Herbivore response to vegetational diversity: spatial interaction of resources and natural enemies

Vegetational diversity in agricultural systems is predicted to reduce herbivore populations, but we observed the opposite effect: higher nymph population densities of a functionally monophagous herbivore, the squash bug, Anasa tristis (Hemiptera: Coreidae) in a vegetationally diverse squash-bean-corn polyculture than in a squash monoculture. We examined spatial and temporal aspects of squash bug and predator populations in relation to vegetational diversity. Average colonization, oviposition, and mortality rates for the herbivore were similar in monocultures and polycultures. In the polyculture, however, we found that squash bugs eggs were highly aggregated on plants on the outer edges of plots. Predation was also lower on plants near the edges, allowing the large aggregations of eggs found in the polyculture to escape predation and ultimately produce more squash bugs. Spatial interactions between herbivores and natural enemies may underlie some of the general effects of vegetational diversity on herbivores.     

Herbivore attack in Casearia nitida influenced by plant ontogenetic variation in foliage quality and plant architecture

Traits influencing plant quality as food and/or shelter for herbivores may change during plant ontogeny, and as a consequence, influence the amount of herbivory that plants receive as they develop. In this study, differences in herbivore density and herbivory were evaluated for two ontogenetic stages of the tropical tree Casearia nitida. To assess plant ontogenetic differences in foliage quality as food for herbivores, nutritional and defensive traits were evaluated in saplings and reproductive trees. Predatory arthropods were quantified and the foraging preferences of a parasitoid wasp of the genus Zacremnops were assessed. In addition, survival rates of lepidopteran herbivores (Geometridae) were evaluated experimentally. Herbivore density was three times higher and herbivory was 66% greater in saplings than in reproductive trees. Accordingly, concentrations of total foliar phenolics were higher in reproductive trees than in saplings, whereas leaf toughness, water and nitrogen concentration did not vary between ontogenetic stages. Survival rates of lepidopteran larvae exposed to natural enemies were equivalent in reproductive trees and saplings. Given the greater herbivore density on saplings, equal survival rates implied a greater foraging effort of predators on reproductive trees. Furthermore, observed foraging of parasitoid wasps was restricted to reproductive trees. I propose that herbivore density, and as a consequence, leaf damage were lower in reproductive trees than in saplings due to both traits influencing food quality, and architectural or unmeasured indirect defensive traits influencing foraging preference of natural enemies of herbivores.     

Induced defence in detached uninfested plant leaves: effects on behaviour of herbivores and their predators

Summary Induction of plant defence against herbivores may include the attraction by volatile infochemicals of natural enemies of the herbivore. The emitted volatiles that mediate this attraction may also affect the behaviour of the herbivore itself. In this paper we investigate the response of the herbivorous spider miteTetranychus urticae and the predatory mitePhytoseiulus persimilis towards volatiles whose production is induced in detached Lima bean leaves. Detached uninfested Lima bean leaves were incubated on wet cotton wool on which bean leaves infested with spider mites (T. urticae) were present simultaneously or had been present previously. These treatments induce the production of volatile infochemicals in the uninfested bean leaf tissue: predatory mites are attracted and spider mites are deterred. These are the first data on the response of predators and herbivores to plant volatiles whose production was induced in detached uninfested leaves.     

Two heteropteran predators in relation to weed management in herbicide-tolerant corn

Deployment of genetically modified (GM), herbicide-tolerant corn may alter weed flora abundance and composition and may affect pests and their natural enemies. Among on-plant predators, Orius spp. are the prevalent group in Spain and were selected to study the impact of glyphosate use on predators. We also studied Nabis sp. which is commonly recorded on corn in the study area. For this, a 4-year study was conducted in NE Spain. Three different herbicide regimes were compared: two glyphosate (a broad-spectrum herbicide) treatments per season, no herbicide treatment, and one pre-emergence conventional treatment with selective herbicides against broadleaf and grassy weeds. Density of main arthropod herbivores and the above two predatory groups was recorded on plants. Differences between herbicide regimes were observed in the two functional groups studied, herbivores and heteropteran predators. The comparison of glyphosate-treated and untreated plots showed significant differences for both functional groups, but the differences between glyphosate-treated and conventionally treated plots for the two functional groups were lower. For Orius spp., annual density per plot was significantly correlated with annual density of leafhoppers and to a lesser extent, with aphids. Nabis sp. densities were never different between glyphosate-treated and conventionally treated plots, and Nabis sp. density showed no relation to any of the herbivores tested. We concluded that no significant changes in heteropteran predator densities may be expected from moderate alterations in weeds arising from the deployment of herbicide-tolerant corn varieties and that leafhoppers are probably the herbivore prey that most influences Orius spp. densities in corn in our study area.     

Prey‐mediated effects of glucosinolates on aphid predators

1. Plant resistance against herbivores can act directly (e.g. by producing toxins) and indirectly (e.g. by attracting natural enemies of herbivores). If plant secondary metabolites that cause direct resistance against herbivores, such as glucosinolates, negatively influence natural enemies, this may result in a conflict between direct and indirect plant resistance. 2. Our objectives were (i) to test herbivore‐mediated effects of glucosinolates on the performance of two generalist predators, the marmalade hoverfly (Episyrphus balteatus) and the common green lacewing (Chrysoperla carnea) and (ii) to test whether intraspecific plant variation affects predator performance. 3. Predators were fed either Brevicoryne brassicae, a glucosinolate‐sequestering specialist aphid that contains aphid‐specific myrosinases, or Myzus persicae, a non‐sequestering generalist aphid that excretes glucosinolates in the honeydew, reared on four different white cabbage cultivars. Predator performance and glucosinolate concentrations and profiles in B. brassicae and host‐plant phloem were measured, a novel approach as previous studies often measured glucosinolate concentrations only in total leaf material. 4. Interestingly, the specialist aphid B. brassicae selectively sequestered glucosinolates from its host plant. The performance of predators fed this aphid species was lower than when fed M. persicae. When fed B. brassicae reared on different cultivars, differences in predator performance matched differences in glucosinolate profiles among the aphids. 5. We show that not only the prey species, but also the plant cultivar can have an effect on the performance of predators. Our results suggest that in the tritrophic system tested, there might be a conflict between direct and indirect plant resistance.     

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.     

Elevated atmospheric CO2 lowers herbivore abundance, but increases leaf abscission rates

Increased levels of atmospheric carbon dioxide (CO₂) are likely to affect the trophic relationships that exist between plants, their herbivores and the herbivores' natural enemies. This study takes advantage of an open‐top CO₂ fertilization experiment in a Florida scrub oak community at Kennedy Space Center, Florida, consisting of eight chambers supplied with ambient CO₂ (360 ppm) and eight chambers supplied with elevated CO₂ (710 ppm). We examined the effects of elevated CO₂ on herbivore densities and levels of leaf consumption, rates of herbivore attack by natural enemies and effects on leaf abscission. Cumulative levels of herbivores and herbivore damage were significantly lower in elevated CO₂ than in ambient CO₂. This may be because leaf nitrogen levels are lower in elevated CO₂. More herbivores die of host plant‐induced death in elevated CO₂ than in ambient CO₂. Attack rates of herbivores by parasitoids are also higher in elevated CO₂, possibly because herbivores need to feed for a longer time in order to accrue sufficient nitrogen (N), thus exposing themselves longer to natural enemies. Insect herbivores cause an increase in abscission rates of leaves throughout the year. Because of the lower insect density in elevated CO₂, we thought, abscission rates would be lower in these chambers. However, abscission rates were significantly higher in elevated CO₂. Thus, the direct effects of elevated CO₂ on abscission are greater than the indirect effects on abscission mediated via lower insect densities. A consequence of increased leaf abscission in elevated CO₂ is that nutrient deposition rates to the soil surface are accelerated.     

The three criteria for resistance by plant carrion‐provisioning: insect entrapment and predator enrichment on Mimulus bolanderi

1. Many sticky plants provision mutualistic scavenging arthropod predators with carrion, which in turn protect the plant from insect herbivores. While insect entrapment is a common trait across plants, which plants attract these predators and may derive protection is still largely unknown. 2. Three conditions were proposed that must be satisfied for observational data to suggest this defensive strategy: (i) the consistent presence of scavenging predators, (ii) positive correlation between predator numbers and carrion, and (iii) suitability of these predators for controlling known herbivores. 3. As a case study, we examined the fire‐following annual, Mimulus [Diplacus] bolanderi (Phyrmaceae), which is part of a well‐studied radiation of California monkeyflowers. Many monkeyflowers entrap insects, though attraction to predators has not been quantified in this genus. 4. A guild of scavenging arthropod predators on M. bolanderi (condition #1) was found, which correlated positively with carrion abundance (#2) and could consume the primary herbivore (#3), suggesting a carrion‐mediated defensive strategy. Lastly, as M. bolanderi is variable in time and space, these interactions are facultative, and these predators are quick to adopt ephemeral carrion resources on novel host plants.     

Differentiation and adaptation in <Emphasis Type="Italic">Brassica nigra</Emphasis> populations: interactions with related herbivores

Local adaptation and population differentiation of plants are well documented, but studies on interactions with natural enemies are rare. In particular, evidence for plant adaptation to the local biotic environment, such as herbivores remains poor. We used the black mustard Brassica nigra, an annual species of river valley and coastal habitats to (1) analyse population differentiation in plant traits and herbivory in a common garden experiment, (2) examine home versus away differences in a reciprocal transplant experiment and (3) test whether plants are adapted to local herbivores or vice versa under standard greenhouse conditions. In the common garden experiment, we found significant differentiation in plant traits, leaf damage and herbivore number among seven populations of B. nigra from France and Germany (distance 15–1,000 km). Differences were particularly strong among coastal and river valley populations and did not necessarily increase with geographical distance. A herbivore removal treatment did not change population differentiation when compared with the control allowing natural colonisation. The reciprocal transplant experiment at a scale of 15–30 km did not reveal local plant adaptation, whilst one dominant herbivore species (Meligethes aeneus) occurred in significantly higher numbers on local plants. A greenhouse experiment combining three aphid (Brevicoryne brassicae) and plant populations of the same provenance indicated herbivore adaptation to their local plants rather than plant adaptation, but overall contrasts between local and non-local combinations were not significant. The results suggest that herbivores may counteract local plant adaptation to other environmental factors. Our study has important implications for plant translocations in ecological restoration projects.     

Seeing the forest for the trees: long-term exposure to elevated CO2 increases some herbivore densities

The effects of elevated CO₂ on plant growth and insect herbivory have been frequently investigated over the past 20 years. Most studies have shown an increase in plant growth, a decrease in plant nitrogen concentration, an increase in plant secondary metabolites and a decrease in herbivory. However, such studies have generally overlooked the fact that increases in plant production could cause increases of herbivores per unit area of habitat. Our study investigated leaf production, herbivory levels and herbivore abundance per unit area of leaf litter in a scrub‐oak system at Kennedy Space Center, Florida, under conditions of ambient and elevated CO₂, over an 11‐year period, from 1996 to 2007. In every year, herbivory, that is leafminer and leaftier abundance per 200 leaves, was lower under elevated CO₂ than ambient CO₂ for each of three species of oaks, Quercus myrtifolia, Quercus chapmanii and Quercus geminata. However, leaf litter production per 0.1143 m² was greater under elevated CO₂ than ambient CO₂ for Q. myrtifolia and Q. chapmanii, and this difference increased over the 11 years of the study. Leaf production of Q. geminata under elevated CO₂ did not increase. Leafminer densities per 0.1143 m² of litterfall for Q. myrtifolia and Q. chapmanii were initially lower under elevated CO₂. However, shortly after canopy closure in 2001, leafminer densities per 0.1143 m² of litter fall became higher under elevated CO₂ and remained higher for the remainder of the experiment. Leaftier densities per 0.1143 m² were also higher under elevated CO₂ for Q. myrtifolia and Q. chapmanii over the last 6 years of the experiment. There were no differences in leafminer or leaftier densities per 0.1143 m² of litter for Q. geminata. These results show three phenomena. First, they show that elevated CO₂ decreases herbivory on all oak species in the Florida scrub‐oak system. Second, despite lower numbers of herbivores per 200 leaves in elevated CO₂, increased leaf production resulted in higher herbivore densities per unit area of leaf litter for two oak species. Third, they corroborate other studies which suggest that the effects of elevated CO₂ on herbivores are species specific, meaning they depend on the particular plant species involved. Two oak species showed increases in leaf production and herbivore densities per 0.1143 m² in elevated CO₂ over time while another oak species did not. Our results point to a future world of elevated CO₂ where, despite lower plant herbivory, some insect herbivores may become more common.     

Impact of Galerucella birmanica (Coleoptera: Chrysomelidae) on growth and seed production of Trapa natans

Success in biological weed control programs depends upon the ability of host-specific herbivores to suppress populations of their host plant. While pre-release predictions of field host range (i.e., specificity) appear widely accurate, predictions about which agent or agent combination may suppress plant populations have lately been compared to predictions in a lottery. The history of weed biocontrol does not offer immediately obvious approaches to improve the lottery model, however, pre-release assessments of the impact of different herbivore densities on the invasive plant may provide an opportunity to improve predictions of success. In this paper, we report on the impact of the leaf beetle Galerucella birmanica on growth and reproduction of water chestnut, Trapa natans, in the native range in China. At low herbivore densities (10–50 larvae/rosette), plants compensated for leaf herbivory by increasing leaf production at the expense of reproductive effort. Inoculating >50 first instar larvae per rosette greatly suppressed biomass production and plants were unable to grow when three or more G. birmanica pairs were released per seven rosettes. In the native range, similar densities are found in the field, resulting in complete defoliation of T. natans. Our study indicates that G. birmanica feeding has significant negative impacts on T. natans. This chrysomelid species appears to be a promising biological control agent and we would predict that the species will be able to attain sufficiently high populations to control its host plant—if approved for release in North America.     

Second and third trophic level effects of differences in plant species reflect dietary specialisation of herbivores and their endoparasitoids

In natural populations, plants demonstrate an array of indirect and direct defence strategies that help to protect them from their herbivores and pathogens. Indirect defences include the release of odours that attract the natural enemies of herbivores, whereas direct defences may include the production of secondary compounds, allelochemicals that impair herbivore development or repel herbivore attack. Although both strategies have been well studied independently, comparatively little attention has been paid to examining the conflict that may arise between indirect and direct defences, such as when the performance of ‘recruited’ parasitoids or predators is negatively affected by plant allelochemicals. Here, we examine the growth and development of polyphagous and oligophagous folivores and their respective endoparasitoids on three crucifer species. One of the species, Brassica oleracea, was recently cultivated, whereas populations of B. nigra and Barbarea vulgaris occur naturally. Additionally, these species possess contrasting life‐history patterns and are also known to exhibit differences in secondary chemistry. The development of the generalist herbivore–parasitoid system was much more variable over the three crucifers than that exhibited by the specialists. Moreover, generalist herbivore and/or parasitoid fitness‐related traits (survival, development time, pupal, or adult size) were much more negatively affected on the wild crucifers than in the specialist association. Our results suggest that the relative importance of direct and indirect defences in plants may rest on the degree of dietary specialisation exhibited by herbivores and their natural enemies, and on the level of toxicity in the plant species under investigation.     

Does a multi‐plant diet benefit a polyphagous herbivore? A case study with Bemisia tabaci

Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) is a highly polyphagous herbivore. This research was conducted to compare the development of B. tabaci reared in a multi‐plant treatment (polyculture) with those in single‐plant treatments (monocultures). Adult B. tabaci females fed on a mixture of tomato, cabbage, cotton, cucumber, and kidney bean survived longer and laid more eggs than those fed exclusively on one of these plant species. Egg numbers per plant laid in the polyculture treatment were positively correlated with those laid on the same plant species in the monoculture treatments, and egg numbers per plant laid on tomato, cotton, and cucumber in the polyculture were significantly higher than those laid on the same plants in the monocultures. Concentrations of total protein and trehalose in B. tabaci were not significantly different after 7 days of feeding in the respective treatments, but activities of superoxide dismutases (SOD) and alkaline phosphatase (AKP) of B. tabaci in polyculture were lower than those in monoculture. Conversely, activities of trehalase, sucrase, and amylase in B. tabaci kept in polyculture were higher than those of insects from the monoculture. In each of the monoculture treatments, there was a negative correlation between AKP in B. tabaci and oviposition, and also between AKP and amylase. SOD and sucrase activities in B. tabaci were positively correlated with polyphenol oxidase (PPO) and peroxidase (POD) activities in plants. In the plants damaged by whiteflies in the polyculture treatment, activities of SOD in cucumber, PPO in cotton and kidney bean, and POD in tomato and cucumber were lower than those in the monoculture treatments, whereas SOD in cabbage and catalase (CAT) in tomato in the polyculture treatment were higher than those in the monoculture treatments.     

Direct and indirect effects of plant genetic variation on enemy impact

Abstract.

• 1 The Tritrophic and Enemy Impact concepts predict that natural enemy impact varies: (a) among plant genotypes and (b) may depend on the abundance of heterospecific herbivores, respectively. I tested these predictions using three herbivore species on potted, cloned genotypes of Salik sericea Marshall in a common garden experiment.
• 2 Densities of the leaf miner (Phyllonorycter salicifoliella (Chambers)) and two leaf galling sawflies (Phyllocolpa nigrita (Marlatt) and Phyllocolpa eleanorae Smith and Fritz) varied significantly among willow clones, indicating genetic variation in resistance.
• 3 Survival and natural enemy impact caused by egg and larval parasitoids and/or unknown predators differed significantly among willow clones for each of the three herbivore species, indicating genetic variation in survival and enemy impact.
• 4 Survival of Phyllonorycter was negatively density-dependent among clones.
• 5 Survival of Phyllonorycter and Phyllocolpa eleanorae were positively correlated with densities of heterospecific herbivores among clones and parasitism of these species were negatively correlated with densities of the same heterospecific herbivores among clones.
• 6 At least for Phyllonorycter this positive correlation may suggest either facilitation of survival between herbivore species, which do not share natural enemies, or an apparent interaction caused by host plant genetic variation.
• 7 Among clones, egg parasitism of Phyllocolpa eleanorae was weakly positively correlated with density of Phyllocolpa nigrita. Since these species share the same Trichogramma egg parasitoid, this interaction could support the hypothesis of apparent competition.

     

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.     

Herbivory on invasive exotic plants and their non-invasive relatives

The Enemy Release Hypothesis links exotic plant success to escape from enemies such as herbivores and pathogens. Recent work has shown that exotic plants that more fully escape herbivores and pathogens are more likely to become highly invasive, compared to plants with higher enemy loads in their novel ranges. We predicted that highly invasive plants from the Asteraceae and the Brassicaceae would be less acceptable, in laboratory no-choice feeding trials, to the generalist herbivore the American grasshopper, Schistocerca americana. We also compared herbivory on invasive and non-invasive plants from the genus Centaurea in no-choice feeding trials using the red-legged grasshopper Melanoplus femurrubrum and in a common garden in the field. In accordance with our predictions, highly invasive plants were fed on less by grasshoppers in the laboratory. They also received less damage in the field, suggesting that they contain feeding deterrents that render them less acceptable to generalist herbivores than non-invasive plants.     

Plant Size as Determinant of Species Richness of Herbivores,Natural Enemies and Pollinators across 21 Brassicaceae Species

Large plants are often more conspicuous and more attractive for associated animals than small plants, e.g. due to their wider range of resources. Therefore, plant size can positively affect species richness of associated animals, as shown for single groups of herbivores, but studies usually consider intraspecific size differences of plants in unstandardised environments. As comprehensive tests of interspecific plant size differences under standardised conditions are missing so far, we investigated effects of plant size on species richness of all associated arthropods using a common garden experiment with 21 Brassicaceae species covering a broad interspecific plant size gradient from 10 to 130 cm height. We recorded plant associated ecto- and endophagous herbivores, their natural enemies and pollinators on and in each aboveground plant organ, i.e. flowers, fruits, leaves and stems. Plant size (measured as height from the ground), the number of different plant organ entities and their biomass were assessed. Increasing plant size led to increased species richness of associated herbivores, natural enemies and pollinating insects. This pattern was found for ectophagous and endophagous herbivores, their natural enemies, as well as for herbivores associated with leaves and fruits and their natural enemies, independently of the additional positive effects of resource availability (i.e. organ biomass or number of entities and, regarding natural enemies, herbivore species richness). We found a lower R² for pollinators compared to herbivores and natural enemies, probably caused by the high importance of flower characteristics for pollinator species richness besides plant size. Overall, the increase in plant height from 10 to 130 cm led to a 2.7-fold increase in predicted total arthropod species richness. In conclusion, plant size is a comprehensive driver of species richness of the plant associated arthropods, including pollinators, herbivores and their natural enemies, whether they are endophagous or ectophagous or associated with leaves or fruits.     

Comparison of Herbivores and Herbivory in the Canopy and Understory for Two Tropical Tree Species1

The Janzen–Connell model of tropical forest tree diversity predicts that seedlings and young trees growing close to conspecific adults should experience higher levels of damage and mortality from herbivorous insects, with the adult trees acting as either an attractant or source of the herbivores. Previous research in a seasonal forest showed that this pattern of distance‐dependent herbivory occurred in the early wet season during the peak of new leaf production. I hypothesized that distance‐dependent herbivory may occur at this time because the new foliage in the canopy attracts high numbers of herbivores that are limited to feeding on young leaves. As a consequence, seedlings and saplings growing close to these adults are more likely to be discovered and damaged by these herbivores. In the late wet season, when there is little leaf production in the canopy, leaf damage is spread more evenly throughout the forest and distance dependence disappears. I tested three predictions based on this hypothesis: (1) the same species of insect herbivores attack young and adult trees of a given plant species; (2) herbivore densities increase on adult trees during leaf production; and (3) herbivore densities in the understory rise during the course of the wet season. Censuses were conducted on adults and saplings of two tree species, raribea asterolepis and Alseis blackiana. Adults and saplings of both species had largely the same suite of chewing herbivore species. On adults of Q. asterolepis, the density of chewing herbivores increased 6–10 times during leaf production, but there was no increase in herbivore density on adults of A. blackiana. Herbivore densities increased 4.5 times on A. blackiana saplings and 8.9 times on Q. asterolepis saplings during the wet season, but there were no clear trends on the adults of either species. These results suggest that the potential of adult trees as a source of herbivores on saplings depends on the value of new leaves to a tree species' herbivores, which may differ across tree species.     

Effects of integrating companion cropping and nitrogen application on the performance and infestation of collards by Brevicoryne brassicae

Sustainable management of cabbage aphids, Brevicoryne brassicae (L.) (Hemiptera: Aphididae), is a major goal for collard, Brassica oleracea (L.) var. acephala (Brassicaceae), growers globally. Host finding ability of insect pests is significantly affected by diversified cropping systems, and this approach is being utilized currently as a pest management tool. Soil nutrition and its interaction with the cropping systems could have a significant effect on the general performance of collards and the infestation by cabbage aphids. In a search for a sustainable cabbage aphid control, a two‐season field experiment was carried out with two intercrops, collards and chilli, Capsicum frutescens (L.) (Solanaceae), and collards and spring onions, Allium cepa (L.) (Alliaceae), and a collard monoculture. For each of the cropping systems, nitrogen (N) was applied to the soil as a top‐dress at 20, 25, 30, and 35 g per collard plant. The response factors monitored were collard yield (fresh weight) and aphid infestation on collards. Spring onion‐collard intercrop had the lowest aphid density and the highest yield. Collard monoculture had the highest aphid infestation and the lowest yield. High levels of N led to increased infestation of collards by aphids, but also led to a significant increase in the yield of collards. Significant interactions between the N rates and the cropping systems were observed on some sampling dates, with the highest yield being realized under a combination of spring onion‐collard intercrop at a N rate of 30 g per plant. High aphid density led to a decrease in the yield of collards. It was concluded that with a spring onion‐collard intercrop, the soil N level could be raised from the blanket rate of 20–30 g per plant and this would lead to an increase in yield.     

Can plants betray the presence of multiple herbivore species to predators and parasitoids? The role of learning in phytochemical information networks

In response to feeding by phytophagous arthropods, plants emit volatile chemicals. This is shown to be an active physiological response of the plant and the released chemicals are therefore called herbivore-induced plant volatiles (HIPV). One of the supposed functions of HIPV for the plant is to attract carnivorous natural enemies of herbivores. Depending on which plant and herbivore species interact, blends of HIPV show qualitative and quantitative variation. Hence, one may ask whether this allows the natural enemies to discriminate between volatiles from plants infested by herbivore species that are either suitable or unsuitable as a food source for the natural enemy. Another question is whether natural enemies can also recognise HIPV when two or more herbivore species that differ in suitability as a food source simultaneously attack the same plant species. By reviewing the literature we show that arthropod predators and parasitoids can tell different HIPV blends apart in several cases of single plant–single herbivore systems and even in single plant–multiple herbivore systems. Yet, there are also cases where predators and parasitoids do not discriminate or discriminate only after having learned the association between HIPV and herbivores that are either suitable or non-suitable as a source of food. In this case, suitable herbivores may profit from colonising plants that are already infested by another non-suitable herbivore. The resulting temporal or partial refuge may have important population dynamical consequences, as such refuges have been shown to stabilise otherwise unstable predator–prey models of the Lotka-Volterra or Nicholson-Bailey type.