Sap-feeding Herbivores Research Articles

Plant volatiles: Herbivores shush the chatty tree

Plant-insect interactions can be complex and elusive. A new study shows that sap-feeding herbivores reduce tree emissions of specific volatile organic compounds that attract natural enemies. Sap-feeding insects thereby provide enemy-free space for chewing herbivores living on the same tree.

Ecological convergence of secondary phytochemicals along elevational gradients.

Biologists still strive to identify the ecological and evolutionary drivers of phytochemical variation that mediate biotic interactions. We hypothesized that plant species growing at sites characterized by high herbivore pressure would converge to produce highly toxic blends of secondary metabolites, independent of phylogenetic constraints. To address the role of shared evolutionary history and ecological niches in driving variation in plant phytochemistry, we combined targeted metabolomics with insect herbivore bioassays and with a set of growth-related traits of several Cardamine species growing along the entire elevational gradient of the Alps. We observed that Cardamine phytochemical profiles grouped according to previously established growth form categorizations within specific abiotic conditions, independently of phylogenetic relationship. We also showed that novel indices summarizing functional phytochemical diversity better explain plant resistance against chewing and sap-feeding herbivores than classic diversity indices. We conclude that multiple functional axes of phytochemical diversity should be integrated with the functional axis of plant growth forms to study phenotypic convergence along large-scale ecological gradients.

Keystone mutualism strengthens top-down effects by recruiting large-bodied ants.

Determining the impacts of mutualistic interactions and predator diversity on food webs are two important goals in community ecology. In this study, we examined how predator community variation mediates the strength of top-down effects in the presence and absence of mutualistic interactions. We examined the impacts of predatory ant species that simultaneously prey on leaf-chewing herbivores (Lepidoptera) and engage in food-for-protection mutualisms with sap-feeding herbivores (Hemiptera) in the lower canopy of Connecticut forests. In this 2-year study, we examined three hypothetical mechanisms by which mutualisms can alter the top-down effects of ants: (1) sap feeders increase ant abundance, thus strengthening predatory effects; (2) sap feeders increase the relative abundance of a species that has stronger predatory effects; and (3) changes to predator diversity (species richness) are caused by sap feeders mediating top-down effects of the ant community. Experiments revealed that host plants occupied by sap feeders favored large-bodied ant species in the genus Camponotus, but there were no changes to community-wide ant abundance or ant species richness. Fitting predictions of predation strength based on the functional trait of body size, large-bodied Camponotus suppressed caterpillars and reduced leaf herbivory. This work shows that the ant-hemipteran mutualism, which has been characterized as a keystone interaction, can generate strong top-down effects on leaf-chewing herbivores and herbivory via increasing the relative abundance of species with functional traits relevant to predation, such as body size. Therefore, the emergence of specific ants as keystone predators in a community can be contingent upon their mutualism with sap-feeding Hemiptera.

Diversity and specificity of sap-feeding herbivores and their parasitoids on Australian fig trees

The ecology, diversity, and parasitoid complex of plant–sap feeding insects of the family Homotomidae (Hemiptera: Psylloidea) specialised on fig trees (Ficus) have so far received little research attention. They are ecologically important, however, as occasional outbreaks of the homotomid Mycopsylla fici may cause complete defoliation of its host plant, the Moreton Bay fig (Ficus macrophylla). Mycopsylla proxima, the only other species reported from Australia, feeds on F. rubiginosa without any recorded outbreaks. We searched for homotomids and their parasitoids on eight Ficus species on the east coast of Australia, Lord Howe Island (LHI), and in Auckland, New Zealand, and detected them on three Ficus species. Using mitochondrial and nuclear DNA sequences, we delimited three Mycopsylla species, including a putative new species on F. watkinsiana. We also characterised six (including one previously described) parasitoid species of the genus Psyllaephagus (Hymenoptera: Encyrtidae) based on congruent morphological characters and molecular data. Each of the homotomid species was highly host specific to a single fig species, whereas parasitoid species varied in host specificity: three host specific to M. fici and three host generalists. Geographic distribution varied among parasitoid species; e.g. one host-specific species was found on both the mainland and LHI, but a second species only on LHI. Our study revealed previously unrecognised diversity in fig homotomids and especially in their parasitoids. The herbivores and parasitoids showed contrasting patterns of host specificity. Interestingly, M. fici, the only outbreak species, had the highest diversity of associated parasitoid species and was the only species with host-specific parasitoids.

Multiple interaction types determine the impact of ant predation of caterpillars in a forest community.

Direct and indirect effects of predators are highly variable in complex communities, and understanding the sources of this variation is a research priority in community ecology. Recent evidence indicates that herbivore community structure is a primary determinant of predation strength and its cascading impacts on plants. In this study, we use variation in herbivore community structure among plant species to experimentally test two hypotheses in a temperate forest food web. First, variation in the strength of predator effects, such as ant predation of caterpillars, is predicted to be density dependent, exhibiting stronger effects when prey abundance is high (density-dependent predation hypothesis). Second, mutualistic interactions between ants and sap-feeding herbivores are expected to increase the abundance of predatory ants, strengthening predation effects on herbivores with cascading effects on host plants (keystone mutualism hypothesis). Using a large-scale predator exclusion experiment across eight dominant tree species, we tracked changes in insect density on 862 plants across two years, recording 2,322 ants, 1,062 sap-feeders, 5,322 caterpillars, and quantifying herbivory on 199, 338 leaves. In this experiment, density-dependent predation did not explain variation in the direct or indirect effects of ants on caterpillars and herbivory. In partial support of the keystone mutualism hypothesis, sap-feeders strengthened top-down effects of ants on caterpillars under some conditions. However, stronger ant predation of caterpillars did not lead to measurable trophic cascades on trees occupied by sap-feeders. Instead, the presence of sap-feeders was associated with increased per capita feeding damage by caterpillars, and this bottom-up effect attenuated the indirect effects of ants on host plants. These findings demonstrate that examining the multi-trophic impacts of mutualisms and predation in the context of the broader community can reveal patterns otherwise masked by compensatory interactions.

Herbivore specificity and the chemical basis of plant-plant communication in Baccharis salicifolia (Asteraceae).

It is well known that plant damage by leaf-chewing herbivores can induce resistance in neighbouring plants. It is unknown whether such communication occurs in response to sap-feeding herbivores, whether communication is specific to herbivore identity, and the chemical basis of communication, including specificity. We carried out glasshouse experiments using the California-native shrub Baccharis salicifolia and two ecologically distinct aphid species (one a dietary generalist and the other a specialist) to test for specificity of plant-plant communication and to document the underlying volatile organic compounds (VOCs). We show specificity of plant-plant communication to herbivore identity, as each aphid-damaged plant only induced resistance in neighbours against the same aphid species. The amount and composition of induced VOCs were markedly different between plants attacked by the two aphid species, providing a putative chemical mechanism for this specificity. Furthermore, a synthetic blend of the five major aphid-induced VOCs (ethanone, limonene, methyl salicylate, myrcene, ocimene) triggered resistance in receiving plants of comparable magnitude to aphid damage of neighbours, and the effects of the blend exceeded those of individual compounds. This study significantly advances our understanding of plant-plant communication by demonstrating the importance of sap-feeding herbivores and herbivore identity, as well as the chemical basis for such effects.

Scaling and the Effects of Plant, Soil, and Landscape Characteristics on Sap-Feeding Herbivores in Cotton

Soil physical and chemical properties can affect plant growth and nutrition, which in turn can affect a plant’s attractiveness and susceptibility to insect herbivores. A further source of variation in these relationships is the spatial scale at which patterns are measured. Both the size of the area being sampled, or scale, and the distance between measurements, or grain, are parameters that affect interpretation of insect abundance patterns. Our objectives in this study were to determine both the relationship of various landscape, plant, and soil characteristics to densities of sap-feeding insect herbivores in cotton and to determine the effects of sampling scale and sampling grain on these relationships. We included three sap-feeding herbivores in our study: Aphis gossypii Glover, Frankliniella occidentalis (Pergande), and Bemisia tabaci Gennadius. We found that abundance of each insect species was related to several single factors within the cotton field and that these relationships were always dependent on the scale and grain of the measurements. No one variable or set of variables was related to a particular insect density for each scale and grain examined. However, some variables were significantly correlated with insect densities at the larger scale (622 by 31 m), although none were significantly correlated for all plots at the smaller scale (154 by 31 m). In comparing the separate effects of each variable using partial correlations, elevation was negatively correlated with A. gossypii density at both grains (samples taken 25 m apart and samples taken 50 m apart), whereas in multiple regression analyses including all variables, plant moisture and soil nitrates were positively correlated and plant height and clay negatively correlated with A. gossypii density. In examining the separate effect of each variable on F. occidentalis density, plant moisture was negatively associated with F. occidentalis density at each grain. In multiple regression analyses, no variable was associated with F. occidentalis density at each grain. For B. tabaci, soil salinity was positively associated when variables were examined separately or in multiple regression. We discuss the possible reasons for why particular variables are related to the densities of each species.

Temporal fluctuations in throughfall carbon concentrations in a spruce forest

Fluxes of nutrients, energy and ions through forest canopies are spatially and temporally highly heterogeneous and good modelling tools are needed to separate different influential variables. Two important factors affecting the variation in the throughfall fluxes are the amount of precipitation and phytophagous insects in the canopy. In spite of their large abundance, wide distribution and production of large quantities of sugary excreta the role of aphids has not been explored in models of DOC-flux dynamics. We hypothesise that aphids qualify as a crucial factor to better explain the spatial and temporal variability in DOC-fluxes. Here, we build on the available knowledge of aphid population dynamics to explore the relative roles of aphids and precipitation as sources of variability in throughfall fluxes in a mature spruce forest. Peaks in DOC-concentrations usually appeared during June–August and precipitation volumes alone were insufficient to explain DOC-concentrations in throughfall. The developed dynamical model, however, showed a 34% increase in the explained variability in DOC throughfall dynamics if aphids were included. This is the first model, which takes into account the possible influence of sap feeding herbivores on DOC-flux dynamics.

Feeding-Induced Changes in Plant Quality Mediate Interspecific Competition between Sap-Feeding Herbivores

Feeding-Induced Changes in Plant Quality Mediate Interspecific Competition between Sap-Feeding Herbivores

Trophic Control across a Natural Productivity Gradient with Sap-Feeding Herbivores

Experiments examining the role that predators play in controlling herbivore damage to plant biomass are common in ecology. The interactive effect of productivity and predation on standing plant biomass in terrestrial ecosystems, however, remains poorly understood. Here we examine the role of predation and productivity on the damage sap-feeding herbivores inflict on goldenrod. Sap feeders are the dominant herbivores in this community. Sap is poor in nutrients and does not carry many of the chemical defense compounds of goldenrod. Sap-feeders should then be sensitive to changes in productivity and relatively impervious to the production of chemical defenses. We examined the effect of predation and fertilization on goldenrod across a productivity and successional gradient. We found that the effect of predators on controlling herbivore damage increased with increasing productivity. Some of these results are consistent with the Exploitation hypothesis proposed by Oksanen while others seem to reflect alternative mechanisms of trophic control across productivity gradients. Top-down control was strong in high productivity sites while plant competitive interactions may have obscured any pattern of top-down control in intermediate and low productivity sites.

Within-plant variation in concentrations of amino acids, sugar, and sinigrin in phloem sap of black mustard,Brassica nigra (L.) Koch (Cruciferae)

Although within-plant variation in the nutrient and allelochemical composition of phloem sap has been invoked to explain patterns of host use by phloem-feeding insects, little is known about within-plant variation in phloem chemistry. Here I describe a new technique in which I use the green peach aphid,Myzus persicae Sulz., to investigate within-plant variation in the concentrations of chemicals in the phloem sap of black mustard,Brassica nigra (L.) Koch (Cruciferae). Relationships between the concentrations of chemicals in aphid diets and honeydew were established using honeydew from aphids fed on artificial diets with known concentrations of amino acids, sucrose, and sinigrin. These relationships were applied to honeydew from aphids fed on different aged leaves of black mustard to estimate the concentrations of the chemicals in phloem sap. Sinigrin concentration was estimated to be high (>10 mM) in phloem sap in young leaves, calling into question the prevailing opinion that phloem sap contains only low concentrations of allelochemicals. High concentrations may function as defenses against sap-feeding herbivores. Within-plant variation in phloem sap composition was high: (1) young leaves had high concentrations of nutrients (216 mM amino acids, 26% sugar) and sinigrin (>10 mM); (2) mature and presenescent leaves had lower concentrations of nutrients (77-83 mM amino acids, 19-20% sugar) and low concentrations of sinigrin (1-2 mM); and (3) senescing leaves had high concentrations of nutrients (199 mM amino acids, 25% sugar) and low concentrations of sinigrin (3 mM).

Productivity of Creosotebush Foliage and Associated Canopy Arthropods Along a Desert Roadside

-A sampling study was designed to test the hypothesis that creosotebush (Larrea tridentata) shrubs along a roadside were larger, more vigorous and supported greater populations of foliage arthropods than shrubs growing 20 m away from the road margin. Structural and chemical attributes of shrubs and associated foliage arthropods were measured. Roadside shrubs were larger, had denser foliage, more flowers, higher foliar nitrogen contents and lower foliar resin contents than shrubs growing away from the road. Foliage arthropod densities were significantly higher on roadside shrubs. Sap-feeding herbivores dominated numerically and accounted for most of the differences in arthropod abundances between roadside and nonroadside shrubs. Numbers of foliage arthropods were positively correlated with shrub size, density and foliar nitrogen contents, and negatively correlated with foliar resin contents. These findings, in concordance with other studies, indicate that in arid environments productive, vigorous plants are preferred hosts for herbivorous insects.

Trophic diversity, niche breadth and generation times of exopterygote insects in a secondary succession.

Various ecological characteristics have been determined for the exopterygote insect fauna of three sites in a secondary succession, with successional ages zero-two years (ruderal and early successional - Young Field), seven to eight years (mid successional - Old Field) and around 60 years (late successional - Woodland). The taxonomic diversity of the group as a whole (Table 1) and the trophic diversity (major types of feeding habits) (Table 2) increased with successional age. A comparison of the numbers of individuals and species of insect with the abundance and species of plant of each of the main life forms (annuals, biennials, perennials, trees, grasses (and rushes), ferns and lower plants) showed that at the start of secondary succession there was a good correlation between the number of species of insect and number of species of plant. In the second year and later the correlation is between plant abundance and insect abundance (the number of individuals) (Table 3). The sap-feeding insects may be divided into three specialised guilds: phloem feeders, xylem feeders and mesophyll feeders. The proportion of species in each guild was similar in all four successional stages (Table 4). Niche breadth (determined from host plant records) in the sap feeding herbivores was inversely related to the successional age of their habitat (Table 6). Herbivore species colonising the stages early in succession had, on average, shorter generation times than those of later stages (Table 7).