Age-Related Shifts in Leaf Chemistry of Clonal Aspen (Populus tremuloides)

Developmental changes in plant structure and function can influence both mammalian and arthropod feeding preferences for many woody plant species. This study documents age-related changes that occur in the leaf chemistry of trembling aspen (Populus tremuloides Michx., Salicaceae) and discusses implications for the herbivore community and ecosystem processes. We collected leaves from replicate ramets from six age classes (1–25+ yr) in each of seven aspen clones growing in south central Wisconsin, USA. Chemical analyses were conducted to determine concentrations of condensed tannins, phenolic glycosides (salicortin and tremulacin), nitrogen, starch, and soluble sugars. Each variable differed significantly among clones and among age classes. On average, condensed tannin concentrations doubled in the first five years and then remained fairly constant among older age classes. Combined phenolic glycoside (salicortin + tremulacin) concentrations were high in the youngest ramets (ca. 19%) and decreased sharply with age. Developmental changes in tannin, salicortin, and tremulacin concentrations exceeded those of nitrogen and carbohydrates. Developmental shifts of this magnitude, and the age-related tradeoff that occurs between condensed tannins and phenolic glycosides, are likely to have significant influence on the herbivore community of aspen and may influence leaf litter decomposition and nutrient cycling.     

Root Chemistry in Populus tremuloides: Effects of Soil Nutrients,Defoliation, and Genotype

Although genetic, environmental, and G x E effects on aboveground phytochemistry have been well documented in trembling aspen (Populus tremuloides), little work has focused on the same factors affecting tissues underground. Belowground plant defenses are likely important mediators of root-feeding herbivores that can strongly influence plant fitness. We used a common garden of potted aspen trees to explore the individual and interactive effects of soil nutrient availability, foliar damage, genotype, and their interactions, on concentrations of phytochemicals in aspen roots. Our common garden experiment employed 12 aspen genotypes that were planted into either low- or high-nutrient soil environments. Half of the trees were subjected to defoliation for two successive years, while the others were protected from damage. At the end of the growing season after the second defoliation, we harvested the trees to obtain root samples for which we assessed levels of phenolic glycosides, condensed tannins, nitrogen, and starch. Phenolic glycosides were most affected by genotype, while the other root phytochemicals were most responsive to soil nutrient conditions. The effects of defoliation were observed in interaction with soil nutrient environment and/or genotype. Interestingly, the effect of defoliation on phenolic glycosides was mediated by soil nutrients, whereas the effect of defoliation on condensed tannins was observed in concert with effects of both soil nutrients and genotype. Comparison of data from this study with an earlier, related study revealed that concentrations of phenolic glycosides and condensed tannins are lower in roots than leaves, and less responsive to defoliation. That soil nutrient environment affects root phytochemical concentrations is not unexpected given the intimate association of roots and soil, but the complex interactions between soil nutrients, aboveground damage, and genotype, and their effects on root phytochemistry, are intriguing. Variation in root chemistry could have wide-reaching effects on soil microbial communities, nutrient cycling, and herbivores. Additionally, the response of phytochemicals to damage across organs can link different, spatially separated herbivores as they use different parts of the same plant resource.     

Phytochemical Variation in Quaking Aspen: Effects on Gypsy Moth Susceptibility to Nuclear Polyhedrosis Virus

The performance of gypsy moths (Lymantria dispar) feeding on quaking aspen (Populus tremuloides) is strongly influenced by host foliar chemistry and susceptibility to a nuclear polyhedrosis virus (LdNPV), but the relationship of susceptibility to chemistry is poorly understood. We investigated the effects of genetic and resource-mediated variation in phytochemistry on viral pathogenicity. Trees were grown in pots in a common garden. Disks were punched from aspen leaves, inoculated with LdNPV and fed to third instars. Additional leaves were analyzed for levels of nitrogen, starch, phenolic glycosides, and condensed tannins. Despite marked variation among trees in levels of phenolic glycosides and tannins, we observed minimal variation in larval susceptibility to LdNPV. Viral pathogenicity was only weakly (inversely) correlated with tannin concentrations in one of two experiments. These results suggest that differential defoliation of aspen by gypsy moths in the field is due to the direct effects of host chemistry on larval performance rather than to the indirect effects of host chemistry on efficacy of this natural enemy.     

Performance and Secondary Chemistry of Two Hybrid Aspen (<Emphasis Type="Italic">Populus tremula</Emphasis> L. x <Emphasis Type="Italic">Populus tremuloides</Emphasis> Michx.) Clones in Long-Term Elevated Ozone Exposure

The effects of moderately elevated ozone (ca. 35 ppb) on the growth and secondary chemistry of the leaves of two soil-grown Finnish hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) clones with different ozone sensitivities were studied at an open-air exposure field in Kuopio, Finland. Stomatal conductance, photosynthetic rate, and chlorophyll fluorescence were measured during the third growing season. Foliar phenolic concentrations, ergosterol concentration of fine roots, and final dry mass of the trees were determined at the end of the third growing season. Elevated ozone increased the ectomycorrhizal status of the fine roots but had no effect on gas exchange or on the final biomass of either of the clones, indicating equal sensitivity to ozone and no effect of elevated ozone on the intraspecific competitive ability of the clones after three growing seasons. However, in agreement with the data from potted plants of the same clones after two growing seasons, significant differences between the clones were found in all parameters measured. A negative correlation between growth and high concentrations of foliar phenolics indicated that allocation to secondary chemistry also was costly in terms of growth under high resource availability.     

<Emphasis Type="Italic">De novo</Emphasis> Synthesis vs. Sequestration: Negatively Correlated Metabolic Traits and the Evolution of Host Plant Specialization in Cyanogenic Butterflies

Larvae of Heliconius butterflies (Nymphalidae: Heliconiinae) feed exclusively on cyanogenic leaves of Passiflora (passion vine). Most Heliconius manufacture cyanogenic glycosides (cyanogens) and some species sequester cyanogens from host plants. We compare ability to sequester simple monoglycoside cyclopentenyl (SMC) cyanogens and manufacture aliphatic cyanogens in 12 Heliconius species, including larvae that are specialized (single host species) and generalized (many host species). All butterflies tested higher for cyanide concentrations when reared on plants that larvae can sequester from (SMC plants) than when reared on plants that larvae do not sequester from (non-SMC plants). Specialists in the sara–sapho clade sequestered SMC cyanogens from specific host plants at seven times that of Passiflora generalists fed the same hosts. In contrast, sara–sapho clade species reared on non-SMC plants had significantly lower cyanide concentrations from de novo synthesis than generalists fed the same plants. Furthermore, cyanogen analyses indicated that Heliconius sara butterflies reared on an SMC host had a greater proportion of sequestered SMC cyanogens (95.0%) than de novo-synthesized aliphatic cyanogens (5.0%). Thus, sequestration and de novo synthesis are negatively correlated traits. Results suggest that losing the ability to synthesize cyanogens has restricted sara–sapho clade species to specific hosts containing SMC cyanogens and explains dietary restriction in this clade. We dedicate our paper to the memory of the Honorable Miriam Rothschild, F.R.S., a friend, colleague and supporter of many young scientists. Her enthusiasm and delight in chemical ecology and natural history continued to the last weeks of her life.     

Combined Effect of Intercropping and Turnip Root Fly (<Emphasis Type="Italic">Delia floralis</Emphasis>) Larval Feeding on the Glucosinolate Concentrations in Cabbage Roots and Foliage

The effects of plant competition and herbivory on glucosinolate concentrations in cabbage root and foliage were investigated in a cabbage-red clover intercropping system. Cabbage plants were grown under different competitive pressures and with varying degrees of attack by root-feeding Delia floralis larvae. Glucosinolate concentrations in cabbage were affected both by intercropping and by D. floralis density. Glucosinolate concentrations in foliage generally decreased as a response to intercropping, while the responses to insect root damage of individual glucosinolates were weaker. Root glucosinolates responded more strongly to both intercropping and egg density. Total root glucosinolate concentration decreased with clover density, but only at high egg densities. Increased egg density led to opposite reactions by the indole and aliphatic glucosinolates in roots. The responses of individual root glucosinolates to competition and root damage were complex and, on occasion, nonlinear. Reduced concentrations of several glucosinolates and the tendency towards a decrease in total concentration in cabbage foliage caused by intercropping and larval damage suggest that competing plants or plants with root herbivory do not allocate the same resources as unchallenged plants towards sustaining levels of leaf defensive compounds. This could also be true for root glucosinolate concentrations at high egg densities. In addition, the results suggest that changes occurring within a structural group of glucosinolates may be influenced by changes in a single compound, e.g., glucobrassicin (indol-3-ylmethyl) in foliage or sinigrin (2-propenyl) in roots.     

Response to Host Volatiles by Native and Introduced Populations of Dendroctonus valens (Coleoptera: Curculionidae, Scolytinae) in North America and China

Bark beetles (Coleoptera: Curculionidae, Scolytinae) have specialized feeding habits, and commonly colonize only one or a few closely related host genera in their geographical ranges. The red turpentine beetle, Dendroctonus valens LeConte, has a broad geographic distribution in North America and exploits volatile cues from a wide variety of pines in selecting hosts. Semiochemicals have been investigated for D. valens in North America and in its introduced range in China, yielding apparent regional differences in response to various host volatiles. Testing volatiles as attractants for D. valens in its native and introduced ranges provides an opportunity to determine whether geographic separation promotes local adaptation to host compounds and to explore potential behavioral divergence in native and introduced regions. Furthermore, understanding the chemical ecology of host selection facilitates development of semiochemicals for monitoring and controlling bark beetles, especially during the process of expansion into new geographic ranges. We investigated the responses of D. valens to various monoterpenes across a wide range of sites across North America and one site in China, and used the resulting information to develop an optimal lure for monitoring populations of D. valens throughout its Holarctic range. Semiochemicals were selected based on previous work with D. valens: (R)-(+)-α-pinene, (S)-(−)-α-pinene, (S)-(−)-β-pinene, (S)-(+)-3-carene, a commercially available lure [1:1:1 ratio of (R)-(+)-α-pinene:(S)-(−)-β-pinene:(S)-(+)-3-carene], and a blank control. At the release rates used, (+)-3-carene was the most attractive monoterpene tested throughout the native range in North America and introduced range in China, confirming results from Chinese studies. In addition to reporting a more effective lure for D. valens, we present a straightforward statistical procedure for analysis of insect trap count data yielding cells with zero counts, an outcome that is common but makes the estimation of the variance with a Generalized Linear Model unreliable because of the variability/mean count dependency.