Sampling and interpretation of psyllid nymph counts in potatoes

Development of effective management practices for insect pests relies heavily on sampling methods to accurately detect and quantify emerging populations. Herein we describe a novel method to extract and count nymphs of potato psyllid, Bactericera cockerelli (?ulc) (Hemiptera: Triozidae), from leaves in commercial fields of potato, Solanum tuberosum L. (Solanaceae). The proposed sampling method (referred to as the leaf washing method, LWM) consists of: (1) immersing samples of infested leaves in cold water to remove dust and sand; (2) immersing leaves in >85 °C water for 5 s; (3) extracting psyllid nymphs from heated water by passing it through a sampling unit composed of a funnel with a fine mesh‐organza fabric inside and attached to a vacuum pump; and (4) removing organza fabric and count psyllid nymphs under a stereoscope. With five sampling units operating simultaneously, one person is able to process and count immatures from about 400 leaf samples in two normal work days. As examples of applications, the LWM was used to characterize: (1) the vertical distribution of potato psyllid nymphs in the potato canopy; and (2) the spatio‐temporal distribution of potato psyllid nymphs in three potato varieties. Using LWM, we showed that psyllid nymphs were most predominant in the middle portion of the canopy and that spatio‐temporal distributions of nymphs varied among potato varieties.     

Feeny revisited: condensed tannins as anti-herbivore defences in leaf-chewing herbivore communities of Quercus

Abstract. 1. Community level oak–tannin–insect patterns have been largely unexplored since Paul Feeny's ground‐breaking research. Two hypotheses were tested for Quercus velutina and Q. alba in the Missouri Ozarks: abundance and richness of leaf‐chewing herbivores are negatively correlated with foliar condensed tannin concentrations and variation in condensed tannin concentrations explains variation in herbivore community structure. 2. In 2001, foliar condensed tannins in the understorey and canopy of these two oak species were quantified simultaneously with censuses of herbivores in May, during leaf expansion, and in June and August, when leaves were fully expanded. Thirty‐eight of the 134 species encountered had densities sufficient to be analysed individually (n = 10). Of those, Acronicta increta (Noctuidae) and Attelabus sp. (Curculionidae), both oak specialists, were negatively correlated with condensed tannins in the canopy of Q. alba. One additional specialist, Chionodes pereyra (Gelechiidae), was marginally negatively correlated with condensed tannins in the understorey of Q. velutina. Understorey species richness of May Q. velutina herbivores was negatively correlated with condensed tannins, as were total canopy insect density and species richness of August herbivores on Q. alba. 3. Principal component analysis (PCA) of insect abundances indicated that understorey and canopy Q. velutina and Q. alba had different communities of leaf‐chewing insects. Furthermore, condensed tannin levels contributed significantly to variation in PCA scores for Q. velutina, explaining 25% of the total variation. 4. Overall, these results indicate that specialists were more likely than generalists both to correlate negatively with condensed tannins and to occur in lower tannin habitats; abundance and richness of both early and late season fauna correlated negatively with tannins; and species were more likely to correlate negatively with condensed tannins when feeding on Q. alba than on Q. velutina and when feeding in the canopy than in the understorey. Future studies of tannin–insect interactions should manipulate leaf quality in combination with manipulations of other factors that likely influence community structure.     

Effect of defoliation on concentrations of allelochemicals and soluble sugars in leaves of weeping birch (<Emphasis Type="Italic">Betula pendula</Emphasis> roth)

Effect of strong (75%) and complete (100%) artificial defoliation of weeping birch Betula pendula Roth on the dynamics of soluble sugars and phenols—flavonols, catechins, and tannins in leaves of damaged plants was investigated. Within the first 15 days after strong defoliation of birch, no changes were found in leaf contents of flavonol, catechin, and tannin. The concentration of sugars first increased but, on the 10th day after defoliation, it returned to the normal level. One year after strong defoliation, the lead concentrations of catechins and tannins in damaged trees increased, while the concentrations of flavonols and sugars did not differ from that in leaves of control trees. In two years after strong damage, the increased concentration of tannins was retained, while catechins and sugars remained at the control level. One year after complete (100%) artificial defoliation, the leaf concentrations of flavonols and sugars in damaged plants did not differ from that in control plants, while the leaf concentrations of catechins and tannins exceeded those in control plants. Two years after complete damage, the leaves contained an increased amount of tannins, whereas the amounts of catechins, flavonols, and sugars did not differ from the control levels.     

Gall-forming and free-feeding herbivory along vertical gradients in a lowland tropical rainforest: the importance of leaf sclerophylly

In contrast to most insect guilds, gall-forming insects are thought to reach highest diversity on sclerophyllous vegetation, such as Neotropical savannas and Mediterranean vegetation types. The water and nutrient stress endured by meristems of canopy trees in tall wet tropical rainforests may cause leaf sclerophylly. Hence, the upper canopies of such ecosystems may represent a suitable habitat for gall-forming insects. At the San Lorenzo Protected Area, Panama, we estimated free-feeding herbivory and gall densities within five sites in 2003 and 2004, by surveying leaves in vertical and horizontal transects. In each sample, we recorded leaf density (mature and young foliage), free-feeding herbivore damage and number of galls, including the presence of live larvae, parasitoids or fungi. We surveyed 43 994 leaves, including 231 plants and 73 tree and liana species. We collected 5014 galls from 17 host-plant species, including 32 gall species of which 59% were restricted to the canopy (overall infestation rates: 2.4% in 2003, 5.5% in 2004). In 2003, 16% of the galls were occupied by live larvae, against 5% in 2004. About 17–20% of leaves surveyed suffered from free-feeding herbivory. Leaf sclerophylly increased significantly with sampling height, while free-feeding herbivory decreased inversely. Conversely, the number of live galls collected in the canopy was 13–16 times higher than in the understorey, a pattern consistent across sites and years. Hence, the probability of gall survivorship increased with increasing leaf sclerophylly as death by fungi, parasitoids or accidental chewing were greater in the understorey. Increasing harsh ecophysiological conditions towards the upper canopy appear favourable to galls-forming population maintenance, in support of the hypothesis of harsh environment. Hence, gall diversity and abundance in the upper canopy of tall tropical forests are perhaps among the highest in the world.     

Effects of eucalypt nutritional quality on the Bog gum‐Victorian metapopulation of Ctenarytaina bipartita and implications for host and range expansion

1. Novel hosts present phytophagous insects with nutritional challenges which can cause host‐associated divergence. 2. The performance of the Bog gum‐Victorian metapopulation of Ctenarytaina bipartita Burckhardt et al. (Psylloidea:Aphalaridae), a psyllid whose range is being expanded by tree planting, on five species and genotypes of Eucalyptus endemic to south‐eastern Australia was quantified. Settling of females on two non‐hosts was also tested. 3. Female C. bipartita exhibited significant host‐associated plasticity in proctiger length (the body part used for oviposition into apical buds). Psyllids with longer wings and proctigers arose from a primary host. Fecundity varied significantly among hosts and was highest on a novel host. Hosts did not differ significantly in free amino acids (FAAs) but did differ in concentrations of essential amino acids (EAAs). However, nymphs did not differ significantly in EAAs. Surprisingly, fecundity was not related to total FAAs, availability of EAAs or concentrations of EAAs but was related to concentrations of four non‐EAAs. Mean fecundity per host was also positively related to the relative abundance of galloyl groups (associated with hydrolysable tannins). 4. Leaf age was more important to settling than eucalypt species; females settled on young leaves but this response was not related to leaf water content. 5. Planting the rare Eucalyptus kitsoniana in new habitats will expand the range of Bog gum‐Victorian C. bipartita and provide a bridge for the colonisation of novel eucalypts with buds suitable for oviposition. Host expansion by this metapopulation is not constrained by nutritional quality and may result in morphological divergence.     

Differences between understorey and canopy in herbivore community composition and leaf quality for two oak species in Missouri

1. From July 1994 to September 1995, at six censuses, the herbivore community associated with understorey (< 2.5 m height) and canopy (15–20 m) leaves of Quercus alba and Q. velutina was sampled in south-eastern Missouri, U.S.A. 2. Across all censuses, herbivore densities were not significantly different between canopy and understorey for Q. alba and Q. velutina, except in August 1994 when herbivore densities were 60% higher in the canopy on Q. alba. Little significant spatial variation in herbivore densities or community composition was found during the study years. 3. The herbivore community was diverse, consisting of 138 species of leaf-chewing insects. Species richness was significantly greater (by 5–20%) in the understorey than in the canopy for both tree species, and the relative abundance of the main families, different feeding guilds, and most common species differed significantly between understorey and canopy. 4. To determine the extent to which leaf quality explained the observed patterns, percentage nitrogen and protein binding capacity were measured in canopy and understorey leaves of Q. alba and Q. velutina. Per cent nitrogen was higher in canopy leaves for Q. velutina while protein binding capacity was higher in canopy leaves for Q. alba. 5. These results suggest that the herbivore community associated with these two species of Quercus comprises species that appear to respond individually to environmental and biological conditions encountered in the understorey and the canopy.     

Seasonal changes in the density of the symbionts Buchnera and Wolbachia of the aphid Tuberculatus macrotuberculatus on Quercus dentata

Seasonal changes in leaf traits and tannins, including hydrolysable tannins (HTs) and condensed tannins (CTs), affect aphid body size and demographic patterns. The aphid Tuberculatus macrotuberculatus Essig & Kuwana (Hemiptera: Aphididae) feeds on the leaves of the daimyo oak, Quercus dentata Thunberg (Fagaceae), does not alternate hosts, and is attended continuously by ants. Buchnera aphidicola Munson et al. (a γ‐proteobacterium, hereafter Buchnera) is the primary symbiont of most aphid species. It provides essential amino acids to host aphids. Wolbachia sp. (an α‐proteobacterium) is present in some aphid species. This study investigated the effects of seasonal tannin and leaf trait fluctuations in Q. dentata on aphid performance and Buchnera and Wolbachia densities in T. macrotuberculatus. As the season progressed, the water content and HT concentration in Q. dentata leaves decreased, CT concentration in Q. dentata leaves increased, and T. macrotuberculatus performance decreased. Buchnera density varied in accordance with host aphid performance, whereas Wolbachia density did not vary throughout the season, suggesting that although Buchnera depends on the host plant and host aphid performance for growth, Wolbachia may have a high tolerance for oligotrophic environments and may exist independent of the nutritional metabolism of the host aphid. Although the role of Wolbachia in T. macrotuberculatus remains unclear, it may be associated with resistance to parasitoid wasps and plant secondary metabolites.     

Boom and bust: rapid feedback responses between insect outbreak dynamics and canopy leaf area impacted by rainfall and CO2

Frequency and severity of insect outbreaks in forest ecosystems are predicted to increase with climate change. How this will impact canopy leaf area in future climates is rarely tested. Here, we document function of insect outbreaks that fortuitously and rapidly occurred in an ecosystem under free‐air CO₂ enrichment. Over the first 2 years of CO₂ fumigation of a naturally established mature Eucalyptus woodland, we continuously assessed population responses of three sap‐feeding insect species of the psyllid genera Cardiaspina, Glycaspis and Spondyliaspis for up to ten consecutive generations. Concurrently, we quantified changes in the canopy leaf area index (LAI). Large and rapid shifts in psyllid community composition were recorded between species with either flush (Glycaspis) or senescence‐inducing (Cardiaspina, Spondyliaspis) feeding strategies. Within the second year, two psyllid species experienced significant and rapid population build‐up resulting in two consecutive outbreaks: first, rainfall stimulated Eucalyptus leaf production increasing LAI, which supported population growth of flush‐feeding Glycaspis without impacting LAI. Glycaspis numbers then crashed and were followed by the outbreak of senescence‐feeding Cardiaspina fiscella that led to significant defoliation and reduction in LAI. For all three psyllid species, the abundance of lerps, protective coverings excreted by the sessile nymphs, decreased at e[CO₂]. Higher lerp weight at e[CO₂] for Glycaspis but not the other psyllid species provided evidence for compensatory feeding by the flush feeder but not the two senescence feeders. Our study demonstrates that rainfall drives leaf phenology, facilitating the rapid boom‐and‐bust succession of psyllid species, eventually leading to significant defoliation due to the second but not the first outbreaking psyllid species. In contrast, e[CO₂] may impact psyllid abundance and feeding behaviour, with psyllid species‐specific outcomes for defoliation severity, nutrient transfer and trophic cascades. Psyllid populations feeding on Eucalyptus experience rapid boom‐and‐bust cycles depending on availability of suitable foliage driven by rainfall patterns and leaf phenology.     

Trophic cascades in bell miner‐associated dieback forests: Quantifying relationships between leaf quality,psyllids and Psyllaephagus parasitoids

Coteries of the meliphagid bird Manorina melanophrys are associated with a form of eucalypt defoliation and recovery called bell miner‐associated dieback (BMAD). Through their defence of cooperative colony boundaries against other insectivorous birds, bell miners may foster greater abundances of lerp‐forming psyllids (Hemiptera: Aphalaridae), some of which reduce the lifespan of leaves. Trophic cascades in BMAD forests need to be understood to have a complete picture of regulatory processes. We studied relationships between leaf quality, psyllid and Psyllaephagus parasitoid/hyperparasitoid abundances within the Gondwana Rainforest World Heritage Area, NSW, Australia; our focal tree species were Eucalyptus propinqua and E. biturbinata. Eucalyptus biturbinata had tougher leaves than E. propinqua; leaf toughness of both species varied with site and tree. We found a statistically significant, negative relationship between toughness (surrogate for leaf age) and foliar nitrogen content; younger leaves had higher nitrogen contents. Both bell miner abundance and foliar nitrogen were positively correlated with psyllid abundance. The abundance of Glycaspis species (the psyllid that produces lerps with the highest sugar content) was more closely correlated with foliar nitrogen content than was the abundance of all five psyllid genera combined. We identified 14 Psyllaephagus spp./morphospecies, comprising 11 primary parasitoids and three hyperparasitoids. The abundance of all Psyllaephagus combined was positively correlated with the abundance of lerps. However, psyllid parasitism was not correlated with the abundance of lerps. The abundance of the three hyperparasitoids was positively correlated with the abundance of Psyllaephagus hosts. The availability of epicormic foliage (young, morphologically juvenile leaves produced following defoliation) is likely to alter the nutritional ecology underpinning the diversity and abundance of psyllid populations. Higher quality epicormic foliage should favour populations of Glycaspis species (by enhancing nymphal survival) creating lerp hotspots that induce residency by opportunistic bell miners. The positive contribution of induced amelioration, interacting with feedbacks from parasitoids and hyperparasitoids, to BMAD requires longitudinal investigation.     

Differential leaf traits of a neotropical tree Cariniana legalis (Mart.) Kuntze (Lecythidaceae): comparing saplings and emergent trees

Cariniana legalis is an emergent tree that reaches the upper canopy in Brazilian Semideciduous Forest. Spatial contrasts in microclimatic conditions between the upper canopy and understorey in a forest may affect morpho-physiological leaf traits. In order to test the hypothesis that the upper canopy is more stressful to leaves than a gap environment we compared emergent trees of C. legalis, 28–29 m in height to gap saplings, 6–9 m in height, for the following parameters: leaf area, leaf mass area (LMA the dry weight:leaf area ratio), leaf thickness, leaf anatomical parameters, stomata conductance, and chlorophyll a fluorescence. Leaves from emergent trees had smaller leaf areas but greater LMA compared to saplings. Leaf thickness, palisade layer thickness, and stomatal density were higher for emergent trees than for saplings. The opposite pattern was observed for spongy layer thickness and spongy/palisade ratio. Stomatal conductance was also higher for emergent tree leaves than for sapling leaves, but the magnitude of depression on stomatal conductance near midday was more pronounced in emergent trees. The potential quantum yield of photosystem II, as determined by the F _v/F _m ratio was lower for leaves from saplings. The lower values of stomatal conductance, indicating restriction in CO₂ diffusion into the mesophyll can be related to higher photoinhibition observed in the saplings. Leaves from emergent trees and saplings exhibited similar values for apparent electron transport rates and non-photochemical quenching. Our results suggest that changes in leaf traits could be associated to dry conditions at the upper canopy as well as to the ontogenetic transition between sapling/emergent tree life stages.     

A gall-inducing arthropod drives declines in canopy tree photosynthesis

Mature forest canopies sustain an enormous diversity of herbivorous arthropods; however, with the exception of species that exhibit large-scale outbreaks, canopy arthropods are thought to have relatively little influence on overall forest productivity. Diminutive gall-inducing mites (Acari; Eriophyoidae) are ubiquitous in forest canopies and are almost always highly host specific, but despite their pervasive occurrence, the impacts of these obligate parasites on canopy physiology have not been examined. We have documented large declines in photosynthetic capacity (approx. 60%) and stomatal conductance (approx. 50%) in canopy leaves of mature sugar maple (Acer saccharum) trees frequently galled by the maple spindle gall mite Vasates aceriscrumena. Remarkably, such large impacts occurred at very low levels of galling, with the presence of only a few galls (occupying approx. 1% of leaf area) compromising gas-exchange across the entire leaf. In contrast to these extreme impacts on the leaves of adult trees, galls had no detectible effect on the gas-exchange of maple saplings, implying large ontogenetic differences in host tolerance to mite galling. We also found a significant negative correlation between canopy tree radial increment growth and levels of mite galling. Increased galling levels and higher physiological susceptibility in older canopy trees thus suggest that gall-inducing mites may be major drivers of “age-dependent” reductions in the physiological performance and growth of older trees.     

The role of psyllids (Hemiptera: Psyllidae) and bell miners (Manorina melanophrys) in canopy dieback of Sydney blue gum (Eucalyptus saligna Sm.)

Abstract The presence of the aggressive, colony-forming honeyeater, Manorina melanophrys (bell miner), in the canopies of unhealthy eucalypts has been well reported. There is, however, some debate as to the actual mechanisms producing these unhealthy trees. To investigate further some of the processes that may be contributing to this form of canopy dieback, two field trials were carried out in Olney State Forest, near Wyong, New South Wales. The study site contained Eucalyptus saligna (Sydney blue gum) with canopy dieback and was occupied by a large colony of bell miners. Close examination of the foliage revealed a large and diverse suite of phytophagous insects, including at least 16 species of psyllid (Hemiptera: Psyllidae). In the first trial, the use of bird exclusion cages over selected branches significantly improved leaf survival compared to leaves exposed to a relatively high density of bell miners. It is proposed that colonization by bell miners may interfere with the efficacy of both other insectivorous birds (through aggressive interspecific territoriality) and the invertebrate predators and parasitoids. Interference with such regulatory factors may enable some phytophagous insect populations to rise to sustained damaging levels. In the second trial, an insecticide application combined with reduced competition from the dense understorey and neighbouring trees was required to significantly improve trunk diameter and crown condition scores. After 12 months, neither treatment, by itself, significantly improved both growth measures. Possibly both treatments were required because the E. saligna trees were suffering from another source of stress (e.g. drought) in addition to the relatively high level of insect attack.     

Interactions between oak tannins and parasite community structure: Unexpected benefits of tannins to cynipid gall-wasps

Plant species vary tremendously in the number of phytophagous species they support. May (1979) and Price (1980) proposed that some of this variation may be due to variation in biochemical defenses. We find that variation between oak species in leaf tannin levels is positively correlated with 1) variation in the numbers of species of leaf-galling cynipid wasps those trees host; and 2) the density of individual galls per oak leaf. We hypothesize that leaf and gall tannins serve a protective function for cynipids, decreasing the amount of cynipid larval mortality due to fungal infestation. This defensive function would explain the observed positive relationships between oak tannin levels and cynipid diversity as well as cynipid abundance.     

Effects of host shading on consumption and growth of the geometrid Epirrita autumnata: interactive roles of water, primary and secondary compounds

Shading is assumed to reduce allocation to plant phenolics and to defense in general. We here report the results of experimental shading of individual branches or whole canopies in mountain birch on foliar chemistry and on the growth and consumption of a geometrid, Epirritaautumnata. Branch‐wide shading tended to have at least as strong effects on both leaf chemistry and herbivore performance as canopy‐wide shading, indicating local responses of the host to shading. Responses to shading varied among the key leaf traits. Leaf water content was higher and toughness lower in shaded than in non‐shaded leaves. Leaf sugars were lower and protein‐bound and free amino acids higher in shaded than in control leaves. Sucrose and galactose were at high levels in unshaded branches adjacent to shaded ones, suggesting that partial shading enhanced translocation of sugars within canopies. Total phenolics and soluble proanthocyanidins were low in both shading treatments. Of the other phenolic groups, concentrations of gallotannins and cell‐wall‐bound proanthocyanidins did not differ between shaded and non‐shaded leaves. Epirrita larvae grew better in both types of shading treatments compared to either unshaded control trees or to unshaded branches in the branch‐shading trees. By far the most important correlate of larval growth was the amount of water consumed with leaf mass (r=0.94). When variance in water intake was standardized (also largely eliminating parallel variation in proteins), fructose and glucose still had significant positive correlations and proanthocyanidins negative with larval growth on control but not on shade leaves. Concentrations of several phenolic compounds correlated negatively with intake of dry matter and especially water, and different phenolics were important in shaded (gallotannins) and in control (flavonoids) leaves. Our findings strongly suggest that the effects of putatively defensive leaf traits on insect consumption and growth interact with nutritive leaf traits, particularly with water.     

Climbing plants in a temperate rainforest understorey: searching for high light or coping with deep shade?

Background and Aims

While the climbing habit allows vines to reach well-lit canopy areas with a minimum investment in support biomass, many of them have to survive under the dim understorey light during certain stages of their life cycle. But, if the growth/survival trade-off widely reported for trees hold for climbing plants, they cannot maximize both light-interception efficiency and shade avoidance (i.e. escaping from the understorey). The seven most important woody climbers occurring in a Chilean temperate evergreen rainforest were studied with the hypothesis that light-capture efficiency of climbers would be positively associated with their abundance in the understorey.

Methods

Species abundance in the understorey was quantified from their relative frequency and density in field plots, the light environment was quantified by hemispherical photography, the photosynthetic response to light was measured with portable gas-exchange analyser, and the whole shoot light-interception efficiency and carbon gain was estimated with the 3-D computer model Y-plant.

Key Results

Species differed in specific leaf area, leaf mass fraction, above ground leaf area ratio, light-interception efficiency and potential carbon gain. Abundance of species in the understorey was related to whole shoot features but not to leaf level features such as specific leaf area. Potential carbon gain was inversely related to light-interception efficiency. Mutual shading among leaves within a shoot was very low (<20 %).

Conclusions

The abundance of climbing plants in this southern rainforest understorey was directly related to their capacity to intercept light efficiently but not to their potential carbon gain. The most abundant climbers in this ecosystem match well with a shade-tolerance syndrome in contrast to the pioneer-like nature of climbers observed in tropical studies. The climbers studied seem to sacrifice high-light searching for coping with the dim understorey light.     

Arboreal Ant Abundance and Leaf Miner Damage in Coffee Agroecosystems in Mexico

Shaded coffee agroecosystems traditionally have few pest problems potentially due to higher abundance and diversity of predators of herbivores. However, with coffee intensification (e.g., shade tree removal or pruning), some pest problems increase. For example, coffee leaf miner outbreaks have been linked to more intensive management and increased use of agrochemicals. Parasitic wasps control the coffee leaf miner, but few studies have examined the role of predators, such as ants, that are abundant and diverse in coffee plantations. Here, we examine linkages between arboreal ant communities and coffee leaf miner incidence in a coffee plantation in Mexico. We examined relationships between incidence and severity of leaf miner attack and: (1) variation in canopy cover, tree density, tree diversity, and relative abundance of Inga spp. shade trees; (2) presence of Azteca instabilis, an arboreal canopy dominant ant; and (3) the number of arboreal twig‐nesting ant species and nests in coffee plants. Differences in vegetation characteristics in study plots did not correlate with leaf miner damage perhaps because environmental factors act on pest populations at a larger spatial scale. Further, presence of A. instabilis did not influence presence or severity of leaf miner damage. The proportion of leaves with leaf miner damage was significantly lower where abundance of twig‐nesting ants was higher but not where twig‐nesting ant richness was higher. These results indicate that abundance of twig‐nesting ants in shaded coffee plantations may contribute to maintenance of low leaf miner populations and that ants provide important ecosystem services in coffee agroecosystems.     

Covariation of fluctuating asymmetry, herbivory and chemistry during birch leaf expansion

Fluctuating asymmetry (FA) is used to describe developmental instability in bilateral structures. In trees, high FA of leaves has been assumed to indicate the level of environmental or genetic stress, and for herbivores leaves from such trees have been shown to be in some cases (though not invariably) of higher quality compared to trees with symmetrical leaves. We demonstrated that FA of birch leaves correlated positively with growth rate of leaves, and with the amount of leaf biomass consumed by larvae of the geometrid Epirrita autumnata. Since asymmetry per se cannot define leaf quality for a herbivore, we determined the biochemical compounds which covary with the degree of foliage FA, in order to elucidate relationships between leaf FA, chemistry and herbivory. High foliar FA was characteristic of birches with high initial concentrations, and rapid seasonal decline in the concentrations of gallic acid and hydrolysable tannins, and with rapid seasonal changes in the concentrations of flavonoid-glycosides and sugars. In contrast, leaf FA was not related to concentrations of proanthocyanidins, protein-bound amino acids or soluble phenylalanine, the precursor of proanthocyanidins and proteins with aromatic amino acids. The positive correlation between leaf FA and consumption by E. autumnata was presumably related to the previously demonstrated compensatory consumption of E. autumnata to high concentrations of foliar gallotannins. Furthermore, sugars are well-known feeding stimulants. We propose that the variable results in studies correlating leaf FA and herbivory may stem from variable chemical associations of FA in different plants and of species-specific effects of compounds on insects. Received: 15 July 1999 / Accepted: 24 September 1999     

Altitudinal patterns in host suitability for forest insects

Conspecific trees growing at high and low-elevations encounter different growing conditions and may vary in their suitability as hosts for herbivorous insects. Mountain tree populations may be more resistant to herbivory if low temperatures constrain growth more than they constrain photosynthesis, resulting in increased secondary metabolism (temperature hypothesis). Alternatively, mountain trees may be fertilized by atmospheric nitrogen deposition and become more palatable to insects (atmospheric deposition hypothesis). We evaluated these two hypotheses by comparing high- and low-elevation trees with insect bioassays and analyses of foliar nitrogen and condensed tannin. Contrary to the temperature hypothesis, high-elevation foliage had higher leaf nitrogen (six of six tree species) and allowed higher growth rates of Lymantria dispar larvae (five of six tree species). The nitrogen deposition hypothesis was broadly supported by measurements from two mountains showing that high-elevation trees tended to have higher leaf nitrogen, lower leaf tannins, and support higher insect growth performance than conspecific trees from lower elevations. The deposition hypothesis was further supported by fertilization studies showing that simulated atmospheric nitrogen deposition changed the foliar chemistry of valley trees to resemble that of high-elevation trees. Predictions that the altitudinal gradient in foliar chemistry and host suitability should be steepest on mountains receiving more deposition were largely not supported, but interpretations are complicated by lack of replication among mountains. In the northeastern United States, increased host suitability of high-elevation trees seems sufficient to influence the population dynamics and community composition of herbivores. Atmospheric nitrogen deposition offers a promising hypothesis to explain and predict some important spatial patterns in herbivory. Received: 21 September 1997 / Accepted: 12 June 1998     

Differential effects of host plant hybridization on herbivore community structure and grazing pressure on forest canopies

Interspecific hybridization in plants is known to have ecological effects on associated organisms. We examined the differences in insect herbivore community structure and grazing pressure on tree canopy leaves among natural hybrids and their parental oak species. We measured leaf traits, herbivore community structure, and grazing pressure on leaves of two oak species, Quercus crispula and Q. dentata, and their hybrids. The concentration of nitrogen in canopy leaves was greater in hybrids and in Q. dentata than in Q. crispula. The concentration of total phenolics was lower in hybrids than in Q. crispula. The concentration of condensed tannin was greater in hybrids than in Q. crispula. Relative herbivore abundance and species richness were greater on oak hybrids than on either parental species; herbivore species diversity and composition on hybrids were close to those on Q. crispula. Herbivore grazing pressure was lower on hybrids and Q. dentata than on Q. crispula. There was a negative correlation between herbivore grazing pressure and leaf nitrogen, suggesting that interspecific variation among oak taxa in herbivore pressure may be explained by leaf nitrogen; variation in herbivore community structure among oak taxa is likely to be controlled by polygenic leaf traits. Differing responses of (1) herbivore community structure and (2) herbivore grazing pressure to host plant hybridization may play important roles in regulating herbivore biodiversity in cool‐temperate forest canopies.     

Changes in Leaf Phenology are Dependent on Tree Height inAcer mono,a Deciduous Broad-leaved Tree

Ontogenetic changes in leaf phenology of a hardwood tree,Acer mono, were investigated in individuals in different size classes in a temperate forest. Leaf emergence was earliest in current-year seedlings, and was increasingly delayed with increasing height of the individual. The shorter the tree, the longer the duration of leaf emergence. Timing of leaf emergence of the dominant heterospecific canopy trees was almost identical to that of conspecific adults; understorey light then gradually decreased with expansion of canopy leaves. These traits indicate that smaller individuals that receive the least light in summer can acquire favourable light for a longer period in spring than taller plants even in a forest understorey, but the advantage decreases with increasing plant height. Changes in the duration of leaf emergence and leaf longevity in response to environmental light regime [sun (forest edge)vs. shade (forest understorey)], were greatest for current year seedlings but decreased with increasing plant height. These results suggest that the plastic response of leaf phenology in juvenile stages may reduce the risk of losing an entire cohort in spatially heterogenous environments in the understorey of temperate forests.