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.     

Emission of Plutella xylostella-induced compounds from cabbages grown at elevated CO2 and orientation behavior of the natural enemies

Several plant species defend themselves indirectly from herbivores by producing herbivore-induced volatile compounds that attract the natural enemies of herbivores. Here we tested the effects of elevated atmospheric CO(2) (720 micromol mol(-1)) concentration on this indirect defense, physiological properties, and constitutive and induced emissions of white cabbage (Brassica oleracea ssp. capitata, cvs Lennox and Rinda). We monitored the orientation behavior of the generalist predator Podisus maculiventris (Heteroptera: Pentatomidae) and the specialist parasitoid Cotesia plutellae (Hymenoptera: Braconidae) to plants damaged by Plutella xylostella (Lepidoptera: Plutellidae) in the Y-tube olfactometer. Elevated CO(2) levels did not affect stomatal densities but reduced specific leaf area and increased leaf thickness in cv Lennox. In addition to enhanced constitutive monoterpene emission, P. xylostella-damaged cabbages emitted homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene, sesquiterpene (E,E)-alpha-farnesene, and (Z)-3-hexenyl acetate. Growth at elevated CO(2) had no significant effect on the emissions expressed per leaf area, while minor reduction in the emission of homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene and (E,E)-alpha-farnesene was observed at elevated CO(2) in one of two experiments. The generalist predator P. maculiventris discriminated only between the odors of intact and P. xylostella-damaged cv Rinda plants grown at ambient CO(2) concentration, preferring the odor of the damaged plants. The specialist parasitoid C. plutellae preferred the odor of damaged plants of both cultivars grown at ambient CO(2) but did not detect damaged cv Lennox plants grown at elevated CO(2). The results suggest that elevated atmospheric CO(2) concentration could weaken the plant response induced by insect herbivore feeding and thereby lead to a disturbance of signaling to the third trophic level.     

The effect of herbivore density on host plant mediated interactions between two insects

Recent work suggests that foliar- and root-feeding insects can interact in a novel plus-minus fashion. However, the effects of herbivores may be different at different densities. This paper describes two laboratory experiments investigating the effect of increasing insect density on plant performance and the host-plant mediated interaction between a leaf-mining fly and root-feeding chafer larvae. Above- and below-ground insect herbivory decreased plant performance. The effects of the root feeder were, for some parameters, different between the two densities of chafer larvae. Leaf mining significantly decreased the performance of the chafer larvae, while root herbivory was found to increase the pupal weight (related to fecundity) of the leaf miner. The effects of root herbivory suggest that the relationships between herbivore density and plant performance may be curved as the greater chafer density had no significant increased effect on the plants or foliar-feeding insects.     

Plant-insect herbivore interactions in elevated CO(2) environments

The increasing concentration of CO(2) in the atmosphere is expected to lead to global changes in the physical environment of terrestrial organisms. We are beginning to understand how these changes are transmitted into pervasive effects on the interactions between plants and their leaf-feeding insect herbivores. An elevated CO(2) atmosphere often stimulates plant carbon assimilation and growth and alters carbon allocation patterns. This, in turn, determines the quality of plants as resources for herbivorous insects. These 'quality' factors include: the concentrations of water, nitrogen and allelochemicals in host-plant leaves, and the toughness and starch and fiber content of leaf tissue. Because these parameters change in plants grown in enriched CO(2) environments, the doubled CO(2) levels anticipated for the next century will alter the dynamics of plant-insect herbivore interactions because herbivore consumption, growth and fitness are affected by the typically lower quality of plants grown under these conditions.     

Different mechanisms explain feeding type-specific patterns of latitudinal variation in herbivore damage among diverse feeding types of herbivorous insects

To determine whether latitudinal variation in herbivore impact exists, we examined three major herbivorous insect feeding types (chewers, gallers, and miners) on/in leaves of Japanese beech. Herbivores were collected with litter traps deployed in forests across a latitudinal gradient of 10°. Leaf litter analyses demonstrated that chewing herbivory increased with increasing latitude of collection site. However, the densities of miners and gallers decreased with latitude. To test whether latitudinal variation in herbivore damage occurs in the absence of geographically differentiated environmental cueing (e.g., physical stresses or herbivore damage), we measured both genetically determined constitutive leaf traits and herbivore damage in a common-garden experiment. In this experiment, miner density decreased with latitude, but chewing herbivory did not vary latitudinally. Galler density was higher on trees from native provenances than on trees from unrelated provenances likely because of local adaptations. Leaf mass per unit area (LMA), tannin, and phenolics all decreased with latitude of provenance. The latitudinal variation in one constitutive leaf trait (LMA) best explained latitudinal variation in chewing herbivory. Thus, different mechanisms account for feeding type-specific patterns of latitudinal variation in herbivore damage among different herbivore feeding types.     

How does elevated carbon dioxide (CO2) affect plant–herbivore interactions? A field experiment and meta‐analysis of CO2‐mediated changes on plant chemistry and herbivore performance

First, we report the results of the longest‐known field study (9 years) to examine the effects of elevated carbon dioxide (CO₂) on leaf miner densities in a scrub‐oak community at Kennedy Space Center, Florida. Here, the densities of all leaf miner species (6) on all host species (3) were lower in every year in elevated CO₂ than they were in ambient CO₂. Second, meta‐analyses were used to review the effects of elevated CO₂ on both plants (n=59 studies) and herbivores (n=75 studies). The log of the response ratio was chosen as the metric to calculate effect sizes. Results showed that elevated CO₂ significantly decreased herbivore abundance (−21.6%), increased relative consumption rates (+16.5%), development time (+3.87%) and total consumption (+9.2%), and significantly decreased relative growth rate (−8.3%), conversion efficiency (−19.9%) and pupal weight (−5.03%). No significant differences were observed among herbivore guilds. Host plants growing under enriched CO₂ environments exhibited significantly larger biomass (+38.4%), increased C/N ratio (+26.57%), and decreased nitrogen concentration (−16.4%), as well as increased concentrations of tannins (+29.9%) and other phenolics. Effects of changes on plant primary and secondary chemistry due to elevated CO₂ and consequences for herbivore growth and development are discussed.     

The relative importance of resources and natural enemies in determining herbivore abundance: thistles, tephritids and parasitoids

1. The relative importance of host-plant resources and natural enemies in influencing the abundance of insect herbivores was investigated in potted plant and natural population experiments, using tephritid (Diptera: Tephritidae) flies, their host plant, creeping thistle Cirsium arvense, and their Hymenoptera parasitoids. 2. Experimental manipulation of host-plant quality (i.e. levels of host-plant nutrients) and resource availability (i.e. the number of buds) increased tephritid abundance. There was no evidence that the seed-feeding tephritid fly Xyphosia miliaria preferentially oviposited on fertilized C. arvense. 3. At low thistle densities, X. miliaria showed a constant rate of resource exploitation. At higher thistle densities, a threshold was detected, above which additional buds were not attacked. 4. Parasitism attack was variable across host (tephritid) densities but levels of parasitism were consistently higher on the fertilized thistles. 5. Experimental manipulation of host-plant quality and resource availability (quantity) not only directly affects the tephritid population but also, indirectly, leads to high rates of parasitism. Both chemical and physical characteristics of host plants affect the performance of natural enemies. 6. Both top-down and bottom-up forces act to influence tephritid abundance, with bottom-up influences appearing to be the most important.     

CO2‐mediated changes of plant traits and their effects on herbivores are determined by leaf age

1. Concentration of atmospheric CO₂ is predicted to double during the 21st century. However, quantitative effects of increased CO₂ levels on natural herbivore–plant interactions are still little understood. 2. In this study, we assess whether increased CO₂ quantitatively affects multiple defensive and nutritive traits in different leaf stages of cyanogenic wildtype lima bean plants (Phaseolus lunatus), and whether plant responses influence performance and choice behaviour of a natural insect herbivore, the Mexican bean beetle (Epilachna varivestis). 3. We cultivated lima bean plants in climate chambers at ambient, 500, 700, and 1000 ppm CO₂ and analysed cyanogenic precursor concentration (nitrogen‐based defence), total phenolics (carbon‐based defence), leaf mass per area (LMA; physical defence), and soluble proteins (nutritive parameter) of three defined leaf age groups. 4. In young leaves, cyanide concentration was the only parameter that quantitatively decreased in response to CO₂ treatments. In intermediate and mature leaves, cyanide and protein concentrations decreased while total phenolics and LMA increased. 5. Depending on leaf stage, CO₂‐mediated changes in leaf traits significantly affected larval performance and choice behaviour of adult beetles. We observed a complete shift from highest herbivore damage in mature leaves under natural CO₂ to highest damage of young leaves under elevated CO_2. Our study shows that leaf stage is an essential factor when considering CO₂‐mediated changes of plant defences against herbivores. Since in the long run preferred consumption of young leaves can strongly affect plant fitness, variable effects of elevated CO₂ on different leaf stages should receive highlighted attention in future research.     

Does low nutritional quality act as a plant defence? An experimental test of the slow‐growth,high‐mortality hypothesis

Abstract.  1. The slow-growth, high-mortality hypothesis was experimentally tested in this study by investigating the effects of plant quality and natural enemies on leaf-miner growth, performance, and survivorship. Two leaf miners ( Acrocercops albinatella and Brachys tesselatus ) occurring on the turkey oak Quercus laevis were studied using a factorial design that manipulated predation/parasitism pressure and plant nutritional quality.
2. Forty trees were randomly divided into four treatments: (1) control plants (nutrients and natural enemies unaltered); (2) nutrients added, natural enemies unaltered; (3) nutrients unaltered, natural enemies reduced; and (4) nutrients added and natural enemies reduced. Water content, leaf toughness, tannin concentration, and foliar nitrogen were quantified monthly for each plant, and mine growth and survivorship were assessed by tracing mines on a 2–3-day interval and by following the fates of 50 mines per species per treatment combination.
3. Fertilised plants exhibited significantly higher amounts of nitrogen, but no significant differences among treatments were observed for water content, leaf toughness, and tannin concentration. These results only partially support the slow-growth, high-mortality hypothesis, as mines were significantly smaller and developed faster on fertilised plants, but neither fertilisation nor natural enemy exclusion significantly affected mine survivorship or mortality caused by natural enemies.     

The impact of weed diversity on insect population dynamics and crop yield in collards, Brassica oleraceae (Brassicaceae)

Vegetational diversity within agricultural fields is often suggested as a means to reduce insect herbivore populations and to increase their natural enemies. In this paper we compare population densities of herbivores, predators, and parasitoids on collards in monocultures and on collards interplanted with two different groups of weeds, one with weed species from the same plant family as the collards (Brassicaceae) and one with weed species from unrelated plant families (non-Brassicaceae). The collards in the Brassicaceae weed polyculture had higher densities (number of herbivores/mean leaf area (cm²) per plant) of specialist herbivores than collards in the non-Brassicaceae weed polyculture and in collard monoculture. The “resource concentration” hypothesis is supported by the observation of higher populations of Phyllotreta spp., acting as facultative polyphages, in the Brassicaceae weed polyculture than in the non-Brassicaceae weed polyculture where Phyllotreta spp. are facultative monophages. Population densities of natural enemies (mostly coccinellids, carabids, and staphylinids) were higher in the polycultures than in the monoculture: carabid and staphylinid predators may be responsible for larval mortality in the imported cabbage worm, Pieris␣rapae, and in the diamondback larvae, Plutella xylostella. In spite of differences in densities of specialist herbivores across treatments, crop yield, leaf area (cm²), the proportion of leaf area damaged, and the number of leaves undamaged did not differ. These findings suggest that plant competition may interfere with attempts to reduce herbivore damage. We conclude that the use of weedy cultures can provide effective means of reducing herbivores if the crop and weed species are not related and plant competition is prevented. Received: 25 December 1995 / Accepted: 24 February 1997     

大气CO2浓度增加对昆虫的影响

大气CO2浓度增加已经受到国内外的极大关注.CO2浓度升高不但影响植物的生长发育,而且还改变植物体内的化学成分的组成与含量,从而间接地影响到植食性昆虫,并进而通过食物链影响到以之为食的天敌.根据国内外研究进展,结合多年的研究,系统介绍了CO2浓度变化对植物-昆虫系统影响的研究方法,论述了CO2浓度变化对植食性昆虫、天敌的作用规律及作用机理,探讨了CO2浓度变化对植物-植食性昆虫系统影响的特征,分析了未来研究发展的趋势及其存在的问题.     

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.     

De Novo Biosynthesis of Volatiles Induced by Insect Herbivory in Cotton Plants

In response to insect feeding on the leaves, cotton (Gossypium hirsutum L.) plants release elevated levels of volatiles, which can serve as a chemical signal that attracts natural enemies of the herbivore to the damaged plant. Pulse-labeling experiments with [13C]CO2 demonstrated that many of the volatiles released, including the acyclic terpenes (E,E)-[alpha]-farnesene, (E)-[beta]-farnesene, (E)-[beta]-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, and (E/E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene, as well as the shikimate pathway product indole, are biosynthesized de novo following insect damage. However, other volatile constituents, including several cyclic terpenes, butyrates, and green leaf volatiles of the lipoxygenase pathway are released from storage or synthesized from stored intermediates. Analysis of volatiles from artificially damaged plants, with and without beet armyworm (Spodoptera exigua Hubner) oral secretions exogenously applied to the leaves, as well as volatiles from beet armyworm-damaged and -undamaged control plants, demonstrated that the application of caterpillar oral secretions increased both the production and release of several volatiles that are synthesized de novo in response to insect feeding. These results establish that the plant plays an active and dynamic role in mediating the interaction between herbivores and natural enemies of herbivores.     

Seasonal shift from bottom-up to top-down impact in phytophagous insect populations

Although many studies now examine how multiple factors influence the dynamics of herbivore populations, few studies explicitly attempt to document where and when each is important and how they vary and interact. In fact, how temporal variation in top-down (natural enemies) and bottom-up (host plant resources) factors affect herbivore dynamics has been suggested as a particularly important yet poorly understood feature of terrestrial food webs. In this study we examined how temporal changes in predator density (wolf spiders, sheet-web builders, and mirid egg predators) and host-plant resources (plant quality and structural complexity) influence the population dynamics of the dominant phytophagous insects on Atlantic-coast salt marshes, namely Prokelisia planthoppers (Homoptera: Delphacidae). We designed a factorial experiment in meadows of Spartina alterniflora to mimic natural variation in vegetation quality and structure by establishing two levels of plant nutrition (leaf nitrogen content) by fertilization, and two levels of habitat complexity by adding leaf litter (thatch). We then assessed seasonal changes in the strength of bottom-up (plant quality) and top-down (predator) impacts on planthopper populations. Planthopper populations responded positively to increased plant quality treatments in late summer. Despite the greater number of planthopper adults colonizing fertilized Spartina plots compared to unfertilized controls, the offspring of these colonists were much less abundant at the end of the season in fertilized plots, particularly those with thatch. The initial colonization effect was later erased because arthropod predators selectively accumulated in fertilized plots where they inflicted significant mortality on all stages of planthoppers. Predators rapidly colonized fertilized plots and reached high densities well in advance of planthopper colonization, a response we attribute to their rapid aggregation in complex-structured habitats with readily available alternative prey. Our results suggest that plant resources not only mediate the strength of predator impacts on herbivore populations, but they also promote the coupling of predator and prey populations and thus influence when enemy impacts are realized.     

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.     

Correlations between leaf constituent levels and the densities of herbivorous insect guilds in an Australian forest

Abstract Significant correlations were found between herbivorous insect densities and leaf constituent levels across 18 species of understorey shrub in an Australian forest. In general, insect densities were positively correlated with nitrogen and water and negatively correlated with fibre and its constituents (cellulose and lignin), but not all insect guilds were significantly correlated with all leaf constituents. When mature leaf traits were considered, total herbivore density among plant species was most strongly correlated with levels of nitrogen and water. However, when insects were divided into functional guilds, only densities of sucking insects were significantly correlated with mature leaf nitrogen and water, whereas chewing insect densities were significantly correlated with mature leaf fibre and lignin. Low leaf nitrogen levels were recorded for all plants surveyed (among the lowest reported for woody angiosperms), and many plant species also had high levels of leaf fibre. These features are characteristic of Australian forests and have been linked to the generally nutrient‐poor soils of this continent. Levels of new leaf nitrogen (% fresh weight) were generally less than or equal to levels of mature leaf nitrogen (% fresh weight). Hence the new leaf preference of several herbivore guilds could not be explained by higher levels of leaf nitrogen. However, significant negative correlations were found between chewer densities and levels of leaf fibre, cellulose and lignin for new and mature leaves, suggesting that higher levels of fibre, cellulose and lignin in mature leaves may be contributing more strongly to the new leaf preference of chewers than leaf nitrogen levels. Despite the significant correlations between leaf constituent levels and the densities of individual guilds, multivariate analyses found no significant relationship between leaf constituent levels and the taxonomic or guild composition of herbivorous insect assemblages.     

Leaf phenology mediates provenance differences in herbivore populations on valley oaks in a common garden

1. Plants from different populations often display a variation in herbivore resistance. However, it is rarely understood what plant traits mediate such differences. 2. It was tested how leaf phenology affects herbivore populations in a 15‐year‐old common garden of valley oaks (Quercus lobata Née) with different populations and maternal parents from throughout the Q. lobata range. 3. The abundance of leaf miners (Stigmella sp. Shrank) and leaf phenology of oaks in the common garden was measured. 4. Leaf miner abundance varied among provenance locations (population), but not among maternal parents within populations. Leaf phenology varied by provenance location and maternal parent, and trees that leafed out earlier accrued higher leaf‐miner abundance. Path analysis indicated that leaf phenology was the likely driver of provenance and parental differences in resistance to leaf miners. 5. Understanding population differences is particularly important when considering transport of genotypes for ornamental or restoration purposes. The present study suggests that similarity in leaf phenology may be one factor that could be used to find genotypes with a similar herbivore resistance to local genotypes.     

Compensation behaviour by insect herbivores and natural enemies: its influence on community structure

Insect herbivores feeding on low-quality plants often compensate by increasing their consumption of plant tissue. This usually results in a longer developmental time leading to a higher vulnerability to natural enemies. This has been termed the slow-growth, high-mortality hypothesis. To explore how compensation may shape the species composition of herbivore and natural enemy populations, we present a mathematical model of a tri-trophic system incorporating both the nutritional quality of plants and herbivores, and the compensatory ability of herbivores and their natural enemies. Using this model we predict the abundance of herbivores and natural enemies, and some characteristics of the composition of species of insect communities along a gradient of plant nutritional quality. Specifically, we make the following predictions: 1) In the absence of natural enemies, the abundance of the juvenile herbivores increases with plant quality, and only highly compensating herbivores persist at low plant nutritional quality. 2) If natural enemies are present, the abundance of the juvenile herbivores decreases with increasing plant quality due to more effective suppression by the natural enemies. Poorly compensating herbivores increase while their highly compensating counterparts decrease with lowered plant quality. 3) When the plants have low nutritional quality, natural enemies will only persist when either very highly compensating herbivores are present or if the natural enemy itself is highly compensating. 4) The abundance of adult herbivores in a community with natural enemies can either increase or decrease with increasing plant quality depending on the compensatory abilities of herbivores and natural enemies.     

Abiotic factors promote plant heterogeneity and influence herbivore performance and mortality in Gambel's oak (Quercus gambelii)

Environmental gradients are expected to alter the relative effects of host‐plants and natural enemies on phytophagous insects. Moreover, studies of gradients may assist in an identification of the factors important to the outcomes of trophic interactions. We investigated the role of elevation‐based variation in environmental conditions (temperature and relative humidity) and foliar nitrogen on tri‐trophic interactions in Quercus gambelii Nutt. (Fagaceae) during 2001 and 2002. Quercus gambelii displayed significant elevational and seasonal fluctuation in foliar nitrogen content, and sites with similar environmental conditions produced similar foliar quality. However, leaf‐miners, Phyllonorycter spec. (Lepidoptera: Gracillariidae) and Cameraria spec. (Lepidoptera: Gracillariidae), did not perform better on trees with a greater nitrogen content. Overall densities of both species declined significantly in 2002, most likely due to severe drought conditions in the south‐western USA. Both species exhibited significant, but distinct, patterns in emergence rate with elevation. While environmentally based fluctuation in foliar nitrogen failed to predict the result of trophic interactions, site environmental conditions, as measured by temperature and relative humidity, were strongly related to differences in leaf‐miner performance and mortality. The ordination of sites by variation in environmental conditions accurately predicts the relative effect of unexplained vs. natural‐enemy sources of mortality for leaf‐miners.     

Consequences for a host–parasitoid interaction of host-plant aggregation, isolation, and phenology

Abstract.  1. Spatial habitat structure can influence the likelihood of patch colonisation by dispersing individuals, and this likelihood may differ according to trophic position, potentially leading to a refuge from parasitism for hosts.
2. Whether habitat patch size, isolation, and host-plant heterogeneity differentially affected host and parasitoid abundance, and parasitism rates was tested using a tri-trophic thistle–herbivore–parasitoid system.
3.  Cirsium palustre thistles ( n = 240) were transplanted in 24 blocks replicated in two sites, creating a range of habitat patch sizes at increasing distance from a pre-existing source population. Plant architecture and phenological stage were measured for each plant and the numbers of the herbivore Tephritis conura and parasitoid Pteromalus elevatus recorded.
4. Mean herbivore numbers per plant increased with host-plant density per patch, but parasitoid numbers and parasitism rates were unaffected. Patch distance from the source population did not influence insect abundance or parasitism rates. Parasitoid abundance was positively correlated with host insect number, and parasitism rates were negatively density dependent. Host-plant phenological stage was positively correlated with herbivore and parasitoid abundance, and parasitism rates at both patch and host-plant scales.
5. The differential response between herbivore and parasitoid to host-plant density did not lead to a spatial refuge but may have contributed to the observed parasitism rates being negatively density dependent. Heterogeneity in patch quality, mediated by variation in host-plant phenology, was more important than spatial habitat structure for both the herbivore and parasitoid populations, and for parasitism rates.