Search

The specificity of woody plant defense responses to different attacking herbivores is poorly known. We investigated the responses of black poplar (Populus nigra) to leaf feeding by three lepidopteran species (Lymantria dispar, Laothoe populi and Amata mogadorensis) and two leaf beetle species (Phratora vulgatissima and Chrysomela populi). Of the direct defenses monitored, increases in trypsin protease inhibitor activity and the salicinoid salicin were triggered by herbivore damage, but this was not herbivore-specific. Moreover, the majority of leaf salicinoid content was present constitutively and not induced by herbivory. On the other hand, volatile emission profiles did vary among herbivore species, especially between coleopterans and lepidopterans. Monoterpenes and sesquiterpenes were induced in damaged and adjacent undamaged leaves, while the emission of green leaf volatiles, aromatic and nitrogen-containing compounds (known to attract herbivore enemies) was restricted to damaged leaves. In conclusion, indirect defenses appear to show more specific responses to attacking herbivores than direct defenses in this woody plant.

Plant volatiles influence host selection of herbivorous insects. Since volatiles often vary in space and time, herbivores (especially polyphagous ones) may be able to use these compounds as cues to track variation in host plant quality based on their innate abilities and previous experience. We investigated the behavioral response of naïve (fed on artificial diet) and experienced (fed on poplar) gypsy moth (Lymantria dispar) caterpillars, a polyphagous species, towards constitutive and herbivore-induced black poplar (Populus nigra) volatiles at different stages of herbivore attack. In Y-tube olfactometer assays, both naïve and experienced caterpillars were attracted to constitutive volatiles and volatiles released after short-term herbivory (up to 6 hr). Naïve caterpillars also were attracted to volatiles released after longer-term herbivory (24–30 hr), but experienced caterpillars preferred the odor of undamaged foliage. A multivariate statistical analysis comparing the volatile emission of undamaged plants vs. plants after short and longer-term herbivory, suggested various compounds as being responsible for distinguishing between the odors of these plants. Ten compounds were selected for individual testing of caterpillar behavioral responses in a four-arm olfactometer. Naïve caterpillars spent more time in arms containing (Z)-3-hexenol and (Z)-3-hexenyl acetate than in solvent permeated arms, while avoiding benzyl cyanide and salicyl aldehyde. Experienced caterpillars avoided benzyl cyanide and preferred (Z)-3-hexenyl acetate and the homoterpene (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) over solvent. Only responses to DMNT were significantly different when comparing experienced and naïve caterpillars. The results show that gypsy moth caterpillars display an innate behavioral response towards constitutive and herbivore-induced plant volatiles, but also that larval behavior is plastic and can be modulated by previous feeding experience. Electronic supplementary material The online version of this article (doi:10.1007/s10886-016-0698-7) contains supplementary material, which is available to authorized users.

Species of the Salicaceae produce phenolic compounds that may function as anti-herbivore defenses. Levels of these compounds have been reported to increase upon herbivory, but only rarely have these changes in phenolics been studied under natural conditions. We profiled the phenolics of old-growth black poplar (Populus nigra L.) and studied the response to gypsy moth (Lymantria dispar L.) herbivory in two separate field experiments. In a first experiment, foliar phenolics of 20 trees were monitored over 4 weeks after caterpillar infestation, and in a second experiment the bark and foliar phenolics of a single tree were measured over a week. Of the major groups of phenolics, salicinoids (phenolic glycosides) showed no short term response to caterpillar feeding, but after 4 weeks they declined up to 40 % in herbivore damaged and adjacent undamaged leaves on the same branch when compared to leaves of control branches. Flavonol glycosides, low molecular weight flavan-3-ols, and condensed tannins were not affected by herbivory in the first experiment. However, in the single-tree experiment, foliar condensed tannins increased by 10-20 % after herbivory, and low molecular weight flavan-3-ols decreased by 10 % in the leaves but increased by 10 % in the bark. Despite 15 % experimental leaf area loss followed by a 5-fold increase in foliar jasmonate defense hormones, we found no evidence for substantial induction of phenolic defense compounds in old growth black poplar trees growing in a native stand. Thus, if phenolics in these trees function as defenses against herbivory, our results suggest that they act mainly as constitutive defenses.

Honey bees harvest resins from various plant species and use them in the hive as propolis. While there have been a number of studies concerning the chemical composition of this antimicrobial product, little is known about selective behavior and bee preference when different potential plant sources of resin are available. The main objective of this paper was to investigate some aspects of behavioral patterns of honeybees in the context of resin acquisition. Samples of propolis originating from temperate zones of Europe and the supposed botanical precursors of the product were analyzed. Taxonomical markers of bud resins of two white birch species, aspen, black poplar, horse-chestnut, black alder, and Scots pine were determined through GC-MS analysis. All these trees have been reported as sources of propolis, but comparisons of the chemical composition of their bud resins with the compositions of propolis samples from seven European countries have demonstrated the presence of taxonomical markers only from black poplar, aspen, and one species of birch. This suggests selective behavior during the collection of bud resins by honeybees. To examine the causes of such selectivity, the antimicrobial properties of bud resins were determined. Horse-chestnut resins had lower antimicrobial activity than the other resins which did not differ significantly. Electronic supplementary material The online version of this article (doi:10.1007/s10886-016-0708-9) contains supplementary material, which is available to authorized users.

The growth-differentiation balance hypothesis (GDBH) postulates that variation in resource availability can increase or decrease allocation to secondary metabolism, depending on how growth is affected relative to carbon assimilation. Growth and leaf area of black poplar (Populus nigra) increased substantially in response to increased nutrient availability, while net assimilation rate and photosynthesis were less strongly affected. In response, total phenolic glycoside concentrations declined, which is consistent with GDBH. Drought stress decreased net assimilation rate and photosynthesis as well as growth, while increasing total phenolic glycoside concentrations. This pattern does not follow GDBH, which predicts lower secondary metabolism when resource limitation decreases both growth and carbon assimilation. However, there was a strong negative correlation between growth and total phenolic glycoside concentration consistent with a trade-off between primary and secondary metabolism, a key premise of GDBH. Drought decreased the growth of gypsy moth (Lymantria dispar) larvae but had no effect on whitemarked tussock moth (Orgyia leucostigma). Increased nutrient availability had a positive linear effect on growth of whitemarked tussock moth, but no effect on gypsy moth. Treatment effects on gypsy moth corresponded closely with effects on total phenolic glycosides, whereas effects on whitemarked tussock moth more closely tracked changes in nutritional quality. Localized gypsy moth herbivory elicited rapid induced resistance to gypsy moth, with the effect being independent of water and nutrient availability, but did not affect whitemarked tussock moth, indicating that the effects of biotic and abiotic stress on insect resistance of trees can be species-specific.

To evaluate methods for promoting consumption of tamarisk plants by beavers (Castor canadensis), we determined the feeding responses by captive beavers to diets that contained tannins and sodium chloride (hereafter referred to as tamarisk diet). In two-choice tests, beavers consumed equivalent quantities of tamarisk diet and control diet. Treatment with polyethylene glycol and fructose did not increase beaver preferences for the tamarisk diet. When offered the choice of control diet and casein hydrolysate-treated control diet, beavers strongly avoided the latter, showing feeding deterring activity of casein hydrolysate. However, when tamarisk diet was the alternative to the deterrent treatment, beavers consumed similar quantities of the two diets. Finally, beaver foraging preferences for actual plant cuttings were assessed. Casein hydrolysate application to cuttings of black poplar (Populus nigra) and Scouler's willow (Salix scouleriana) reduced browsing of these highly preferred species and promoted a marked increase in browsing of tamarisk (Tamarix ramosissima). These results suggest that casein hydrolysate treatment of desirable riparian plant species such as Salix and Populus may promote beaver foraging of invasive tamarisk.