The role of the plant in attracting parasitoids: response to progressive mechanical wounding

Based on the model system of Brussels sprouts [Brassica oleracea var. gemmifera (Brassicaceae)], the herbivore cabbage white caterpillar, Pieris brassicae (L.) (Lepidoptera: Pieridae), and the parasitoid wasp, Cotesia glomerata (L.) (Hymenoptera: Braconidae), the influence of plant damage type, and damage duration were assessed on plant volatile emission and subsequent recruitment of natural antagonists of the herbivore. Plants were damaged by three methods for a period of either 3 or 8 h: herbivore damage (HD), progressive mechanical damage, and final mechanical damage inflicted in a single event. Wind‐tunnel bioassays evaluated whether the mode of damage affected female parasitoid oriented flight. After both periods of damage, all treatments were highly significantly preferred by naïve C. glomerata to undamaged control plants. After 3 h, herbivore‐damaged plants were significantly preferred to plants with final damage (FD). Most remarkably, following 8‐h damage, the parasitoid preferred both herbivore‐damaged and progressively damaged plants to plants with FD and did not significantly discriminate between herbivore and progressively damaged plants, thus indicating a similarity in plant response to herbivore and progressive mechanical damage. In addition to wind‐tunnel bioassays, emitted plant volatiles were collected and analysed by thermal desorption gas chromatography/mass spectrometry, following 3 and 8 h of damage in order to correlate volatiles released from different damage types with the attraction of the parasitoid. Differences in volatile profiles from all damage types were similar following both 3 and 8 h of damage, with only (Z)‐3‐hexenyl acetate found to be emitted in significantly higher quantities by final mechanical damage compared with HD after 3 h. In conclusion, the plant's response to progressive mechanical damage was more similar to HD than final mechanical damage deployed at a single point in time, irrespective of damage duration, and C. glomerata did not significantly discriminate between progressive damage and HD.     

Sequential rapid adaptation of indigenous parasitoid wasps to the invasive butterfly Pieris brassicae

The introduction of a new species can change the characteristics of other species within a community. These changes may affect discontiguous trophic levels via adjacent trophic levels. The invasion of an exotic host species may provide the opportunity to observe the dynamics of changing interspecific interactions among parasitoids belonging to different trophic levels. The exotic large white butterfly Pieris brassicae invaded Hokkaido Island, Japan, and quickly spread throughout the island. Prior to the invasion, the small white butterfly P. rapae was the host of the primary parasitoid Cotesia glomerata, on which both the larval hyperparasitoid Baryscapus galactopus and the pupal hyperparasitoid Trichomalopsis apanteroctena depended. At the time of the invasion, C. glomerata generally laid eggs exclusively in P. rapae. During the five years following the invasion, however, the clutch size of C. glomerata in P. rapae gradually decreased, whereas the clutch size in P. brassicae increased. The field results corresponded well with laboratory experiments showing an increase in the rate of parasitism in P. brassicae. The host expansion of C. glomerata provided the two hyperparasitoids with an opportunity to choose between alternative hosts, that is, C. glomerata within P. brassicae and C. glomerata within P. rapae. Indeed, the pupal hyperparasitoid T. apanteroctena shifted its preference gradually to C. glomerata in P. brassicae, whereas the larval hyperparasitoid B. galactopus maintained a preference for C. glomerata in P. rapae. These changes in host preference may result from differential suitability of the two host types. The larval hyperparasitoid preferred C. glomerata within P. rapae to C. glomerata within P. brassicae, presumably because P. brassicae larvae attacked aggressively, thereby hindering the parasitization, whereas the pupal hyperparasitoid could take advantage of the competition-free resource by shifting its host preference. Consequently, the invasion of P. brassicae has changed the host use of the primary parasitoid C. glomerata and the pupal hyperparasitoid T. apanteroctena within a very short time.     

Testing the efficiency of three 15N-labeled nitrogen compounds for indirect labeling of grasshoppers via plants in the field

In field studies of plant–insect herbivore interactions it is often difficult to establish which herbivore has fed on a particular plant. We investigated the suitability of three different ¹⁵N‐labeled nitrogen compounds (ammonium, nitrate, and glycine) for indirect marking of three grasshopper species [Omocestus viridulus (L.), Chorthippus parallelus (Zett.), and Chorthippus biguttulus (L.) (Orthoptera: Acrididae)] through labeling their food plants in the field. In two short‐term experiments grassland plots of 1 m² were separately labeled with either one of the different nitrogen compounds. Grasshoppers were caged on three food‐plant species [Dactylis glomerata L., Holcus lanatus L. (Poaceae), and Trifolium repens L. (Fabaceae)] present in these plots for 72 h. Significantly enriched δ¹⁵N values in grasshoppers were found in all plant/grasshopper combinations. Enrichment in grasshoppers was positively correlated with the enrichment of plants and labeling with nitrate resulted in highest ¹⁵N enrichment. In a long‐term experiment, individuals of C. biguttulus were placed in a cage covering an area of 1 m² for 37 days, with sampling of grasshoppers at regular intervals. δ¹⁵N values of the grasshopper and a common food plant, D. glomerata, increased steadily over time, up to 40‐fold by the end of the experiment. Our results demonstrate that ¹⁵N‐labeling of plants is an appropriate tool for the investigation of insect–plant interactions under natural conditions.     

INFLUENCE OF VARIOUS EXPOSURE PERIODS ON THE BIOMASS AND CHLOROPHYLL A OF CLADOPHORA GLOMERATA (CHLOROPHYTA)1

The biomass of Cladophora glomerata (L.) Kütz. was estimated at selected sites in the Colorado River between Glen Canyon Dam, Arizona, and River Kilometer 354. C. glomerata biomass was significantly higher at sites above Lees Ferry (25 km downstream from the Dam) than sites below the Ferry. Biomass and chlorophyll a were significantly reduced when C. glomerata was subjected to one-time exposures to the atmosphere for 12 daylight h in more. Repeated 12/12 h and 24/24 h (exposure/submergence) cycles over a two-week period also showed a significant reduction in biomass. The adaptations of C. glomerata to “stranding” during regulated flows are discussed.     

Feulgen Densitometry: Importance of a Stringent Hydrolysis Regime

Abstract: Feulgen densitometry is still a widely used method for DNA content measurements, but experimental procedures and results are often controversial. The present note is concerned with a recent report in the literature that optimum Feulgen staining required a remarkably longer hydrolysis time with 5 M HCI in Dactylis glomerata L. than in Hordeum vulgare L. (i.e., 62 min versus 20 min at 25 C). As this result is prone to question the usual practice of maintaining unified hydrolysis times for test material and internal standard, we established hydrolysis curves for D. glomerata, H. vulgare, Pisum sativum L. and Allium cepa L. at 20 C and 25C for 0 to 100 min. No striking differences between the species and, in particular, between Doctylis and Hordeum were found. Optimum staining occurred after 60 min with hydrolysis at 20 C and after 25 min at 25 C. It is strongly recommended to conduct the quantitative Feulgen reaction at a precisely controlled temperature instead of an inexact room temperature. The broader plateau of optimum staining at 20 C makes this regime preferable.     

Increasing growth temperature reduces the stimulatory effect of elevated CO2 on photosynthesis or biomass in two perennial species

We examined how anticipated changes in CO₂ concentration and temperature interacted to alter plant growth, harvest characteristics and photosynthesis in two cold-adapted herbaceous perennials, alfalfa ( Medicago sativa L. cv. Arc) and orchard grass ( Dactylis glomerata L. cv. Potomac). Plants were grown at two CO₂ concentrations (362 [ambient] and 717 [elevated] μmol mol⁻¹ CO₂) and four constant day/night temperatures of 15, 20, 25 and 30°C in controlled environmental chambers. Elevated CO₂ significantly increased total plant biomass and protein over a wide range of temperatures in both species. Stimulation of photosynthetic rate, however, was eliminated at the highest growth temperature in M. sativa and relative stimulation of plant biomass and protein at high CO₂ declined as temperature increased in both species. Lack of a synergistic effect between temperature and CO₂ was unexpected since elevated CO₂ reduces the amount of carbon lost via photorespiration and photorespiration increases with temperature. Differences between anticipated stimulatory effects of CO₂ and temperature and whole plant single and leaf measurements are discussed. Data from this study suggest that stimulatory effects of atmospheric CO₂ on growth and photosynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.     

Strategies providing success in a variable habitat: I. Relationships of environmental factors and dominance of Cladophora glomerata

Dominance of macrophytes and their response to environmental factors were studied in the river Ilm, Thuringia/ Germany with special reference to Cladophora glomerata (L.) Kütz. Macroalgae showed a growth peak in spring with C. glomerata, Ulothrix zonata Kütz., Lemanea fluviatilis (L.) C. Agardh and Audouinella sp. being the dominant species. Shortly after this peak, a rapid decline of macrophyte substrate coverage was observed. Only C. glomerata revealed a second growth peak in late summer/early autumn. Frequent disturbances of the macrophyte assemblage by floods resulted repeatedly in an almost complete wash out of benthic organisms. After summer floods C. glomerata was the species that recolonized the substrate. At high‐light sites, faster recovery of C. glomerata was observed as compared to low‐light sites. This is discussed in relation to the life cycle of C. glomerata. Among the physical and chemical parameters that were analysed, irradiance, current velocity, pH, soluble reactive phosphorus and ammonia‐nitrogen accounted for most of the observed patterns of dominance of C. glomerata.     

Potential allelopathic interference of Melilotus neapolitana metabolites on three coexisting species of Mediterranean herbaceous plant community

Abstract

This study is focused on the allelopathic interference of Melilotus neapolitana, an annual stenomediterranean species frequently occurring in the herbaceous plant community of the Mediterranean macchia, on three coexisting species of the Mediterranean herbaceous plant community. The ethereal and methanol extracts of the plant led to the isolation of 11 allelochemicals: coumarin, cis- and trans-melilotoside, the dihydromelilotoside, seven flavone glycosides, as well as the inactive β-sitosterol, a C13-norterpene glucoside and a polyphenol. All the structures have been elucidated on the basis of their spectroscopic features. The organic extracts and all the pure compounds have been assayed for their allelopathic activity on three coexisting species, Petrorhagia velutina, Dactylis hispanica, and Phleum subulatum. The cluster analysis has shown a clear structure-activity relationship, by evidencing four main groups of compounds with a specific effect patterns. Coumarin, the most abundant compound both in the plant extracts and soil, is found to be highly phytotoxic on all the tested species. As for the flavone glycosides, it is shown that aglycones play an important role due to their bioactivity.     

Root–shoot growth responses during interspecific competition quantified using allometric modelling

Background and Aims

Plant competition studies are restricted by the difficulty of quantifying root systems of competitors. Analyses are usually limited to above-ground traits. Here, a new approach to address this issue is reported.

Methods

Root system weights of competing plants can be estimated from: shoot weights of competitors; combined root weights of competitors; and slopes (scaling exponents, α) and intercepts (allometric coefficients, β) of ln-regressions of root weight on shoot weight of isolated plants. If competition induces no change in root : shoot growth, α and β values of competing and isolated plants will be equal. Measured combined root weight of competitors will equal that estimated allometrically from measured shoot weights of each competing plant. Combined root weights can be partitioned directly among competitors. If, as will be more usual, competition changes relative root and shoot growth, the competitors'' combined root weight will not equal that estimated allometrically and cannot be partitioned directly. However, if the isolated-plant α and β values are adjusted until the estimated combined root weight of competitors matches the measured combined root weight, the latter can be partitioned among competitors using their new α and β values. The approach is illustrated using two herbaceous species, Dactylis glomerata and Plantago lanceolata.

Key Results

Allometric modelling revealed a large and continuous increase in the root : shoot ratio by Dactylis, but not Plantago, during competition. This was associated with a superior whole-plant dry weight increase in Dactylis, which was ultimately 2·5-fold greater than that of Plantago. Whole-plant growth dominance of Dactylis over Plantago, as deduced from allometric modelling, occurred 14–24 d earlier than suggested by shoot data alone.

Conclusion

Given reasonable assumptions, allometric modelling can analyse competitive interactions in any species mixture, and overcomes a long-standing problem in studies of competition.