Effect of leaf age on epicuticular wax alkanes in Rhododendron

Alkane distribution pattern in epicuticular wax of Rhododendron fortunei cv Admiral Piet Hein leaves remains unchanged from the age of 6 weeks to abscission.     

Climate change and cereal aphids: the relative effects of increasing CO2 and temperature on aphid population dynamics

Experimental evidence regarding the responses of cereal aphids to rising atmospheric CO₂ has been ambiguous. Some studies suggest increased population sizes under future CO₂ levels, others suggest decreased population sizes, and still others suggest little or no difference. Recently, Newman et al. (2003) constructed a general mathematical model of the aphid–grass interaction to investigate whether or not we should, in fact, expect a general aphid response to rising CO₂. They concluded that aphid populations are likely to be larger under future CO₂ concentrations if soil N levels are high, the aphid species' nitrogen requirement is low and the aphid species' density‐dependent response in winged morph production is weak. In that model, and in field experiments, CO₂ concentration influences aphid population dynamics through the effect it has on plant quality. However, future CO₂ concentrations are also likely to be accompanied by higher ambient temperatures, a combination that has received little focus to date. In the present paper, the Newman et al. model is used to consider the combined effects of increased CO₂ concentrations and temperature on aphid population sizes. It is concluded that, when both factors are elevated, aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone. This result has important implications for future experimentation.     

Resistance in sorghum to the shoot fly, Atherigona soccata: epicuticular wax and wetness of the central whorl leaf of young seedlings

Sorghum genotypes known to be resistant or susceptible to shoot fly, Atherigona soccata Rondani were examined by scanning electron microscopy for differences in epicuticular wax structure and wetness of the central leaf whorl. Two major types of wax structures were observed: shoot fly resistant and moderately resistant genotypes were characterised by a smooth amorphous wax layer and sparse wax crystals while susceptible genotypes possessed a dense meshwork of crystalline epicuticular wax. The density of wax crystals decreased from the third leaf to the seventh leaf stage and was related to both seedling age and leaf position. Water droplets on susceptible genotypes with dense wax crystals showed spreading at the edges indicating a tendency to wet easily. In resistant genotypes with less dense wax crystals the droplets remained intact and did not spread.     

Chemical composition of the epicuticular and intracuticular wax layers on the adaxial side of Ligustrum vulgare leaves

Previous research has shown that cuticular triterpenoids are exclusively found in the intracuticular wax layer of Prunus laurocerasus. To investigate whether this partitioning was species-specific, the intra- and epicuticular waxes were identified and quantified for the glossy leaves of Ligustrum vulgare, an unrelated shrub with similar wax morphology. Epicuticular wax was mechanically stripped from the adaxial leaf surface using the adhesive gum arabic. Subsequently, the organic solvent chloroform was used to extract the intracuticular wax from within the cutin matrix. The isolated waxes were quantified using gas chromatography with flame ionization detection and identified by mass spectrometry. The results were visually confirmed by scanning electron microscopy. The outer wax layer consisted entirely of homologous series of very-long-chain aliphatic compound classes. By contrast, the inner wax layer was dominated (80%) by two cyclic triterpenoids, ursolic and oleanolic acid. The accumulation of triterpenoids in the intracuticular leaf wax of a second, unrelated species suggests that this localization may be a more general phenomenon in smooth cuticles lacking epicuticular wax crystals. The mechanism and possible ecological or physiological reasons for this separation are currently being investigated.     

Effects of light and temperature on the composition of epicuticular wax of barley leaves

The amount of wax/cm² on expanding primary leaves of Bonus barley depends on both the photo- and thermoperiods in which the seedlings are grown. With a temperature cycle of 15–10°, transfer of dark grown leaves to the light stopped leaf expansion and after 24 hr yielded 2·5 times more wax/cm² than is characteristic for light grown leaves. This demonstrates that wax synthesis and extrusion is not directly correlated with leaf expansion. The relative amounts of the wax classes formed by the decarboxylation pathways (< 1%), the reductive pathways (89%) or only by elongation (10%) are the same in light and dark. Within the reductive pathways, however, light stimulates aldehyde formation. Both environmental parameters can strongly influence the chain length composition of the wax classes. In the light one chain length or one group of chain lengths dominates a given wax class. In the dark two prominent chain lengths or groups thereof are found. The major chain length in these two groups differs by two or more carbons.     

Effects of GNA and other mannose binding lectins on development and fecundity of the peach-potato aphid Myzus persicae

Three mannose-binding lectins were assayed in artificial diets for their toxic and growth-inhibitory effects on nymphal development of the peach-potato aphid Myzus persicae. The snowdrop (Galanthus nivalis) lectin GNA was the most toxic, with an induced nymphal mortality of 42% at 1500 g ml^–1 (30 M) and an IC₅₀ (50% growth inhibition) of 630 g ml^–1 (13 M). The daffodil (Narcissus pseudonarcissus) lectin NPA and a garlic (Allium sativum) lectin ASA induced no significant mortality in the range 10–1500 g ml^–1, but did result in growth inhibition of 59% (NPA) and 26% (ASA) at 1500 g ml^–1 (40 M for NPA, 63 M for ASA). All three lectins were responsible for a slight but significant growth stimulation when ingested at 10 g ml^–1, reaching +26%, +18% and +11% over the control values for the garlic lectin, the daffodil lectin and the snowdrop lectin, respectively. GNA, as well as the glucose/mannose binding lectin Concanavalin A, were also provided at sublethal doses throughout the life cycle of the aphids, and effects on adult performance were monitored. Adult survival was not significantly altered, but both lectins adversely affected total fecundity and the dynamics of reproduction, resulting in significant reduction in calculated r _ms (population intrinsic rate of natural increase) on lectin-containing diets. These effects are discussed in relation to the use of transgenic plants expressing these toxic lectins for potential control of aphid populations.     

Influence of temperature and daylength on population development ofAphis gossypii onCucurbita pepo

The life table parameters ofAphis gossypii Glover were evaluated in tow sets of experiments onCucurbita pepo. The first set was conducted at six constant temperatures (5°C increments from 10–35°C) with 12 h photophase. The second set of experiments was conducted at 6, 12, and 18 h photophase at 25°C. Nymphal survivorship was 100% at 15, 20, 25 and 30°C. However, it was 80 and 0% at 10 and 35°C, respectively. The optimum temperature for body length was 10°C, and body length decreased with increasing temperature.A. gossypii attained its optimum growth and reproduction at 25°C. At this temperature, the net reproductive rate (79.7), intrinsic rate of increase (0.496) and finite rate of increase (1.6) were largest while generation time (6.6 d) and population doubling time (1.4 d) were smallest. Temperatures above and below 25°C reduced the net reproductive rate, and the intrinsic and finite rates of increase. The intrinsic and finite rates of increase ofA. gossypii reared at 18 h photophase (0.53 and 1.7) were significantly higher than at 12 (0.43 and 1.5) and 6 h daylength (0.49 and 1.6).     

Influence of leaf epicuticular waxes from cerrado species on substrate selection byAtta sexdens rubropilosa

The influence of leaf epicuticular waxes of nine woody species from cerrado (a savanna ecosystem of Brazil) on the foraging behavior of the leaf-cutting antAtta sexdens rubropilosa Forel, 1908 (Formicidae, Attini) was investigated. Some waxes showed neither inhibitory nor attractive effect over a series of tests, while a second group caused a delayed rejection. Waxes of two species were shown to be strongly deterrent to the ants. Since different waxes caused different responses, it is concluded that their chemical composition, rather than hydrophobicity or mechanical constraints, accounts for the observed effects. Plant species with leaves rejected by the ants did not necessarily have repellent waxes. On the other hand, no species with acceptable leaves yielded waxes with deterrent effects.     

Glandular trichomes on perennial alfalfa affect host-selection behavior of Empoasca fabae

Selected behavior of Empoasca fabae (Harris) (Homoptera: Cicadellidae) was examined to elucidate resistance of commercially-available glandular-haired alfalfa to this key forage pest. The overall objective was to assess the effects of the glandular trichomes on the behavior of nymphs and adults. Studies of host-plant acceptance by E. fabae nymphs found first and third instars to show a higher degree of change in settling location on the glandular-haired FGplh13 alfalfa than on the nonglandular P5373 alfalfa. Nymphs also cleaned their tarsi more frequently when in contact with the glandular trichomes on FGplh13 alfalfa, and in addition a larger number of nymphs jumped/fell off the surface of FGplh13 alfalfa. The glandular trichomes on FGplh13 also impeded nymphal mobility more effectively than the nonglandular trichomes on P5373. No choice, time-course analysis of adult host-plant acceptance behavior found that adults settled less frequently on FGplh13 alfalfa with the glandular trichomes intact, than on either FGplh13 with the glandular trichomes removed, or P5373 alfalfa with the nonglandular trichomes intact or removed. Free choice, time-course analysis of adult host-preference behavior determined that at each observation, stems of the nonglandular P5373 alfalfa were preferred over FGplh13 alfalfa. Similarly, at each observation, stems of FGplh13 alfalfa with the glandular trichomes removed were preferred over FGplh13 with the glandular trichomes intact. These data provide additional evidence for the localization of a resistance factor in the glandular trichomes of FGplh13 alfalfa. An antixenotic resistance mechanism also appears to be present, which may function, in part, through a tactile avenue.     

Probing behaviour of Diuraphis noxia on five cereal species with different hydroxamic acid levels

The feeding behaviour of Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae) was electronically monitored on five cereal species (Triticum aestivum L., T. turgidum L., Secale cereale L. and x Triticosecale Wittmack) containing different hydroxamic acids levels and on Hordeum vulgare L., lacking these compounds, by means of a DC-System. With increasing DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) levels D. noxia showed less probing particularly before reaching the phloem phase and a lower percentage of aphids achieved sustained phloem ingestion. However, no significant correlation was found between DIMBOA levels and the total time of phloem feeding. These findings indicate that factors from the mesophyll and vascular tissues appear to be involved in the effects of resistance. The underlying mechanism, however, remains unclear. Thus, DIMBOA seems to be only part of the chemical defenses of the cereal plants that might be implicated in D. noxia resistance.     

Inhibitory effects of Hymenaea and Copaifera leaf resins on the leaf fungus, Pestalotia subcuticularis

Leaf resins in the leguminous genera Hymenaea and Copaifera may play a role in restricting infection by the associated leaf fungus, Pestalotia. We tested this hypothesis by assessing growth of the geographically widespread Pestalotia subcuticularis presented with different compositions of resins. Leaf resins of Hymenaea and Copaifera are composed of the same suite of sesquiterpene hydrocarbons, but these vary quantitatively to form discrete compositional patterns among individuals and populations. Leaf resins may also contain sesquiterpene oxides; caryophyllene oxide, which may have been formed from the caryophyllene precursor common in these resins, inhibited fungal growth in vitro.     

Effect of experience on in-flight orientation to host-associated cues in the generalist parasitoidLysiphlebus testaceipes

The effect of experience on the responsiveness of the aphidiid parasitoidLysiphlebus testaceipes (Cresson) (Hymenoptera: Aphidiidae) to host-associated cues was investigated using a wind-tunnel bioassay. Naive females were able to discriminate between uninfested wheat (Triticum aestivum L.) and wheat infested withSchizaphis gramimum (Rondani) (Homoptera: Aphididae), but oviposition experience significantly increased the parasitoid's propensity to respond to aphid-infested plants with upwind, targeted flight. The behavioural change associated with such experience was acquired rapidly (within five minutes) and persisted for at least 24 h. The parasitoid could be successfully conditioned to associate a novel odour with the presence of hosts, suggesting that the increase in response to aphid-infested plants which occurs as a result of experience is probably due to associative learning of olfactory cues from the plant-aphid complex.     

Influence of epicuticular waxes from white spruce and balsam fir on feeding behavior of the eastern spruce budworm

Fourth-instar larvae of the eastern spruce bud-worm Choristoneura fumiferanaClem. (Lepidoptera: Tortricidae) were shown in two-choice feeding tests to respond differently to epicuticular waxes from different white spruce and balsam fir trees. Larvae also showed different preferences for various wax fractions obtained by separation on silicic acid.Deceased.     

Leaf puncturing, feeding and oviposition behavior of Liriomyza trifolii

The leaf puncturing, ovipositional and feeding behavior of Liriomyza trifolii was investigated on a preferred and non-preferred host (chrysanthemum and tomato, respectively). The sequence of leaf-puncturing behavior was stereotypic on both hosts, with a high probability of transition from one event to the next once the sequence is initiated. Two types of leaf punctures were made: a large fan-shaped puncture without an egg and tubular-shaped punctures with or without an egg. Regardless of the type of leaf puncture created, the female always backed over the puncture to feed. Therefore, all leaf punctures can be considered as feeding punctures. Fewer slow abdominal thrusts were noted for flies during oviposition compared to leaf puncturing only on chrysanthemum and tomato. The major differences noted between hosts were that L. trifolii fed for a significantly shorter period and spent less time during the leaf puncturing sequence on tomato vs. chrysanthemum. This suggests a repellent/antifeedant present in the former host which is detected through gustatory mechanisms. The deposition of an epideictic pheromone during oviposition was not detected.

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Résumé L'étude des comportements de piqûre, de ponte et d'alimentation de Liriomyza trifolii a été réalisée sur hôtes préférés ou non (Chrysanthème ou tomate). Les séquences du comportement de piqûre de feuilles sont stéréotypées sur les deux plantes, avec une forte probabilité de transmission d'une séquence à la suivante quand le comportement a débuté.II y avait deux types de piqûres de feuilles: de larges piqûres coniques sans oeufs et des piqûres tubulaires avec ou sans oeuf. Sans considération du type de piqûre, la femelle retourne sur la piqûre pour s'alimenter. Ainsi, toutes les piqûres peuvent être considérées comme des piqûres alimentaires. Moins de lentes contractions abdominales ont été observées chez les mouches pondant que chez celles piquant uniquement, tant sur chrysanthème que tomate. Les principales différences observées entre hôtes ont été que L. trifolii s'alimentait pendant moins longtemps et passait moins de temps pendant la perforation sur tomate que sur chrysanthème. Ceci suggère la présence d'un répulsif/dissuadant chez la tomate qui serait décelé par la gustation. Aucun dépôt de phéronome épidéictique n'a été mis en évidence pendant la ponte.

     

Incidence of apple fruit and leaf surface metabolites on Cydia pomonella oviposition

Water soluble metabolites identified from surfaces of apple tree fruit and leaf stimulate oviposition in Cydia pomonella L. The effects of two artificial blends of primary metabolites representing fruit and leaf surfaces, respectively, and of components within the blends were examined on egg-laying after two time periods: 3 min and 25 min of darkness. An artificial mixture of six metabolites, viz., three sugar alcohols (sorbitol, quebrachitol, and myo-inositol) and three sugars (glucose, fructose, and sucrose) did stimulate egg-laying. Fructose, sorbitol, and myo-inositol are important components of the stimulatory blend. Contact durations may induce variations in egg-laying responses. After 3 min of darkness, there were no differences in numbers of females laying eggs nor in the numbers of eggs laid on cloths treated with the complete blends and the controls. There were, however, clear effects of groups of compounds and of individual compounds. Reduced blends without sugars and sugar alcohols were in many cases significantly less stimulatory than the complete blends and the controls. After 25 min of darkness, the proportions of females laying eggs as well as the numbers of eggs were higher after treatment with the complete blends than on the controls. The proportions of females laying eggs on cloths treated with the reduced blends were rather similar to the controls, whereas there were still significant effects on the numbers of eggs laid after treatment with reduced blends derived from fruits but not from leaves.     

Influence of polishing and abrasion on the diffusive conductance of leaf surface of Festuca arundinacea Schreb.

Abstract Leaf surfaces of Festuca arundinacea Schreb. were subjected to controlled polishing and abrasion on a test-bed designed to simulate components of abrasive wind damage. Both treatments substantially increased the leaf surface conductance, particularly the polishing treatment. Scanning electron micrographs of cryo-fixed leaf surfaces showed displacement and smoothing of the epicuticular waxes, damage to the cuticle, collapse of epidermal cells and fracture of trichomes. The importance of the epicuticular waxes in determining leaf diffusivity and permeability is discussed.     

Influence of epiphytic micro-organisms on leaf wettability: wetting of the upper leaf surface of Juglans regia and of model surfaces in relation to colonization by micro-organisms

Wetting of the upper leaf surface of Juglans regia L. and of model surfaces colonized by epiphytic micro-organisms was investigated by measuring contact angles of aqueous solutions buffered at different pH values. During June to October 1995, contact angles of aqueous solutions on the leaf surface of J. regia decreased by angles ranging from 12° (low pH values) to 25° at high pH values. At the end of this vegetation period, wetting was strongly dependent on pH showing significantly lower contact angles with alkaline solutions (pH 9·0) than with acidic solutions (pH 3·0). Contact angle titration measured angles on the leaf surface as a function of the pH of buffered aqueous solutions, covering a pH range from 3·0 to 11·0. Titration curves revealed inflection points around 7·5, indicating the existence of ionizable carboxylic groups at the interface of the phylloplane. Altered leaf-surface wetting properties observed on the intact leaf surface could be simulated in model experiments by measuring contact angles on artificial surfaces colonized by Pseudomonas fluorescens and by epiphytic micro-organisms isolated from the phylloplane of J. regia . Strong evidence is provided that interfacial carboxylic groups derive from epiphytic micro-organisms present on the phylloplane. Results suggest that the age-dependent increase in, and pH dependence of, wetting as leaves mature are related to the presence of epiphytic micro-organisms on the phylloplane. Ecological consequences of increased leaf-surface wetting, concerning the structure of the leaf surface as a microhabitat for epiphytic micro-organisms, are discussed.     

Influence of aqueous leaf extract of common lambsquarters and NaCl salinity on the germination, growth, and nutrient content of wheat

Aqueous leaf extract of common lambsquarters (Chenopodium album L.) was evaluated alone or in combination with NaCl salinity for its influence on germination, seedling growth and contents of Na, K, and Ca by shoots and roots of wheat. The leaf extract and NaCl alone or in combination did not have any significant effect on germination, but the shoot and root lengths of seedlings and their dry weight decreased significantly by the treatments. Root growth was affected more than the shoot. The combination of leaf extract and NaCl drastically reduced the growth more than the separate effects of these stress treatments. The incorporation of leaf extract in the growth media decreased the content of Na by shoot, whereas the contents of K and Ca increased. NaCl treatments in combination with leaf extract increased the content of Na. Similar increases were observed for K and Ca in shoot, while these nutrients were increased in roots compared to control.     

Influence of Experience on the Response of Bathyplectes curculionis (Hymenoptera: Ichneumonidae), a Nonaphidophagous Parasitoid, to Aphid Odor

Bathyplectes curculionis (Thomson) is an introduced natural enemy of the alfalfa weevil in North America. The wasp requires carbohydrate foods as an adult. Adult wasps have increased longevity and fecundity when provided access to pea aphid, Acyrthosiphon pisum (Harris), honeydew in the laboratory, and adults respond positively to the presence of pea aphids in alfalfa fields. However, it is unknown how these wasps find aphid honeydew in the field. In a series of Y-tube olfactometer experiments, we evaluated the response of naïve and experienced adult female B. curculionis to odors from pea aphids, alfalfa, and pea aphids on alfalfa. Naïve adult females did not respond positively to pea aphid odor even when hungry. But adult females were able to learn aphid odor, and the mechanism of learning appears to be associative rather than by sensitization. Naïve females also showed no preference for alfalfa odor but learned alfalfa odor through sensitization. The wasps did not distinguish between alfalfa with aphids and alfalfa without aphids, even after exposure to aphids or alfalfa with aphids. However, they preferred pea aphid odor to alfalfa odor after a feeding experience in the presence of pea aphid odors. But after exposure to mixed odors of aphids and alfalfa while feeding, B. curculionis females preferred the odor of alfalfa to the odor of pea aphids. These results suggest that alfalfa odors mask or override aphid odors when aphids are associated with alfalfa (as happens naturally), thus interfering with the wasp's ability to respond to learned aphid odors. Therefore, although the wasps are capable of learning to find pea aphids and their honeydew in a simplified laboratory setting, it appears unlikely that they do so in the field.     

Fungal endophytes of wild barley and their effects on Diuraphis noxia population development

Laboratory experiments were conducted to compare the expression of Diuraphis noxia (Mordvilko) (Homoptera: Aphididae) resistance in four plant introduction (PI) lines of wild barley (Hordeum) infected with different species or strains of endophytic fungi (tribe Balansieae, family Clavicipitaceae, Neotyphodium gen. nov. [formerly Acremonium]). Aphid densities were significantly lower on endophyte-infected plants of PI 314696 (H. bogdanii Wilensky) and PI 440420 (H. brevisubulatum subsp. violaceum (Boissier & Hohenacker)), compared with densities on endophyte-free plants of both PI lines in population growth experiments. This endophyte- associated resistance was the result of antibiosis effects or starvation. In other experiments, endophyte-free plants of PI 269406 and PI 440413 (H. bogdanii) were not superior to endophyte-infected conspecifics as host plants of D. noxia. Our results demonstrate the influence of host plant species/genotype and endophyte species/strain on expression of aphid resistance, provide an explanation of the high levels of D. noxia resistance in PI 314696 and PI 440420 previously reported in the literature, and underscore the potential importance of endophytic fungi in conferring insect resistance in wild barley.