The role of foliar microfungi in mountain birch - insect herbivore relationships

We studied the effects of epiphytic and endophytic phyllosphere fungi and pathogenic birch rust fungus infection of mountain birch Betula pubescens ssp czerepanovit trees on the larval performance of leaf beetle Phratora potaris We assessed the effects of epiphytic fungi by growing larvae on leaves from trees with manipulated fungal densities We also monitored larval perfonnance and endophytic fungal densities among tree groups classified by herbivory or rust fungus densities The differences in expenmentally manipulated epiphytic fungal densities did not affect larval relative growth rates (RGR) of the species, instead we found significant tree effects Phratora polaris RGR was higher on trees with high level of herbivory than on trees with low herbivory, nevertheless, endophyte densities between these groupings did not differ In the rust fungus expenment, P polarts performance was lowest on trees with low infection compared to no and high infection trees We also did not find correlations among tree-specific endophyte densities and P polaris performance on high and low herbivory trees and trees classified by rust fungus infection Although antagonism among fungi and induction of tree defences cannot be excluded, we suggest that epiphytic and endophytic fungi of mountain birch have negligible effects on P polaris larval performance under natural conditions, probably due to mountain birch variability and a loose ecological connection between mountain birch and its epi- and endophytes Mountain birch and pathogenic birch rust have a more tightly linked relationship, which may also affect insect herbivores Still, leaf properties may play an important role and the effects will depend on the relative timing of the rust infection, herbivore development and changes in leaf quality     

Multiplicity of biochemical factors determining quality of growing birch leaves

Due to rapidly changing physical and biochemical characteristics of growing leaves, correlations between traits of foliage biochemistry and the performance indices of flush feeding herbivores may vary considerably following relatively minor changes in experimental conditions. We examined the effects of the seasonal and inter-tree variation of a comprehensive array of biochemical compounds on the success of an early season geometrid, Epirrita autumnata, feeding on maturing foliage of mountain birch, Betula pubescens ssp. czerepanovii. We monitored the concentrations of individual phenolics, sugars, total nitrogen, nitrogen of proteins, and nitrogen of soluble compounds, water and acetone-insoluble residue. Simultaneously we recorded larval consumption, physiological performance, growth, and pupal mass of E. autumnata. We found significant phenological changes in almost all leaf traits measured. In bioassays with half-grown leaves, leaf gallotannin concentrations showed a nonlinear effect: in trees with high foliar gallotannin concentrations (over 10 mg g⁻¹), physiological performance was strongly reduced by high gallotannin concentrations. In trees with lower gallotannin concentrations, on the other hand, larval growth was reduced by soluble proanthocyanidins, not gallotannins. Differences between high and low gallotannin trees largely depended on phenology, i.e., on the age of leaves. However, not all the differences in leaf traits between late (with high gallotannin concentrations at the time of the bioassay) and early flushing trees disappeared with leaf maturation, indicating that there is also phenology-independent variance in the tree population. In the full-grown leaves of all the study trees, low concentrations of water and of nitrogen of proteins (but not nitrogen of soluble compounds) were the main factors reducing pupal masses of E. autumnata, while neither gallotannin nor proanthocyanidins now played a significant role. The observed change in the factors underlying leaf quality (from gallotannins and proanthocyanidins to nitrogen and water) relate to the activity of the shikimate pathway and the formation of cell walls: gallotannins and proanthocyanidins are both produced in the pathway, and these tannins are assumed to contribute – via binding into cell walls – to tough and durable cell walls. Interestingly, low quality of leaves did not automatically translate into low foliar consumption (i.e., benefits to the tree). On the trees with young, high gallotannin leaves, larvae actually increased consumption on low quality foliage. In the group of trees with slightly more developed, low gallotannin leaves, the quality of leaves did not clearly modify amounts consumed. In full-grown leaves, low leaf quality strongly reduced leaf consumption. These results emphasize the strong influence of tree phenology on the relationships between biochemical compounds and the herbivore. Received: 30 November 1998 / Accepted: 1 March 1999     

Within-tree variability of mountain birch leaves causes variation in performance for Epirrita autumnata larvae

Within-tree variability in leaf characteristics of the mountain birch (Betula pubescens ssp. tortuosa) was bioassayed for the autumnal moth (Epirrita autumnata) by rearing larvae on birch leaves in a laboratory and measuring their growth, consumption rate, approximate digestibility, efficiency of conversion of ingested food and efficiency of conversion of digested food. Only short shoot leaves, i.e. leaves of the same age, were used. The highest hierarchical level, which included trees and ramets within trees, accounted for most of the total variance in almost all the measured traits. Short shoots (within branches) accounted for more of the variance than branches (within trees/ramets) in most of the traits. The results suggest that differences in leaf quality were reflected in larval growth mainly by differences in food utilization efficiencies (postingestive effects) and less by differences in consumption rate (preingestive effects). The observed within-tree variation is probably a consequence of the modular structure, sectoriality and partial functional independence of tree parts.     

Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores

Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g^?1), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully‐grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing‐induced changes in food quality.     

Larval gregariousness and neonate establishment of the eucalypt-feeding beetle Chrysophtharta agricola (Coleoptera: Chrysomelidae: Paropsini)

The selective advantage offered to individuals living within groups may relate to natural enemy defence, but in leaf feeding insects may also relate to overcoming plant defences, especially with respect to feeding establishment. We conducted a series of experiments focusing on neonate larval survival, examining the effect of group size and leaf age on the survival of a eucalypt-feeding beetle, Chrysophtharta agricola , which formed groups of up to 43 larvae on the foliage of Eucalyptus nitens in the field. In the laboratory, in the absence of natural enemies, we found that initial density, leaf age and damage to the leaf margin significantly affected larval survival. Survival of solitary first-instar larvae on young foliage was around 80% whereas on older foliage it was around 11%. Prior damage to the leaf margin significantly increased survival on older leaves to around 61%. Initial larval density also affected survival, although mortality was always significantly higher on older leaves. On older leaves the larval group size above which mortality increased no further was over two-fold that on young leaves. Observations of group feeding behaviour at each instar showed that the majority of larvae (75.7%) were aligned facing away from the feeding site and that only around 7.5%, or just 1–2 larvae per group, fed at any one time. Feeding larvae chewed the leaf edge by straddling the leaf margin. Measurements of leaf margins showed that older leaves had significantly thicker leaf margins and 'thickness' ratios (leaf margin to leaf lamina proper). In the field, approximately 85% of all larvae occurred on the first two expanded leaf pairs, and larval mortality was highest between eclosion and establishment of the first instar. However, beetles apparently did not adjust clutch size according to leaf age.     

Choice of feeding sites by moose during summer, the influence of forest structure and plant phenology

We observed forage and habitat selection in radio-collared moose at feeding sites in southeast Norway. Use of older forest increased from spring to autumn. Birch Betula spp. and bilberry Vaccinium myrtillus accounted for c. 75% of the diet. Occurrence of important forage plants, height of browse, and difference in phenology between plant species all appeared to play a role in moose selection of feeding sites. Shading influences moose forage by delaying plant phenology and possibly through its effect on leaf content of water and secondary compounds. On single birch trees, feeding was concentrated to the top branches at midsummer; during spring and autumn more leaves from side branches were eaten. Greater discrimination as vegetation matured was also evident from the wider variety of forage species used at midsummer. Selection of feeding sites was not related to density of important browse species. We propose that variations in light/shade conditions may play a role in moose choice of feeding sites and that moose on summer range will benefit from a heterogeneous mixture of plantations and older forest stands.     

Continuous and Fluctuating Leaf Phenology of Adults and Seedlings of a Shade‐tolerant Emergent Tree,Dipterocarpus sublamellatus,in Malaysia

Continuous leaf phenology, with sequential production of metamers, is usually found in pioneer species, but is rare in shade‐tolerant species. Because of the nature of ‘continuous’ growth, continuous phenology has seldom been related to meteorological factors. We studied the leaf demography of seedlings and tall trees of a shade‐tolerant emergent species, Dipterocarpus sublamellatus (Dipterocarpaceae), in Malaysia. Although leaf phenology of D. sublamellatus was classified as continuous, the rates of leaf production and leaf fall were not constant and greatly fluctuated during the census period. Seasonal patterns of leaf production and fall were not synchronized across tall trees in most combinations or between seedlings and tall trees. Mean number of leaves produced per month and its seasonal fluctuation were greater in tall trees than in seedlings. Moreover, relationships between leaf phenology and meteorological factors (monthly rainfall, net radiation, maximum and minimum temperature, and minimum relative humidity) differed among trees. This suggests that endogenous rhythms, i.e., differences in allocation of resources to growth and storage among trees, are important to fluctuations in leaf phenology. These patterns are in contrast to patterns found in various pioneer species with continuous phenology, in which rates of leaf production and fall are generally constant. Different patterns might be caused by differences in allocation to growth and storage between pioneer and shade‐tolerant species, and might be related to their regeneration strategies.     

Life history traits of pest and non-pest populations in the phytophagous ladybird beetle,Epilachna niponica (coleoptera, coccinellidae)

Life history traits of the phytophagous ladybird beetle,Epilachna niponica were compared between a non-pest population feeding on wild thistle and a pest population feeding on cultivated solanaceous crops, mainly potato. The pest population had larger females, a higher population growth rate, a more continuous oviposition schedule, and a shorter developmental period in the immature stages, compared with the non-pest population. The two populations showed no clear differences in phenology from spring to autumn, egg mass size, hatchability, and larval survival rate. Significant differences were found in number of eggs laid per female during the first and second periods, and in the developmental period of the immature stages. These life history traits were influenced primarily by food plant. A higher fecundity and shorter immature period appear to be readily altered by the host shift from thistle to potato.     

Poleward increase in feeding efficiency of leafminer Stigmella lapponica (Lepidoptera: Nepticulidae) in a latitudinal gradient crossing a boreal forest zone

Damage to plant communities imposed by insect herbivores generally decreases from low to high latitudes. This decrease is routinely attributed to declines in herbivore abundance and/or diversity, whereas latitudinal changes in per capita food consumption remain virtually unknown. Here, we tested the hypothesis that the lifetime food consumption by a herbivore individual decreases from low to high latitudes due to a temperature-driven decrease in metabolic expenses. From 2016 to 2019, we explored latitudinal changes in multiple characteristics of linear (gallery) mines made by larvae of the pygmy moth, Stigmella lapponica, in leaves of downy birch, Betula pubescens. The mined leaves were larger than intact leaves at the southern end of our latitudinal gradient (at 60°N) but smaller than intact leaves at its northern end (at 69°N), suggesting that female oviposition preference changes with latitude. No latitudinal changes were observed in larval size, mine length or area, and in per capita food consumption, but the larval feeding efficiency (quantified as the ratio between larval size and mine size) increased with latitude. Consequently, S. lapponica larvae consumed less foliar biomass at higher latitudes than at lower latitudes to reach the same size. Based on space-for-time substitution, we suggest that climate warming will increase metabolic expenses of insect herbivores with uncertain consequences for plant–herbivore interactions.     

Growth performance of Epirrita autumnata (Lepidoptera: Geometridae) on mountain birch: trees,broods, and tree x brood interactions

Summary The outcome of herbivore-host plant interactions is partly a function of variation within the two populations. We partitioned variance in herbivore growth performance into components attributable to differences between trees, differences between (full-sib) insect broods, and tree x brood interactions. Growth performance of Epirrita larvae feeding on a small (0.25 ha) population of mountain birch was greatly influenced by differences between individual trees. Up to 49% of the variation in insect growth rate was due to tree effects; 5th instar growth rates ranged from 0.38 to 0.56 mg·mg^-1·day^-1 across a sample of 8 trees. About 25% of the variation in pupal weights and larval periods was due to tree effects; on low quality trees larvae required a longer time to attain lower pupal weights. Differences between trees were also evident in physical and chemical characteristics of the leaves. Insect broods differed in the duration of the larval period (15% of the variance) which led to differences in the pupal weight attained (13% of the variance). However, brood-specific differences in growth rate were modest (6% in the 4th instar) or nonexistent (5th instar). There was no evidence for tree x brood interactions, which refutes the possibility of fine scale adaptation to particular tree phenotypes. Hypotheses to explain the existence of this variability, and to predict its evolutionary and ecological consequences, are advanced.     

Systematic within-tree variation in mountain birch leaf quality for a geometrid, Epirrita autumnata

Abstract.

• 1 We studied within-tree variation in leaf quality of the mountain birch, Betula pubescens ssp. tortuosa, for larvae of the autumnal moth, Epirrita autumnata.
• 2 The purpose of the study was to determine the possible occurrence of systematic differences in larval growth on short shoot leaves (i.e. leaves of the same age): among leaves facing in different compass directions, between leaves of lower and upper branches, among leaves on different positions within a branch and among leaves of different sizes within a short shoot. We also measured larval growth between short shoot and long shoot leaves (i.e. between leaves of different age).
• 3 The larvae grew best on leaves on the north side of trees and most poorly on south side leaves, the east and west sides being intermediate. Leaves from the upper branches supported larval growth better than leaves from the lower ones. The larvae grew better on the smallest leaf of each short shoot and were able to utilize it more efficiently than the two larger leaves. Short shoot leaves from the basal and middle parts of the upper branches of the trees were of better quality for the larvae than short shoot leaves from the tip part of the branches. The larval growth rate did not differ between short shoot and long shoot leaves. In general, within-tree variation in the larval growth rate was lower than variation among different trees.
• 4 Damage to leaves can decrease leaf quality for herbivores in the same year (rapidly inducible responses) or the following year(s) (delayed inducible responses). Our results show that systematic within-tree variation in larval growth can be as great as the effects of rapidly inducible responses and that variation among individual trees can be as great as the mean effects of delayed inducible responses.

     

Delayed inducible resistance against a leaf-chewing insect in four deciduous tree species

Summary Both mechanical damage to mountain birch foliage and rearing of moth larvae on the trees reduced the growth of Epirrita autumnata larvae reared on these trees in the following year. The effects of physical damage and some other cues from insects were additive. On bird cherry the performance of Epirrita larvae was equal on untreated trees and on trees artificially defoliated in the previous year, but larval growth was reduced on previously insect-damaged branches. With mountain ash just physical damage per se reduced the performance of Epirrita larvae. On Salix phylicifolia there were no significant differences in the growth or survival of Epirrita on untreated control bushes and on bushes with partial larval damage during the previous year. Among untreated control trees the growth and survivorship of Epirrita were higher on fast-growing willow and bird cherry than on the slow-growing mountain birch. Mountain birch and mountain ash, the two deciduous tree species adapted to nutrient-poor soils, showed delayed inducible resistance triggered by defoliation (artificial or insect-made). This supports the hypothesis that delayed inducible resistance may be a passive response due to nutrient-stress caused by defoliation. On the other hand, the additional increase in the resistance of mountain birch triggered by specific cues from insects suggests that this response may be an evolved defense against leaf-eating insects.     

Spatial responses of two herbivore groups to a geometrid larva on mountain birch

Direct or plant-mediated interactions between herbivores may modify their spatial distribution among and within plants. In this study, we examined the effect of a leaf-chewing geometrid, the autumnal moth (Epirrita autumnata), on two different herbivore groups, leaf rolling Deporaus betulae weevils and Eriocrania spp. leafminers, both feeding on mountain birch (Betula pubescens ssp. czerepanovii). The exact locations of herbivores within tree canopies were mapped during three successive summers. In the first 2 years, some trees were artificially colonized by eggs of the autumnal moth to induce both rapid and delayed resistance in the foliage. The natural infection levels of the pathogenic rust fungus (Melampsoridium betulinum), potentially involved in species interactions, were also recorded. At the level of the whole tree, the density of D. betulae leaf rolls was lower in trees infested by the autumnal moth in the same year. However, the feeding locations within trees were partly segregated: D. betulae favoured shadier branches, while E. autumnata preferred the sunny parts of the canopy. The autumnal moth did not affect current- or following-year density of leafminers at the tree or branch level. Trees infected by rust had fewer leafminers in the same summer than noninfected trees. There were no interaction effects between defoliation by the autumnal moth and rust infection, and no delayed effects on the abundance of other herbivores the following year. Taken together, these findings suggest that the autumnal moth has a negative, partially plant-mediated impact on D. betulae, and can reduce the extent of current-year defoliation caused by D. betulae. This may be beneficial for the mountain birch, since the greater part of D. betulae damage occurs around or after the end of the larval period of the autumnal moth, which may be a critical time for tree recovery after moth outbreaks.     

Experimental warming alters spring phenology of certain plant functional groups in an early successional forest community

Experimental study of the effects of projected climate change on plant phenology allows us to isolate effects of warming on life‐history events such as leaf out. We simulated a 2 °C temperature increase and 20% precipitation increase in a recently harvested temperate deciduous forest community in central Pennsylvania, USA, and observed the leaf out phenology of all species in 2009 and 2010. Over 130 plant species were monitored weekly in study plots, but due to high variability in species composition among plots, species were grouped into five functional groups: short forbs, tall forbs, shrubs, small trees, and large trees. Tall forbs and large trees, which usually emerge in the late spring, advanced leaf out 14–18 days in response to warming. Short forbs, shrubs, and small trees emerge early in spring and did not alter their phenology in response to warming or increased precipitation treatments. Earlier leaf out of tall forbs and large trees coincided with almost 3 weeks of increased community‐level leaf area index, indicating greater competition and a condensed spring green‐up period. While phenology of large trees and tall forbs appears to be strongly influenced by temperature‐based growth cues, our results suggest that photoperiod and chilling cues more strongly influence the leaf out of other functional groups. Reduced freeze events and warmer temperatures from predicted climate change will interact with nontemperature growth cues to have cascading consequences throughout the ecosystem.     

Host tree resistance against the polyphagous wood-boring beetle Anoplophora glabripennis

Anoplophora glabripennis (Motschulsky) (Coleoptera: Cerambycidae: Lamiini) is an invasive wood‐boring beetle with an unusually broad host range and a proven ability to increase its host range as it colonizes new areas and encounters new tree species. The beetle is native to eastern Asia and has become an invasive pest in North America and Europe, stimulating interest in delineating host and non‐host tree species more clearly. When offered a choice among four species of living trees in a greenhouse, adult A. glabripennis fed more on golden‐rain tree (Koelreuteria paniculata Laxmann) and river birch (Betula nigra L.) than on London planetree (Platanus × acerifolia (Aiton) Willdenow) or callery pear (Pyrus calleryana Decaisne). Oviposition rate was highest in golden‐rain tree, but larval mortality was also high and larval growth was slowest in this tree species. Oviposition rate was lowest in callery pear, and larvae failed to survive in this tree species, whether they eclosed from eggs laid in the trees or were manually inserted into the trees. Adult beetles feeding on callery pear had a reduced longevity and females feeding only on callery pear failed to develop any eggs. The resistance of golden‐rain tree against the larvae appears to operate primarily through the physical mechanism of abundant sap flow. The resistance of callery pear against both larvae and adults appears to operate through the chemical composition of the tree, which may include compounds that are toxic or which otherwise interfere with normal growth and development of the beetle. Unlike river birch or London planetree, both golden‐rain tree and callery pear are present in the native range of A. glabripennis and may therefore have developed resistance to the beetle by virtue of exposure to attack during their evolutionary history.     

SEASONAL AND INDIVIDUAL VARIATION IN LEAF QUALITY OF TWO NORTHERN HARDWOODS TREE SPECIES

Seasonal trends in five traits of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula allegheniensis Britt.) leaves thought to influence feeding by herbivores were measured from 17 May through 19 September, 1979. Total nitrogen and water contents declined and toughness increased through the growth season. These seasonal changes were more pronounced in sugar maple than in yellow birch. Total polyphenol contents and tanning coefficients of leaf extracts from both species reached a season high by the end of May and changed very little after that date; these patterns differ from those reported by several other investigators. Sugar maple leaf extracts exhibited much higher tanning coefficients than did those of yellow birch, a finding which is consistent with current plant defense theory. Significant differences in total polyphenol content and tanning coefficients were found between individual trees in yellow birch, but not sugar maple. The relationship between successional status, leaf quality traits, and variability in these traits in forest trees is discussed.     

Growth and nitrogen utilization in seedlings of mountain birch (Betula pubescens ssp. tortuosa ) as affected by ultraviolet radiation (UV-A and UV-B) under laboratory and outdoor conditions

 Growth patterns and nitrogen economy were studied in pot-grown seedlings of mountain birch subjected to different ultraviolet radiation under both laboratory and outdoor conditions at Abisko in northern Sweden. In the laboratory, nutrient supply, temperature, humidity, ultraviolet radiation-A (UV-A, 320–400 nm) and B (UV-B, 280–320 nm) were controlled, while photosynthetically active radiation (PAR, 400–700 nm) and photoperiod varied naturally. Under outdoor conditions nutrient supply was controlled, and the irradiation treatments were ambient and above-ambient UV-B using additional fluorescent lamps. Mountain birch nitrogen economy was affected by increased ultraviolet radiation, as reflected by a changed relationship between plant growth and plant nitrogen both in the laboratory and outdoors. In the laboratory enhanced UV-A decreased leaf area per unit plant biomass (leaf area ratio) but increased biomass productivity, both per unit leaf area (leaf area productivity) and per unit leaf nitrogen (leaf nitrogen productivity). Low levels of UV-B affected growth patterns and nitrogen economy in a similar way to enhanced UV-A. High levels of UV-B clearly decreased relative growth rate and nitrogen productivity, as leaf area ratio, leaf area productivity and leaf nitrogen productivity were all decreased. Under outdoor conditions above-ambient levels of UV-B did not alter growth or biomass allocation traits of the seedlings, whilst nitrogen productivity was increased. Mountain birch seedlings originating from different mother trees varied significantly in their responses to different ultraviolet radiation. Received: 10 April 1997 / Accepted: 19 September 1997     

Leaf flushing and shedding,bud and flower production,and stem elongation in tall birch trees subjected to increases in aboveground temperature

Key message

Tall birch trees allocate extra resource due to aboveground temperature elevation to bud and male flower production rather than to plant growth. Saplings increased only plant growth under warming. Size-dependent response should be considered.

Abstract

We experimentally heated canopy organs of tall birch trees (Betula ermanii Cham.; 18–20 m high) growing at a high latitude to determine how leaf phenology, plant growth, and bud and male flower production might shift in response to increases in aboveground temperature during global climate change. We warmed the canopies with infrared heat lamps fixed to steel pipe scaffolds built around the trees. The temperature of the warmed canopies increased by approximately 1 °C. Warming extended the length of the growing season of canopy leaves (by accelerating leaf flush and delaying leaf fall), and significantly increased the numbers of buds and male flowers per shoot. Bud production and shoot length were positively correlated in both warmed and control branches. However, warming did not increase canopy shoot lengths. The intercept value of the positive regression slope between bud production and shoot length for warmed branches was higher than that for control branches. Thus, canopy warming had a direct positive effect on the bud production but had no indirect effect via increases in shoot length. Our experiment showed that tall birch trees allocated extra resources made available by increased aboveground temperature to bud and male flower production rather than to plant growth.

     

Putting the insect into the birch–insect interaction

Leaf maturation in mountain birch (Betula pubescens ssp. czerepanovii) is characterized by rapid shifts in the types of dominant phenolics: from carbon-economic flavonoids aglycons in flushing leaves, via hydrolysable tannins and flavonoid glycosides, to carbon-rich proanthocyanidins (condensed tannins) in mature foliage. This shift accords with the suggested trade-offs between carbon allocation to plant defense and growth, but may also relate to the simultaneous decline in nutritive leaf traits, such as water, proteins and sugars, which potentially limit insect growth. To elucidate how birch leaf quality translates into insect growth, I introduce a simple model that takes into account defensive compounds but also acknowledges insect demand for nutritive compounds. The effects of defensive compounds on insect growth depend strongly on background variation in nutritive leaf traits: compensatory feeding on low nutritive diets increases the intake of defensive compounds, and the availability of growth-limiting nutritive compounds may modify the effects of defenses. The ratio of consumption to larval growth (both in dry mass) increases very rapidly with leaf maturation: from 2.9 to 9.8 over 2 weeks in June-July, and to 15 by August. High concentrations in mature birch leaves of "quantitative" defenses, such as proanthocyanidins (15-20% of dry mass), presumably prevent further consumption. If the same compounds had also protected half-grown leaves (which supported the same larval growth with only one third of the dry matter consumption of older leaves), the same intake of proanthocyanidins would have demanded improbably high concentrations (close to 50%) in young leaves. The model thus suggests an adaptive explanation for the high levels of "quantitative" defenses, such as proanthocyanidins, in low-nutritive but not in high-nutritive leaves because of the behavioral responses of insect feeding to leaf nutritive levels.