foliar N concentrations (N), net assimilation rates, and A/E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the other hand, when instantaneous water-use efficiency and A/N were calculated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budgets was constructed to analyze the relative benefits of nitrogen- and carbonparasitism. The model assumes constant E and includes feedbacks of tissue nitrogen concentration on photosyn-thesis. These results, combined with our earlier observation that net assimilation rates of mistletoes and their hosts are approximately matched (Marshall et al. 1994), support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain. However, the low ratio of A/E is interpreted not as a means of acquiring nitrogen, but as an inevitable consequence of an imbalance in C and N assimilation.This research was supported by the National Science Foundation (grants BSR-8706772 and 8847942).