Influence of elevated CO2 on interspecific interactions at higher trophic levels

Abstract We report the results of a study investigating the influence of elevated CO₂ on species interactions across three trophic levels: a plant (Brassica oleracea), two aphid herbivores (the generalist Myzus persicae and the specialist Brevicoryne brassicae), and two natural enemies (the coccinellid Hippodamia convergens (ladybird) and the parasitoid wasp Diaeretiella rapae). Brassica oleracea plants reared under elevated CO₂ conditions (650 ppmv vs. 350 ppmv) were larger and had decreased water and nitrogen content. Brevicoryne brassicae reared on plants grown in elevated CO₂ were larger and accumulated more fat, while there was no change in M. persicae traits. Fecundity of individual aphids appeared to be increased when reared on plants grown in elevated CO₂. However, these differences were generally lost when aphids were reared in colonies, suggesting that such changes in plant quality will have subtle effects on aphid intraspecific interactions. Nevertheless, CO₂ treatment did influence aphid distribution on plants, with significantly fewer M. persicae found on the shoots, and B. brassicae was only found on senescing leaves, when colonies were reared on plants grown in elevated CO₂. We reared B. brassicae and M. persicae in competition on plants grown at both the CO₂ concentration treatments. We found a significantly lower ratio of M. persicae: B. brassicae on plants grown under elevated CO₂ conditions, strongly suggesting that increasing CO₂ concentrations can alter the outcome of competition among insect herbivores. This was also reflected in the distribution of the aphids on the plants. While the CO₂ treatment did not influence where B. brassicae were found, fewer M. persicae were present on senescing leaves under elevated CO₂ conditions. Changes in plant quality resulting from the CO₂ treatments did not appear to alter aphid quality as prey species, as the number consumed by the ladybird H. convergens, and the number parasitised by the parasitoid wasp D. rapae, did not change. To our knowledge, this study provides the first empirical evidence that changes in host plant quality mediated by increasing levels of CO₂ can alter the outcome of interspecific competition among insect herbivores.     

Drought‐stress and plant resistance affect herbivore performance and proteome: the case of the green peach aphid Myzus persicae (Hemiptera: Aphididae)

Little is known about the simultaneous effects of drought stress and plant resistance on herbivorous insects. By subjecting the green peach aphid Myzus persicae Sulzer to well‐watered and drought‐stressed plants of both susceptible and resistant peach (Prunus persica), the effects of both stressors on aphid performance and proteomics are tested. Overall, the influence of the water treatment on aphid performance is less pronounced than the effect of host plant genetic resistance. On the susceptible cultivar, aphid survival, host acceptance and ability to colonize the plant do not depend on water treatment. On the resistant cultivar, aphid survival and ability to colonize are higher on drought‐stressed than on well‐watered plants. A study examining the pattern of protein expression aiming to explain the variation in aphid performance finds higher protein expression in aphids on the drought‐stressed susceptible cultivars compared with the well‐watered ones. In the susceptible cultivar, the regulated proteins are related to energy metabolism and exoskeleton functionality, whereas, in the resistant cultivar, the proteins are involved with the cytoskeleton. Comparison of the protein expression ratios for resistant versus susceptible plants reveals that four proteins are down‐regulated in well‐watered plants and 15 proteins are down‐regulated in drought‐stressed plants. Drought stress applied to the susceptible cultivar induces the regulation of proteins in M. persicae that enable physiological adaptation to maintain an almost unaltered aphid performance. By contrast, for aphids on the resistant cultivar subjected to drought stress, the down‐regulation of proteins responds to an induced host susceptibility effect.     

Hoverfly oviposition response to aphids in broccoli fields

The oviposition response of predacious hoverflies (Diptera: Syrphidae) to Brevicoryne brassicae L. and Myzus persicae (Sulzer) (Homoptera: Aphididae) in commercial broccoli, Brassica oleracea var. italica L., Plenck (Brassicaceae), fields was investigated at two sites over the course of a growing season. The hoverfly oviposition responses to these aphid species on different parts of the broccoli plant canopy were also examined. There were no hoverfly eggs on broccoli plants without aphids, egg numbers were very low on plants with fewer than 50 aphids, and no peak in oviposition relative to aphid numbers was observed. Within individual plants that were colonized by aphids, there was some oviposition on individual leaves without aphids, and no hoverfly eggs were seen on leaves that had more than 400 aphids. Leaves in the broccoli plant canopy, and the datasets associated with them, were divided into three sections vertically, ‘upper’, ‘middle’, and ‘lower’. Brevicoryne brassicae was more abundant in the upper and middle canopy sections, while M. persicae was found mostly in the lower section. The rate of hoverfly oviposition per aphid was higher in the upper section than in the two other sections. Modeling of the oviposition response using logistic regression showed that the presence of hoverfly eggs was positively correlated with numbers of each aphid species and sampling date.     

UV‐B impact on aphid performance mediated by plant quality and plant changes induced by aphids

Plants face various abiotic and biotic environmental factors and therefore need to adjust their phenotypic traits on several levels. UV‐B radiation is believed to impact herbivorous insects via host plant changes. Plant responses to abiotic challenges (UV‐B radiation) and their interaction with two aphid species were explored in a multifactor approach. Broccoli plants [Brassica oleracea L. convar. botrytis (L.), Brassicaceae] were grown in two differently covered greenhouses, transmitting either 80% (high UV‐B) or 4% (low UV‐B) of ambient UV‐B. Three‐week‐old plants were infested with either specialist cabbage aphids [Brevicoryne brassicae (L.), Sternorrhyncha, Aphididae] or generalist green peach aphids [Myzus persicae (Sulzer), Sternorrhyncha, Aphididae]. Plants grown under high‐UV‐B intensities were smaller and had higher flavonoid concentrations. Furthermore, these plants had reduced cuticular wax coverage, whereas amino acid concentrations of the phloem sap were little influenced by different UV‐B intensities. Cabbage aphids reproduced less on plants grown under high UV‐B than on plants grown under low UV‐B, whereas reproduction of green peach aphids in both plant light sources was equally poor. These results are likely related to the different specialisation‐dependent sensitivities of the two species. The aphids also affected plant chemistry. High numbers of cabbage aphid progeny on low‐UV‐B plants led to decreased indolyl glucosinolate concentrations. The induced change in these glucosinolates may depend on an infestation threshold. UV‐B radiation considerably impacts plant traits and subsequently affects specialist phloem‐feeding aphids, whereas aphid growth forces broccoli to generate specific defence responses.     

Bottom‐up effects mediated by an organic soil amendment on the cabbage aphid pests Myzus persicae and Brevicoryne brassicae

Earthworm‐produced compost or vermicompost has been shown to increase resistance of plants to a variety of insect pests, but it is still unclear whether this resistance is dose dependent and whether the mechanisms responsible are the same for insect species with differing feeding habits and preferences. Therefore, we tested the effects of plants grown in various vermicompost concentrations (0, 20, 40, and 60%) on the preference and performance of generalist, Myzus persicae L., and specialist, Brevicoryne brassicae L. (both Hemiptera: Aphididae), aphid pests. Preference was evaluated with leaf disk (apterous) and whole plant (alate) choice assays. After 24 h of feeding, there was no significant negative effect on the feeding preference noted for apterae of either species of any of the treatments tested. To the contrary, apterae B. brassicae showed a significant preference for vermicompost treatments over control leaf disks. Alate M. persicae preferred alighting on control plants over vermicompost‐grown plants, but B. brassicae showed no preference toward any of the treatments tested. Both aphid species deposited significantly more nymphs on control plants than on those grown in 20% vermicompost. Furthermore, plants grown in soil amended with 20% vermicompost significantly suppressed mass accumulation, as well as numbers of adults and nymphs of both aphid species compared to controls. These data clearly show that vermicompost soil amendments can significantly influence pest aphid preference and performance on plants and that these effects are not dose dependent, but rather species and morph dependent.     

‘Myrosin cells’ are not a prerequisite for aphid feeding on oilseed rape (Brassica napus) but affect host plant preferences

The enzyme myrosinase (EC 3.2.3.1.147) is present in specialised myrosin cells and forms part of the glucosinolate–myrosinase system, also known as ‘the mustard oil bomb’, which has an important role in the defence system of cruciferous plants against insect pests. Transgenic Brassica napus MINELESS have been produced by transgenic ablation of myrosin cells. This prompted us to investigate the importance of myrosin cells in plant–aphid interactions. In order to study this, we challenged transgenic MINELESS and wild‐type cultivar Westar seedlings with the aphids Brevicoryne brassicae (a specialist) and Myzus persicae (a generalist). Our study included aphid free‐choice and aphid fecundity experiments. Data from these experiments showed that B. brassicae prefers wild‐type seedlings and M. persicae prefers MINELESS. B. brassicae and M. persicae showed significant variation in establishment on plants regardless of whether they were wild type or MINELESS and also differed significantly in affecting plant parts. Myrosinase activity in MINELESS control seedlings was 83.6% lower than the wild‐type control seedlings. Infestation with either of the two aphid species induced myrosinase levels in both wild‐type and MINELESS seedlings. Infestation with M. persicae reduced the concentration of most glucosinolates while B. brassicae had the opposite effect. B. brassicae enhanced the formation of glucosinolate hydrolysis products both in wild‐type and MINELESS seedlings. However, M. persicae decreased All ITC but increased 3,4ETBut NIT in wild‐type seedlings. Taken together, the investigation shows that the presence of myrosin cells affects the preference of generalist and specialist aphid species for Brassica napus plants.     

Synomone-mediated interactions between the parasitoidDiaeretiella rapae andBrevicoryne brassicae under field conditions

An interaction apparently mediated by synomones (Nordlund et al., 1981) betweenDiaeretiella rapae (M'Intosh), a primary parasitoid and the cabbage aphid,Brevicoryne brassicae (L.), was investigated under field conditions. Direct application of an allylisothiocyanate emulsion at a rate of 0.25 ml per broccoli plant consistently gave higher aphid parasitization rates and/or number of wasps per plant than those observed on plants treated with 0.25 ml of water or with 0.25 ml of wild mustard extract. These results suggest the existence of a synomonemediated interaction between the species involved, indicating potential avenues to enhance field parasitization rates through manipulation of the chemical environment of cole cropping systems.      

Cascading effects of N input on tritrophic (plant–aphid–parasitoid) interactions

Because N is frequently the most limiting mineral macronutrient for plants in terrestrial ecosystems, modulating N input may have ecological consequences through trophic levels. Thus, in agro‐ecosystems, the success of natural enemies may depend not only from their herbivorous hosts but also from the host plant whose qualities may be modulated by N input. We manipulated foliar N concentrations by providing to Camelina sativa plants three different nitrogen rates (control, optimal, and excessive). We examined how the altered host‐plant nutritional quality influenced the performances of two aphid species, the generalist green peach aphid, Myzus persicae, and the specialist cabbage aphid, Brevicoryne brassicae, and their common parasitoid Diaeretiella rapae. Both N inputs led to increased N concentrations in the plants but induced contrasted concentrations within aphid bodies depending on the species. Compared to the control, plant biomass increased when receiving the optimal N treatment but decreased under the excessive treatment. Performances of M. persicae improved under the optimal treatment compared to the control and excessive treatments whereas B. brassicae parameters declined following the excessive N treatment. In no‐choice trials, emergence rates of D. rapae developing in M. persicae were higher on both optimum and excessive N treatments, whereas they remained stable whatever the treatment when developing in B. brassicae. Size of emerging D. rapae females was positively affected by the treatment only when it developed in M. persicae on the excessive N treatment. This work showed that contrary to an optimal N treatment, when N was delivered in excess, plant suitability was reduced and consequently affected negatively aphid parameters. Surprisingly, these negative effects resulted in no or positive consequences on parasitoid parameters, suggesting a buffered effect at the third trophic level. Host N content, host suitability, and dietary specialization appear to be major factors explaining the functioning of our studied system.     

Associational resistance or susceptibility: the indirect interaction between chemically‐defended and non‐defended herbivore prey via a shared predator

Many organisms possess chemical defences against their natural enemies, which render them unpalatable or toxic when attacked or consumed. These chemically‐defended organisms commonly occur in communities with non‐ or less‐defended prey, leading to indirect interactions between prey species, mediated by natural enemies. Although the importance of enemy‐mediated indirect interactions have been well documented (e.g. apparent competition), how the presence of prey chemical defences may affect predation of non‐defended prey in terrestrial communities remains unclear. Here, an experimental approach was used to study the predator‐mediated indirect interaction between a chemically‐defended and non‐defended pest aphid species. Using laboratory‐based mesocosms, aphid community composition was manipulated to include chemically‐defended (CD) aphids Brevicoryne brassicae, non‐defended (ND) aphids Myzus persicae or a mixed assemblage of both species, on Brassica oleracea cabbage plants, in the presence or absence of a shared predator (Chrysoperla carnea larvae). Aphid population growth rates, aphid distributions on host plants and predator growth rates were measured. In single‐species treatments, C. carnea reduced M. persicae population growth rate, but had no significant impact on B. brassicae population growth rate, suggesting B. brassicae chemical defences are effective against C. carnea. Chrysoperla carnea had no significant impact on either aphid species population growth rate in mixed‐species treatments. Myzus persicae (ND) therefore experienced reduced predation in the presence of B. brassicae (CD) through a predator‐mediated indirect effect. Moreover, predator growth rates were significantly higher in the M. persicae‐only treatments than in either the B. brassicae‐only or mixed‐species treatments, suggesting predation was impaired in the presence of B. brassicae (CD). A trait‐mediated indirect interaction is proposed, consistent with associational resistance, in which the predator, upon incidental consumption of chemically‐defended aphids is deterred from feeding, releasing non‐defended aphids from predatory control.     

Serological Detection of Beet Western Yellows Luteovirus in the Non-vector,Brevicoryne brassicae (Homoptera: Aphididae)

The interaction between beet western yellows luteovirus (BWYV) and the aphid species Brevicoryne brassicae was investigated using virus transmission and serological detection experiments. This species failed to transmit a BWYV isolate from infected to healthy oilseed rape plants, although virus was readily detected by double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) in single B. brassicae adults. When virus-carrying adults were tested by ELISA after different inoculation access periods, the number of virus-positive individuals decreased after 5 days, whereas with the efficient vector Myzus persicae, virus-positive individuals were found even after 10 days. This confirms the inability of B. brassicae to transmit BWYV, even though it may acquire the virus. It is suggested that B. brassicae, as compared with the efficient vector M. persicae, may serve as an experimental model for studying the mechanisms of the luteovirus-vector specificity     

Honeydew feeding in the solitary bee Osmia bicornis as affected by aphid species and nectar availability

Like honey bees (Apis mellifera), non-Apis bees could exploit honeydew as a carbohydrate source. In addition to providing carbohydrates, this may expose them to potentially harmful plant products secreted in honeydew. However, knowledge on honeydew feeding by solitary bees is very scarce. Here we determine whether the polylectic solitary bee Osmia bicornis (=O. rufa) collects honeydew under semi-field conditions, and whether this is affected by aphid species and presence of floral nectar. Bees were provided with oilseed rape plants containing flowers and/or colonies of either Myzus persicae or Brevicoryne brassicae. We used the total sugar level of the bee crop as a measure of the individual's nutritional state and the oligosaccharide erlose as indicator for honeydew consumption. Erlose was present in honeydews from both aphid species, while absent in oilseed rape nectar, nor being synthesized by O. bicornis. When bees were confined to a single honeydew type as the only carbohydrate source, consumption of M. persicae honeydew was confirmed for 47% of the bees and consumption of B. brassicae honeydew for only 3%. Increased mortality in the latter treatment provided further evidence that B. brassicae honeydew is an unsuitable food source for O. bicornis. All bees that were given the choice between honeydew and floral nectar showed significantly increased total sugar levels. However, the fact that no erlose was detected in these bees indicates that honeydew was not consumed when suitable floral nectar was available. This study demonstrates that honeydew exploitation by O. bicornis is dependent on honeydew type and the presence of floral nectar.     

A comparison of the reproduction of Brevicoryne brassicae and Myzus persicae in relation to soluble nitrogen concentration and leaf age (leaf position) in the brussels sprout plant

Wide variations in total soluble nitrogen occurred in different leaves of Brussels sprout plants. These variations were partly the result of leaf age (leaf position) and partly the result of different fertiliser application to the plants. The fecundity of Brevicoryne brassicae and Myzus persicae caged on such leaves also varied greatly and was correlated with total soluble nitrogen within leaves of the same age, but not between leaves of different ages. The value of total soluble nitrogen concentrations, based on analysis of leaf tissue, for indicating plant suitability for aphids is discussed as well as the difference in response of the two aphid species to experimental conditions favouring proteolysis in the leaf.

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Ein vergleich der vermehrung von Brevicoryne brassicae und Myzus persicae in beziehung zum gehalt löslichen stickstoffs und dem blattalter von rosenkohlpflanzen

Zusammenfassung Brevicoryne brassicae (L.) und Myzus persicae (Sulz.) wurden an Kohlblättern gekäfigt, die infolge verschiedener physiologischer Ursachen sehr unterschiedliche Mengen löslichen Stickstoffs enthielten. Den Pflanzen wurden zwei Düngerstoffe ((NH₄)₂SO₄ und KCL) verabreicht und die Blattläuse auf jungen, mittelalten und alten Blättern gekäfigt. Dadurch wurde der Gehalt löslichen Stickstoffs in den Blättern zwischen 0,543% und 1,286% variiert. Die Nachkommenzahl der Blattläuse bei diesen Düngungen und auf verschiedenaltrigen Blättern war sehr unterschiedlich (zwischen 7,3 und 30,2 bei B. brassicae; 17,6 und 59,4 bei M. persicae) und konnte in jedem Blattalter mit dem Stickstoffgehalt korreliert werden. Der Befund, daß die Korrelationen zwischen Blattlausnachkommenzahl und Stickstoffgehalt nicht unabhängig vom Blattalter waren, deutet darauf hin, daß der Stickstoffgehalt in physiologisch aktiven Blattgeweben nicht relativ mit dem der Siebröhren übereinstimmt. M. persicae vermochte in höherem Maße als B. brassicae Blätter auszunützen, in denen der Eiweißabbau durch Düngung oder Alter begünstigt war. Dieser Unterschied zwischen den beiden Arten steht möglicherweise mit ihren verschiedenen Nährstoffbedürfnissen in Zusammenhang.

     

Infection of Arabidopsis by cucumber mosaic virus triggers jasmonate-dependent resistance to aphids that relies partly on the pattern-triggered immunity factor BAK1

Many aphid-vectored viruses are transmitted nonpersistently via transient attachment of virus particles to aphid mouthparts and are most effectively acquired or transmitted during brief stylet punctures of epidermal cells. In Arabidopsis thaliana, the aphid-transmitted virus cucumber mosaic virus (CMV) induces feeding deterrence against the polyphagous aphid Myzus persicae. This form of resistance inhibits prolonged phloem feeding but promotes virus acquisition by aphids because it encourages probing of plant epidermal cells. When aphids are confined on CMV-infected plants, feeding deterrence reduces their growth and reproduction. We found that CMV-induced inhibition of growth as well as CMV-induced inhibition of reproduction of M. persicae are dependent upon jasmonate-mediated signalling. BRASSINOSTEROID INSENSITIVE1-ASSOCIATED KINASE1 (BAK1) is a co-receptor enabling detection of microbe-associated molecular patterns and induction of pattern-triggered immunity (PTI). In plants carrying the mutant bak1-5 allele, CMV induced inhibition of M. persicae reproduction but not inhibition of aphid growth. We conclude that in wildtype plants CMV induces two mechanisms that diminish performance of M. persicae: a jasmonate-dependent and PTI-dependent mechanism that inhibits aphid growth, and a jasmonate-dependent, PTI-independent mechanism that inhibits reproduction. The growth of two crucifer specialist aphids, Lipaphis erysimi and Brevicoryne brassicae, was not affected when confined on CMV-infected A. thaliana. However, B. brassicae reproduction was inhibited on CMV-infected plants. This suggests that in A. thaliana CMV-induced resistance to aphids, which is thought to incentivize virus vectoring, has greater effects on polyphagous than on crucifer specialist aphids.     

Reciprocal grafting separates the roles of the root and shoot in sex‐related drought responses in Populus cathayana males and females

Drought stress responses and sensitivity of dioecious plants, such as Populus cathayana Rehd., are determined by different mechanisms in each sex. In general, males tend to be more resistant while females are more sensitive. Here, we used reciprocal grafting between males and females to determine the relative importance of roots and shoots when plants are exposed to drought stress. Total dry matter accumulation (DMA), photosynthetic capacity, long‐term water‐use efficiency (Δ), water potential and ultrastructure of mesophyll cells were evaluated to determine the different roles of root and shoot in sex‐related drought responses. Plants with male roots were found to be more resistant and less sensitive to water stress than those with female roots under drought conditions. On the contrary, plants with female shoots grew better than those with male shoots under well‐watered conditions. These results indicated that the sensitivity of males and females to water stress is primarily influenced by root processes, while under well‐watered conditions sexual differences in growth are primarily driven by shoot processes. Furthermore, grafting female shoot scion onto male rootstock was proved to be an effective mean to improve resistance to water stress in P. cathayana females.     

Drought increases freezing tolerance of both leaves and xylem of Larrea tridentata

Drought and freezing are both known to limit desert plant distributions, but the interaction of these stressors is poorly understood. Drought may increase freezing tolerance in leaves while decreasing it in the xylem, potentially creating a mismatch between water supply and demand. To test this hypothesis, we subjected Larrea tridentata juveniles grown in a greenhouse under well‐watered or drought conditions to minimum temperatures ranging from ?8 to ?24 °C. We measured survival, leaf retention, gas exchange, cell death, freezing point depression and leaf‐specific xylem hydraulic conductance (k_l). Drought‐exposed plants exhibited smaller decreases in gas exchange after exposure to ?8 °C compared to well‐watered plants. Drought also conferred a significant positive effect on leaf, xylem and whole‐plant function following exposure to ?15 °C; drought‐exposed plants exhibited less cell death, greater leaf retention, higher k_l and higher rates of gas exchange than well‐watered plants. Both drought‐exposed and well‐watered plants experienced 100% mortality following exposure to ?24 °C. By documenting the combined effects of drought and freezing stress, our data provide insight into the mechanisms determining plant survival and performance following freezing and the potential for shifts in L. tridentata abundance and range in the face of changing temperature and precipitation regimes.     

Responses of Myzus persicae and Brevicoryne brassicae to leaf age and water stress in Brussels sprouts grown in pots

Although intermittent water stress is largely beneficial and continuous water stress largely detrimental to the reproduction and survival of Myzus persicae and Brevicoryne brassicae feeding on Brussels sprouts, there are differences between the species on different leaf ages. During intermittent water stress in the old leaves, the normal feeding stratum of M. persicae, this aphid is relatively more successful than B. brassicae. During continuous water stress in the young leaves, the normal feeding stratum of B. brassicae, this aphid is relatively more successful than M. persicae. The high suitability of old leaves for M. persicae and, to a lesser extent, their low suitability for B. brassicae, are also apparent on turgid plants, in terms of reproduction, survival and the proportion of alate progeny. For M. persicae substrates which permit greater reproduction also promote fewer alatae but this is not always so for B. brassicae. However, crowding was not controlled in these experiments.It is suggested that these responses result from variation in the provision of the different nutritional and phagostimulant requirements of the two species by different leaf ages and during water stress.In the field irrigation may delay aphid multiplication during dry weather.

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Zusammenfassung Die Reaktionen ungeflügelter Virginoparer von M. persicae und B. brassicae auf verschiedenes Blattalter von Kohlpflanzen (Brassicae oleracea L. var. bullata gemmifera) in Blumentöpfen mit verschiedener Wasserversorgung wurde an der Reproduktionsrate, der Entwicklungs-und Lebensdauer (des Adultstadiums, der Dauer der reproduktiven wie der post-reproduktiven Phase) und der Geflügeltenbildung der Blattläuse untersucht. Dazu wurden zwei Formen des Wassermangels benutzt: (1) vorübergehender, bei welchem das verfügbare (AW) zwischen der Feldkapazität und einen im voraus festgelegten Defizit (10% = Wasser feucht, 50% = mäßig, 90% = trocken) schwankte, und (2) dauernder, bei welchem eine Abnahme des verfügbaren Wässers von einem bekannten Ausgangspunkt allmählich erfolgte.Obwohl für die Aphiden vorübergehender Wassermangel weitgehend vorteilhaft, beständiger aber schädlich ist, bestehen Unterschiede zwischen den Arten auf verschiedenaltrigen Blättern. Auf jungem und reifem Laub steigen Vermehrung und Lebensdauer beider Arten Durch vorübergehenden Wassermangel an, aber auf alten Blättern leidet B. brassicae in dem Maße wie sich der Mangel verstärkt, während M. persicae bei mäßigem Mangel Vorteile hat und nur leidet, wenn periodisches Welken eintritt. Bei beiden Arten verursacht beständiger Wassermangel Hemmung der Reproduktion. Diese Abnahme beschleunigt sich bei Annäherung an den permanenten Welkepunkt (PWP) und war für die Aphiden auf alten Blättern am stärksten. Im Gegensatz zu M. persicae nimmt die Reproduktionsrate von B. brassicae sogar am permanenten Welkepunkt nicht ab. Die. hohe Eignung alter Blätter für M. persicae und — in geringerem Maße — ihre geringe Eignung für B. brassicae waren auch an turgeszenten Pflanzen bei der Reproduktionsrate, der Überlebensdauer und dem Prozentsatz geflügelter Nachkommen deutlich festzustellen.Es wird vermutet, daß sich diese Reaktionen aus Verschiedenheiten in der Befriedigung unterschiedlicher nutritiver und phagostimulatorischer Bedürfnisse der beiden Arten auf verschieden alten Blättern und bei Wassermangel ergeben. Im Freien dürfte bei trockenem Wetter künstliche Bewässerung die Blattlausvermehrung verzögern.

     

Spectral reflectance from a broccoli crop with vegetation or soil as background: influence on immigration byBrevicoryne brassicae andMyzus persicae

Light reflectance in five wavebands of the spectrum was measured from broccoli (Brassicae oleracea var.botrytis [L].) interplanted with leguminous cover crops (cover crop background) or broccoli grown as monoculture (bare soil background), and fertilized with compost or synthetic fertilizer. AlateBrevicoryne brassicae (L.) andMyzus persicae (Sulzer) (Homoptera: Aphididae) were monitored in yellow pan water traps and on broccoli leaves. Reflectance intensity was higher with a background of bare soil at all wavebands except blue (400–455 nm) in the early-season. Intensity decreased as broccoli canopy cover increased at all wavebands except blue and green (515–550 nm), declining-most dramatically in the yellow (550–590 nm). Highest late-season intensities were in plots with bare soil background and fertilized with compost (those stressed for nitrogen). Few differences in spectral composition, expressed for each waveband as a percentage of total intensity, were recorded. Numbers of alatae were lowest in cover crop background plots in the early season, reached equivalency with bare soil background by mid-season, and showed highest positive correlations with intensity in the yellow (550–590 nm). Results correspond to laboratory findings that aphids are attracted to higher intensity light, especially in the yellow waveband, and support a phototactic explanation for aphid orientation in the field.     

Prey‐mediated effects of glucosinolates on aphid predators

1. Plant resistance against herbivores can act directly (e.g. by producing toxins) and indirectly (e.g. by attracting natural enemies of herbivores). If plant secondary metabolites that cause direct resistance against herbivores, such as glucosinolates, negatively influence natural enemies, this may result in a conflict between direct and indirect plant resistance. 2. Our objectives were (i) to test herbivore‐mediated effects of glucosinolates on the performance of two generalist predators, the marmalade hoverfly (Episyrphus balteatus) and the common green lacewing (Chrysoperla carnea) and (ii) to test whether intraspecific plant variation affects predator performance. 3. Predators were fed either Brevicoryne brassicae, a glucosinolate‐sequestering specialist aphid that contains aphid‐specific myrosinases, or Myzus persicae, a non‐sequestering generalist aphid that excretes glucosinolates in the honeydew, reared on four different white cabbage cultivars. Predator performance and glucosinolate concentrations and profiles in B. brassicae and host‐plant phloem were measured, a novel approach as previous studies often measured glucosinolate concentrations only in total leaf material. 4. Interestingly, the specialist aphid B. brassicae selectively sequestered glucosinolates from its host plant. The performance of predators fed this aphid species was lower than when fed M. persicae. When fed B. brassicae reared on different cultivars, differences in predator performance matched differences in glucosinolate profiles among the aphids. 5. We show that not only the prey species, but also the plant cultivar can have an effect on the performance of predators. Our results suggest that in the tritrophic system tested, there might be a conflict between direct and indirect plant resistance.