大气CO2浓度升高对森林食叶昆虫的潜在影响

评述了大气CO₂浓度升高对森林食叶昆虫的影响,昆虫对森林取食为害水平的潜在变化,以及研究中的主要实验方法.大气CO₂浓度升高通过引起叶片化学变化进而影响食叶昆虫个体的取食和生长;但物种对环境变化反应的特异性、植物化学对高浓度CO₂的反应强度、昆虫对植物生理变化的敏感性和适应性、研究周期的长短、其它环境因子的协同效应以及不同实验中植物生长条件和研究方法的差异均将影响昆虫反应的方向和强度;CO₂气体浓度增高本身可能不足以对食叶昆虫个体的新陈代谢构成影响;大气CO₂浓度升高也可能影响森林食叶昆虫种群的大小.     

Potential effects of elevated carbon dioxide on leaf-feeding forest insects

The elevated concentration of atmospheric CO₂ may result in a decline of leaf nutritional quality (especially N) and an increase in some kinds of defensive secondary components (such as phenolics). The changes in the phytochemistry of trees, combined with the effect of elevated CO₂ per se, have a potential negative influence on insect herbivores. Here, we review the effect of elevated CO₂ on the performance of leaffeeding forest insects at individual-level and community-level. The elevated CO₂ per se have little influence on the metabolism of insects. Over half of the tree-insect experimental systems show that the performance of individual insect become poorer under high-CO₂ grown trees; but the others show that the insects have just little or no response to the treatments. The direction and magnitude of the changes in the performance of insects could be mediated by various factors. The effects of treatment are strongly species-dependent. The magnitude of changes in the phytochemistry, the sensitivity and adaptive capacity of insects to the poorer leaf quality, the differences in plant growth conditions and experimental methods, and the mediated effects of other environmental factors (such as soil nutrient availability, light, temperature, O₃) were all closely related to the final performance of insects. However, the larvae’s consumption usually increased under enriched CO₂ treatment, which was widely thought to be a compensatory response to poorer plant quality. The experiments on forest community-level found identically a reduction in herbivory, which was contrary to the results from small-scale experiments. The changes in insect population and the actual response of consumption by leaf-feeding forest insects under CO₂ enrichment remain unclear, and more field-based experiments need to be conducted. __________ Translated from Chinese Journal of Applied Ecology, 2006, 17(4): 720–726 [译自: 应用生态学报]     

Effects of elevated carbon dioxide and ozone on the phytochemistry of aspen and performance of an herbivore

The purpose of this study was to assess the independent and interactive effects of CO(2), O(3), and plant genotype on the foliar quality of a deciduous tree and the performance of a herbivorous insect. Two trembling aspen (Populus tremuloides Michaux) genotypes differing in response to CO(2) and O(3) were grown at the Aspen FACE (Free Air CO(2) Enrichment) site located in northern Wisconsin, USA. Trees were exposed to one of four atmospheric treatments: ambient air (control), elevated carbon dioxide (+CO(2); 560 microl/l), elevated ozone (+O(3); ambient x1.5), and elevated CO(2)+O(3). We measured the effects of CO(2) and O(3) on aspen phytochemistry and on performance of forest tent caterpillar (Malacosoma disstria Hübner) larvae. CO(2) and O(3) treatments influenced foliar quality for both genotypes, with the most notable effects being that elevated CO(2) reduced nitrogen and increased tremulacin levels, whereas elevated O(3) increased early season nitrogen and reduced tremulacin levels, relative to controls. With respect to insects, the +CO(2) treatment had little or no effect on larval performance. Larval performance improved in the +O(3) treatment, but this response was negated by the addition of elevated CO(2) (i.e., +CO(2)+O(3) treatment). We conclude that tent caterpillars will have the greatest impact on aspen under current CO(2) and high O(3) levels, due to increases in insect performance and decreases in tree growth, whereas tent caterpillars will have the least impact on aspen under high CO(2) and low O(3) levels, due to moderate changes in insect performance and increases in tree growth.     

Foliage of oaks grown under elevated CO2 reduces performance of Antheraea polyphemus (Lepidoptera: Saturniidae)

To understand how the increase in atmospheric CO2 from human activity may affect leaf damage by forest insects, we examined host plant preference and larval performance of a generalist herbivore, Antheraea polyphemus Cram., that consumed foliage developed under ambient or elevated CO2. Larvae were fed leaves from Quercus alba L. and Quercus velutina Lam. grown under ambient or plus 200 microl/liter CO2 using free air carbon dioxide enrichment (FACE). Lower digestibility of foliage, greater protein precipitation capacity in frass, and lower nitrogen concentration of larvae indicate that growth under elevated CO2 reduced the food quality of oak leaves for caterpillars. Consuming leaves of either oak species grown under elevated CO2 slowed the rate of development of A. polyphemus larvae. When given a choice, A. polyphemus larvae preferred Q. velutina leaves grown under ambient CO2; feeding on foliage of this species grown under elevated CO2 led to reduced consumption, slower growth, and greater mortality. Larvae compensated for the lower digestibility of Q. alba leaves grown under elevated CO2 by increasing the efficiency of conversion of ingested food into larval mass. Despite equivalent consumption rates, larvae grew larger when they consumed Q. alba leaves grown under elevated compared with ambient CO2. Reduced consumption, slower growth rates, and increased mortality of insect larvae may explain lower total leaf damage observed previously in plots in this forest exposed to elevated CO2. By subtly altering aspects of leaf chemistry, the ever-increasing concentration of CO2 in the atmosphere will change the trophic dynamics in forest ecosystems.     

Elevated CO2 reduces leaf damage by insect herbivores in a forest community

By altering foliage quality, exposure to elevated levels of atmospheric CO(2) potentially affects the amount of herbivore damage experienced by plants. Here, we quantified foliar carbon (C) and nitrogen (N) content, C : N ratio, phenolic levels, specific leaf area (SLA) and the amount of leaf tissue damaged by chewing insects for 12 hardwood tree species grown in plots exposed to elevated CO(2) (ambient plus 200 microl l(-1)) using free-air CO(2) enrichment (FACE) over 3 yr. The effects of elevated CO(2) varied considerably by year and across species. Elevated CO(2) decreased herbivore damage across 12 species in 1 yr but had no detectable effect in others. Decreased damage may have been related to lower average foliar N concentration and SLA and increased C : N ratio and phenolic content for some species under elevated compared with ambient CO(2). It remains unclear how these changes in leaf properties affect herbivory. Damage to the leaves of hardwood trees by herbivorous insects may be reduced in the future as the concentration of CO(2) continues to increase, perhaps altering the trophic structure of forest ecosystems.     

Experimental climate warming alters aspen and birch phytochemistry and performance traits for an outbreak insect herbivore

Climate change and insect outbreaks are key factors contributing to regional and global patterns of increased tree mortality. While links between these environmental stressors have been established, our understanding of the mechanisms by which elevated temperature may affect tree–insect interactions is limited. Using a forest warming mesocosm, we investigated the influence of elevated temperature on phytochemistry, tree resistance traits, and insect performance. Specifically, we examined warming effects on forest tent caterpillar (Malacosoma disstria) and host trees aspen (Populus tremuloides) and birch (Betula papyrifera). Trees were grown under one of three temperature treatments (ambient, +1.7 °C, +3.4 °C) in a multiyear open‐air warming experiment. In the third and fourth years of warming (2011, 2012), we assessed foliar nutrients and defense chemistry. Elevated temperatures altered foliar nitrogen, carbohydrates, lignin, and condensed tannins, with differences in responses between species and years. In 2012, we performed bioassays using a common environment approach to evaluate plant‐mediated indirect warming effects on larval performance. Warming resulted in decreased food conversion efficiency and increased consumption, ultimately with minimal effect on larval development and biomass. These changes suggest that insects exhibited compensatory feeding due to reduced host quality. Within the context of observed phytochemical variation, primary metabolites were stronger predictors of insect performance than secondary metabolites. Between‐year differences in phytochemical shifts corresponded with substantially different weather conditions during these two years. By sampling across years within an ecologically realistic and environmentally open setting, our study demonstrates that plant and insect responses to warming can be temporally variable and context dependent. Results indicate that elevated temperatures can alter phytochemistry, tree resistance traits, and herbivore feeding, but that annual weather variability may modulate warming effects leading to uncertain consequences for plant–insect interactions with projected climate change.     

Leaf age effects of elevated CO2-grown white oak leaves on spring-feeding lepidopterans

Folivorous insect responses to elevated CO₂-grown tree species may be complicated by phytochemical changes as leaves age. For example, young expanding leaves in tree species may be less affected by enriched CO₂-alterations in leaf phytochemistry than older mature leaves due to shorter exposure times to elevated CO₂ atmospheres. This, in turn, could result in different effects on early vs. late instar larvae of herbivorous insects. To address this, seedlings of white oak (Quercus alba L.), grown in open-top chambers under ambient and elevated CO₂, were fed to two important early spring feeding herbivores; gypsy moth (Lymantria dispar L.), and forest tent caterpillar (Malacosoma disstria Hübner). Young, expanding leaves were presented to early instar larvae, and older fully expanded or mature leaves to late instar larvae. Young leaves had significantly lower leaf nitrogen content and significantly higher total nonstructural carbohydrate:nitrogen ratio as plant CO₂ concentration rose, while nonstructural carbohydrates and total carbon-based phenolics were unaffected by plant CO₂ treatment. These phytochemical changes contributed to a significant reduction in the growth rate of early instar gypsy moth larvae, while growth rates of forest tent caterpillar were unaffected. The differences in insect responses were attributed to an increase in the nitrogen utilization efficiency (NUE) of early instar forest tent caterpillar larvae feeding on elevated CO₂-grown leaves, while early instar gypsy moth larval NUE remained unchanged among the treatments. Later instar larvae of both insect species experienced larger reductions in foliage quality on elevated CO₂-grown leaves than earlier instars, as the carbohydrate:nitrogen ratio of leaves substantially increased. Despite this, neither insect species exhibited changes in growth or consumption rates between CO₂ treatments in the later instar. An increase in NUE was apparently responsible for offsetting reduced foliar nitrogen for the late instar larvae of both species.     

Effects of elevated CO2 and temperature-grown red and sugar maple on gypsy moth performance

Few studies have investigated how tree species grown under elevated CO₂ and elevated temperature alter the performance of leaf‐feeding insects. The indirect effects of an elevated CO₂ concentration and temperature on leaf phytochemistry, along with potential direct effects on insect growth and consumption, may independently or interactively affect insects. To investigate this, we bagged larvae of the gypsy moth on leaves of red and sugar maple growing in open‐top chambers in four CO₂/temperature treatment combinations: (i) ambient temperature, ambient CO₂; (ii) ambient temperature, elevated CO₂ (+ 300 μL L^?1 CO₂); (iii) elevated temperature (+ 3.5°C), ambient CO₂; and (iv) elevated temperature, elevated CO₂. For both tree species, leaves grown at elevated CO₂ concentration were significantly reduced in leaf nitrogen concentration and increased in C: N ratio, while neither temperature nor its interaction with CO₂ concentration had any effect. Depending on the tree species, leaf water content declined (red maple) and carbon‐based phenolics increased (sugar maple) on plants grown in an enriched CO₂ atmosphere. The only observed effect of elevated temperature on leaf phytochemistry was a reduction in leaf water content of sugar maple leaves. Gypsy moth larval responses were dependent on tree species. Larvae feeding on elevated CO₂‐grown red maple leaves had reduced growth, while temperature had no effect on the growth or consumption of larvae. No significant effects of either temperature or CO₂ concentration were observed for larvae feeding on sugar maple leaves. Our data demonstrate strong effects of CO₂ enrichment on leaf phytochemical constituents important to folivorous insects, while an elevated temperature largely has little effect. We conclude that alterations in leaf chemistry due to an elevated CO₂ atmosphere are more important in this plant–folivorous insect system than either the direct short‐term effects of temperature on insect performance or its indirect effects on leaf chemistry.     

Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest

The unabated increase in global atmospheric CO(2) is expected to induce physiological changes in plants, including reduced foliar nitrogen, which are likely to affect herbivore densities. This study employs a field-based CO(2 )enrichment experiment at Kennedy Space Center, Florida, to examine plant-herbivore (insect) interactions inside eight open-topped chambers with elevated CO(2) (710 ppm) and eight control chambers with ambient CO(2). In elevated CO(2) we found decreased herbivore densities per 100 leaves, especially of leaf miners, across all five plant species we examined: the oak trees Quercus myrtifolia, Q. geminata, and Q. chapmanii, the nitrogen-fixing vine Galactia elliottii and the shrub Vaccinium myrsinites. Both direct and indirect effects of lowered plant nitrogen may influence this decrease in herbivore densities. Direct effects of lowered nitrogen resulted in increased host-plant related death and an increase in compensatory feeding: per capita herbivore leaf consumption in elevated CO(2) was higher than in ambient CO(2). Indirectly, compensatory feeding may have prolonged herbivore development and increased exposure to natural enemies. For all leaf miners we examined, mortality from natural enemies increased in elevated CO(2). These increases in host-plant induced mortality and in attack rates by natural enemies decreased leaf miner survivorship, causing a reduction in leaf miner density per 100 leaves. Despite increased leaf production in elevated CO(2) from the carbon fertilization effect, absolute herbivore abundance per chamber was also reduced in elevated CO(2). Because insects cause premature leaf abscission, we also thought that leaf abscission would be decreased in elevated CO(2). However, for all plant species, leaf abscission was increased in elevated CO(2), suggesting a direct effect of CO(2) on leaf abscission that outweighs the indirect effects of reduced insect densities on leaf abscission.     

Elevated CO2 interacts with herbivory to alter chlorophyll fluorescence and leaf temperature in Betula papyrifera and Populus tremuloides

Herbivory can influence ecosystem productivity, but recent evidence suggests that damage by herbivores modulates potential productivity specific to damage type. Because productivity is linked to photosynthesis at the leaf level, which in turn is influenced by atmospheric CO(2) concentrations, we investigated how different herbivore damage types alter component processes of photosynthesis under ambient and elevated atmospheric CO(2). We examined spatial patterns in chlorophyll fluorescence and the temperature of leaves damaged by leaf-chewing, gall-forming, and leaf-folding insects in aspen trees as well as by leaf-chewing insects in birch trees under ambient and elevated CO(2) at the aspen free-air CO(2) enrichment (FACE) site in Wisconsin. Both defoliation and gall damage suppressed the operating efficiency of photosystem II (ΦPSII) in remaining leaf tissue, and the distance that damage propagated into visibly undamaged tissue was marginally attenuated under elevated CO(2). Elevated CO(2) increased leaf temperatures, which reduced the cooling effect of gall formation and freshly chewed leaf tissue. These results provide mechanistic insight into how different damage types influence the remaining, visibly undamaged leaf tissue, and suggest that elevated CO(2) may reduce the effects of herbivory on the primary photochemistry controlling photosynthesis.     

Responses of tree and insect herbivores to elevated nitrogen inputs: A meta-analysis

Increasing atmospheric nitrogen (N) inputs have the potential to alter terrestrial ecosystem function through impacts on plant-herbivore interactions. The goal of our study is to search for a general pattern in responses of tree characteristics important for herbivores and insect herbivorous performance to elevated N inputs. We conducted a meta-analysis based on 109 papers describing impacts of nitrogen inputs on tree characteristics and 16 papers on insect performance. The differences in plant characteristics and insect performance between broadleaves and conifers were also explored. Tree aboveground biomass, leaf biomass and leaf N concentration significantly increased under elevated N inputs. Elevated N inputs had no significantly overall effect on concentrations of phenolic compounds and lignin but adversely affected tannin, as defensive chemicals for insect herbivores. Additionally, the overall effect of insect herbivore performance (including development time, insect biomass, relative growth rate, and so on) was significantly increased by elevated N inputs. According to the inconsistent responses between broadleaves and conifers, broadleaves would be more likely to increase growth by light interception and photosynthesis rather than producing more defensive chemicals to elevated N inputs by comparison with conifers. Moreover, the overall carbohydrate concentration was significantly reduced by 13.12% in broadleaves while increased slightly in conifers. The overall tannin concentration decreased significantly by 39.21% in broadleaves but a 5.8% decrease in conifers was not significant. The results of the analysis indicated that elevated N inputs would provide more food sources and ameliorate tree palatability for insects, while the resistance of trees against their insect herbivores was weakened, especially for broadleaves. Thus, global forest insect pest problems would be aggravated by elevated N inputs. As N inputs continue to rise in the future, forest ecosystem management should pay more attention to insect pest, especially in the regions dominated by broadleaves.     

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.     

No down-regulation of leaf photosynthesis in mature forest trees after three years of exposure to elevated CO2

The photosynthetic responses of six species of mature forest trees to long-term exposure to elevated CO2 (ca. 530 ppm) were determined at the Swiss Canopy Crane (SCC) site near Basel, Switzerland. In the third year of growth in elevated CO2, using web-FACE technology, net photosynthesis (As) in fully sunlit, upper canopy foliage was stimulated by ca. 40% compared to ambient controls. This enhancement did not differ from the instantaneous increase in As found in ambient-grown leaves that were temporarily measured at elevated CO2. A complete lack of down-regulation of photosynthesis was found in all species and in both the early and the late growing season. Neither was leaf nitrogen content significantly affected by long-term exposure to elevated CO2. Our results document a persistent enhancement in leaf level photosynthesis in response to elevated CO2 in mature forest trees over a period of three years. Circumstantial evidence suggests that the additional assimilates feed into large sinks other than stem and shoot growth.     

Seasonal variation in insect abundance among three Australian rain forests,with particular reference to phytophagous types

Monthly sweep net and light trap samples were used to examine seasonal changes in the abundance of insects in subtropical, warm and cool temperate rain forest in New South Wales. Maximum insect abundance, especially of phytophages, coincided with leaf flushing in the canopy trees. Cool temperate insect numbers were highest during the month just following the beech leaf flush, a rapid and synchronous growth event. Conversely, numbers of subtropical insects fluctuated over a longer period, in a pattern similar to the continuous growth of leaves that occurred throughout spring and summer there. The warm temperate was intermediate in its vegetation growth phenology and insect patterns. Rainforest insect abundance varied both temporally and spatially.     

Effects of elevated CO2-grown loblolly pine needles on the growth, consumption, development, and pupal weight of red-headed pine sawfly larvae reared within open-topped chambers

Seedlings of loblolly pine, Pinus taeda , were grown in open-topped chambers under four levels of CO₂: two ambient and two elevated. Larvae of the red-headed pine sawfly, Neodiprion lecontei , were reared from early instar to pupation, primarily on branches within chambers. Larval growth and mortality were assessed and leaf phytochemistry samples of immature and mature leaves collected weekly. Mature leaves grown under elevated CO₂ had significant reductions in leaf nitrogen and increases in non-structural carbohydrate contents, resulting in foliage being a poorer food source for larvae, i.e. higher carbohydrate:nitrogen ratio. Nutritional constituents of immature needles were unaffected by seedling CO₂ treatment. Volatile mono- and sesquiterpenes were unrelated to plant CO₂ treatments for either leaf age class. Larval consumption of immature needles significantly increased on seedlings grown under CO₂ enrichment, while mature needle consumption was not different between the treatments. The average weight gain per larva significantly declined in late instar larvae consuming elevated CO₂-grown needles. In spite of this reduced growth, neither the days to pupation nor pupal weights were different among the CO₂ treatments. This study suggests that enriched CO₂-induced alterations in pine needle phytochemistry can affect red-headed pine sawfly performance. However, compensatory measures by larvae, such as choosing to consume more nutritious immature needles, apparently helps offset enriched CO₂-induced reductions in the leaf quality of mature needles.     

Does large-scale N fertilization have time-delayed effects on insects community structure by changing oak quantity and quality?

The increased atmospheric deposition of nitrogen (N) may indirectly affect herbivorous insects by deposition-induced changes in host quantity and quality. To avoid the “lamp effect” that can occur in small-scale N fertilizations, large-scale N fertilization (ca. 9 ha, 100 kg N ha^?1 year^?1) experiments were performed in a deciduous, broad-leaved, cool temperate forest. The initial responses of mature oak canopy trees (Quercus crispula) and their herbivorous insects to N fertilization were evaluated by measuring the growth and leaf qualities of the trees. The feeding guilds and community structures of the herbivorous insects at control and fertilized sites before (2012) and after (2013–2014) N fertilization were then determined. In 2014, N fertilization enhanced plant growth. In 2013 but not 2014, N fertilization increased N content and decreased the carbon/nitrogen (C/N) ratio in leaves. Despite these changes in plant traits in 2013, N fertilization had no effect on either feeding guilds (chewing herbivory, galler density, and miner density) or community structures (species richness, diversity index, and relative abundance) of herbivorous insects in the same year. However, in 2014, the diversity index decreased significantly, whereas species richness and abundance were unchanged. This suggests that species-specific responses to changes in leaf qualities following N fertilization, in the form of altered insect fecundity, impact the diversity index of herbivorous insects, albeit with a 1-year lag time. Thus, our large-scale N fertilization experiment show the time-delayed bottom-up effects of N fertilization on insect community structure.     

How do herbivorous insects respond to drought stress in trees?

Increased frequency and severity of drought, as a result of climate change, is expected to drive critical changes in plant–insect interactions that may elevate rates of tree mortality. The mechanisms that link water stress in plants to insect performance are not well understood. Here, we build on previous reviews and develop a framework that incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought. Using this framework, we investigate in greater depth how insect performance responds to increasing drought severity for: (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. We outline how intermittent and moderate drought can result in increases of carbon‐based and nitrogen‐based chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. We predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap‐sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. We argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. Current evidence suggests that insect outbreaks are shorter and more spatially restricted in tropical than in temperate forests. We suggest that future research on the impact of drought on insect communities should include (i) assessing how drought‐induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores; (ii) food web implications for other insects and those that feed on them; and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO₂ levels. There is a need for larger, temperate and tropical forest‐scale drought experiments to look at herbivorous insect responses and their role in tree death.     

Interactive effects of elevated CO2 and temperature on the leaf-miner Dialectica scalariella Zeller (Lepidoptera: Gracillariidae) in Paterson's Curse, Echium plantagineum (Boraginaceae)

Doubling of the current atmospheric CO₂ concentration, and an increase in global mean annual temperatures of 1.5–6 °C, have been predicted to occur by the end of this century. Whilst the separate effects of CO₂ and temperature on plant–insect interactions have been examined in a number of studies, few have investigated their combined impact. We carried out a factorial experiment to explore the effect of a doubling of CO₂ concentration and a 3 °C temperature increase on the development of a complete generation of the leaf‐miner, Dialectica scalariella, in the host plant Paterson's Curse, Echium plantagineum. Elevated CO₂ increased biomass, reduced leaf N and increased C:N ratios in the host plants. Leaf thickness also increased under elevated CO₂, but only in the high‐temperature treatment. Female D. scalariella did not discriminate between plants grown at the different CO₂ levels when ovipositing, despite the reduction in foliage quality under elevated CO₂. Overall, the negative response of D. scalariella to elevated CO₂ was greater than for many species of free‐living insects, presumably because of the limited mobility imposed by the leaf‐mining habit. Development was accelerated at the high temperature and slowed under elevated CO₂. The net result was a reduction in development time of ～14 days in the elevated CO₂/high temperature treatment, compared to the ambient CO₂/low temperature treatment. Larval survivorship and adult moth weight were both affected by a significant interaction between CO₂ and temperature. At the low temperature, CO₂ had little effect on survivorship, but at the high temperature, survivorship was significantly reduced under elevated CO₂. Similarly, elevated CO₂ had a stronger negative effect on adult moth weight when combined with the high‐temperature treatment. A possible explanation for these results is that the high temperature accelerated insect development to such an extent that the larvae did not have sufficient feeding time to compensate for the poorer quality of the foliage. The frequency with which interactions between CO₂ and temperature affected both plant and insect performance in this study highlights the need for caution when predicting the effects of future climate change on plant–insect interactions from single‐factor experiments.     

Leaf dynamics and insect herbivory in a Eucalyptus camaldulensis forest under moisture stress

Abstract The influence of soil moisture content on leaf dynamics and insect herbivory was examined between September 1991 and March 1992 in a river red gum (Eucalyptus camaldulensis) forest in southern central New South Wales. Long-term observations of leaves were made in trees standing either within intermittently flooded waterways or at an average of 37. 5m from the edge of the waterways. The mean soil moisture content was significantly (P≤0.05) greater in the waterways than in the non-flooded areas. Trees in the higher soil moisture regime produced significantly larger basal area increments and increased canopy leaf area. This increase in canopy leaf area was achieved, in part, through a significant increase in leaf longevity and mean leaf size. Although a greater number of leaves was initiated and abscissed per shoot from the non-flooded trees, more leaves were collected from litter traps beneath the denser canopies of the flooded trees. Consumption of foliage by insects on the trees subjected to flooding compared to the non-flooded trees was not significantly different. However, the relative impact of insect herbivory was significantly greater on the non-flooded trees. Leaf chewing was the most common form of damage by insects, particularly Chryso-melidae and Curculionidae. No species was present in outbreak during this study. Leaf survival decreased as the per cent area eaten per leaf increased. In addition, irrespective of the level of herbivory, leaf abscission tended to be higher in E. camaldulensis under moisture deficit. The influence of soil moisture content on the balance between river red gum growth and insect herbivory is discussed.     

Leaf quality and insect herbivory in model tropical plant communities after long-term exposure to elevated atmospheric CO2

Results from laboratory feeding experiments have shown that elevated atmospheric carbon dioxide can affect interactions between plants and insect herbivores, primarily through changes in leaf nutritional quality occurring at elevated CO₂. Very few data are available on insect herbivory in plant communities where insects can choose among species and positions in the canopy in which to feed. Our objectives were to determine the extent to which CO₂-induced changes in plant communities and leaf nutritional quality may affect herbivory at the level of the entire canopy. We introduced equivalent populations of fourth instar Spodoptera eridania, a lepidopteran generalist, to complex model ecosystems containing seven species of moist tropical plants maintained under low mineral nutrient supply. Larvae were allowed to feed freely for 14 days, by which time they had reached the seventh instar. Prior to larval introductions, plant communities had been continuously exposed to either 340 l CO₂ l^–1 or to 610 l CO₂ l^–1 for 1.5 years. No major shifts in leaf nutritional quality [concentrations of N, total non-structural carbohydrates (TNC), sugar, and starch; ratios of: C/N, TNC/N, sugar/N, starch/N; leaf toughness] were observed between CO₂ treatments for any of the species. Furthermore, no correlations were observed between these measures of leaf quality and leaf biomass consumption. Total leaf area and biomass of all plant communities were similar when caterpillars were introduced. However, leaf biomass of some species was slightly greater-and for other species slightly less (e.g. Cecropia peltata)-in communities exposed to elevated CO₂. Larvae showed the strongest preference for C. peltata leaves, the plant species that was least abundant in all communites, and fed relatively little on plants species which were more abundant. Thus, our results indicate that leaf tissue quality, as described by these parameters, is not necessarily affected by elevated CO₂ under relatively low nutrient conditions. Hence, the potential importance of CO₂-induced shifts in leaf nutritional quality, as determinants of herbivory, may be overestimated for many plant communities growing on nutrient-poor sites if estimates are based on traditional laboratory feeding studies. Finally, slight shifts in the abundance of leaf tissue of various species occurring under elevated CO₂ will probably not significantly affect herbivory by generalist insects. However, generalist insect herbivores appear to become more dependent on less-preferred plant species in cases where elevated CO₂ results in reduced availability of leaves of a favoured plant species, and this greater dependency may eventually affect insect populations adversely.