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.     

黑带食蚜蝇Episyrphus balteatus De Geer的复眼结构及其调光机制

【目的】观察研究黑带食蚜蝇Episyrphus balteatus De Geer成虫复眼形态、小眼结构和不同光暗条件对小眼结构的影响,以明确其光视觉的结构基础和调光机制。【方法】利用组织切片法和扫描电镜等技术。【结果】1.复眼位于头部两侧,正面观呈半球形,占据除额颜外大部分头部。雄虫与雌虫单个复眼分别有约7 180个、7 230个小眼。各小眼面呈整齐排列的规则六边形。2.小眼由角膜及伪晶锥组成的屈光器、不同水平面分布的8个小网膜细胞及其特化形成的离散型视杆、屏蔽色素细胞和基膜等组成。小眼自远端至近端由主色素细胞和12个附属色素细胞围绕。3.随光暗条件的改变小眼内的附属色素细胞色素和基细胞细胞核沿小眼纵轴移动。光适应时,附属色素细胞色素颗粒沿小眼纵轴均匀分布,基细胞细胞核位于基膜上方。暗适应时,附属色素细胞色素颗粒向伪晶锥近端压缩,基细胞细胞核亦向远端移动,到达视杆中段。【结论】黑带食蚜蝇复眼精密的小眼排列形式和内部结构均显示了其强大的生理功能;屏蔽色素颗粒的移动是其复眼适应外界光环境变化的重要机制。本试验为进一步探究黑带食蚜蝇视觉结构和光调节机制,以及与其飞行行为间的关系提供了一定的理论基础。     

Distribution and abundance of aphidophagous hoverflies (Diptera: Syrphidae) in wildflower patches and field margin habitats

1 The spatial and temporal variations in aphidophagous syrphid abundance were recorded over two seasons in wildflower resource patches sown in a winter barley crop and associated field margins. Standard census techniques and sticky board trapping were used to assess numbers of syrphids, whilst weekly flower head counts were used to quantify the floral resources available in each of the patches. 2 The field margin supported a greater diversity and density of syrphids than the within‐crop wildflower patches, despite having a relatively lower flower head density. Presumably this was in response to other resources that field margins offer, namely additional aphid resources, shelter from predation, lekking sites and suitable flight‐paths. 3 The commonest species of syrphid, Episyrphus balteatus, demonstrated a very positive habitat association with the field margin and was rarely reported in the field patches. Therefore, it may be an unsuitable candidate for the biological control of aphids via augmentation of numbers using non‐host resources. 4 Patch size and shape had little effect on the spatial distributions of syrphids, probably because of the adult syrphids' high mobility. 5 Of greater influence was the number of flowers contained in each habitat patch. Typically, patches with higher numbers of flowers had significantly greater aggregations of hoverflies. Habitat manipulation by the provision of flowers in patches seems to increase the local density of hoverflies. Further work is necessary to establish the importance of flower density in enhancing the control of pest populations.     

Use of selected flowering plants in greenhouses to enhance aphidophagous hoverfly populations (Diptera: Syrphidae)

The addition of floral resources in a crop is the most commonly used conservation biological control strategy. The influence of additional floral resources on the abundance of aphidophagous syrphids has been studied in Mediterranean sweet-pepper greenhouses, in southeast Spain. Sweet alyssum and coriander were the plant species used as flowering plants, distributed in the greenhouse in several monospecific patches. In our first experiment the influence on syrphid pre-imaginal stages (larvae and pupae) was studied and adult stages were studied in a second experiment. A higher number of pre-imaginal syrphids was recorded in two replicated greenhouses where flowers were introduced, compared with two control greenhouses (without additional floral resources). To evaluate the effect on adults, 4 greenhouses were divided into 2 plots in each greenhouse and flowers were introduced in one plot per greenhouse. More hoverfly adults were observed in the plots where flowers had been introduced than in the control. The three most abundant syrphid species found were Eupeodes corollae, Episyrphus balteatus and Sphaerophoria rueppellii. Specimens from these species were dissected, and their pollen content was analysed to assess the food potential of the introduced flowers. The three syrphid species fed on pollen from both the flowering plants, as well as on sweet-pepper pollen. This conservation biological control strategy is an effective method to enhance native syrphid populations in Mediterranean sweet-pepper greenhouses.     

Reproductive rate of the Indian mustard aphid (Lipaphis erysimi pseudobrassicae) on different Brassica oilseeds: colmparisons with Swedish strains of mustard (Lipaphis erysimi erysimf) and cabbage aphid (Brevicoryne brassicae)

The reproductive rate of the Indian mustard aphid (Lipaphis erysimi pseudobrassicae), a serious pest on Brassica oilseeds in India, was compared with that of Swedish strains of mustard aphid (L. e. erysimi) and cabbage aphid (Brevicoryne brassicae) on six common Indian and Swedish Brassica oilseed cultivars. There were clear differences between L. e. erysimi and L. e. pseudobrassicae in the relative suitability of the cultivars, emphasisipg the biological differences between these aphids. B. brassicae had similar reproductive rates on all cultivars.
In addition, the reproductive rate of L. e. pseudobrassicae was measured on both young and old plants of the six Brassica oilseed cultivars and 14 Indian rai (B. juncea) cultivars. Among the six Brassica cultivars, the reproductive rates were highest on turnip rape and toria (B. campestris), and lowest on yellow sarson (B. campestris) and rai (B. juncea). The ranking of the cultivars was similar for young and old plants. There were no significant differences between the rai (B. juncea) cultivars tested, neither among young nor among old plants. These results from laboratory studies are compared with field data from India on the relative susceptibility of different Brassica oilseed cultivars to L. e. pseudobrassicae.     

Effect of eight cauliflower cultivars on biological parameters of the cabbage aphid,Brevicoryne brassicae (L.) (Hem: Aphididae) in laboratory conditions

The cabbage aphid, Brevicoryne brassicae is one of the most important pests with broad host range that does damage to cruciferae plants. In this research, biology parameters of the cabbage aphid was studied on the eight cauliflower cultivars: Smilla, Snow mystique, White cloud, Buris, Galiblanka, Snow crown, SG and Tokita. This study was conducted under controlled conditions (25?±?2?°C, 65?±?5% RH and L-D: 16–8). Statistical analysis showed that there is significant difference between different stages of growth and mean number of laid nymphs period (p?<?0.05), so that the maximum and minimum growth period were observed on the cultivars Galyblanka and Buris, respectively. The longest mean longevity of total nymphal instars was obtained on White cloud, and the shortest on Smilla. The highest pre nymph period, nymph production period, mean number of laid nymphs, Adult longevity and total lifespan on the Galiblanka cultivar was calculated as 8.60, 13.50, 58.60, 17.30 and 25.90, respectively. The results showed that using cultivars that affected on the adult reproductive parameters and provides conditions for reduction of reproduction could be a good solution for the cabbage aphid control management.     

Inter‐specific competition and competition‐free space in the tephritid parasitoids Utetes anastrephae and Doryctobracon areolatus (Hymenoptera: Braconidae: Opiinae)

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Predators in the plant–soil feedback loop: aboveground plant‐associated predators may alter the outcome of plant–soil interactions

Plant–soil feedback (PSF) can structure plant communities, promoting coexistence (negative PSF) or monodominance (positive PSF). At higher trophic levels, predators can alter plant community structure by re‐allocating resources within habitats. When predator and plant species are spatially associated, predators may alter the outcome of PSF. Here, I explore the influence of plant‐associated predators on PSF using a generalised cellular automaton model that tracks nutrients, plants, herbivores and predators. I explore key contingencies in plant–predator associations such as whether predators associate with live vs. senesced vegetation. Results indicate that plant‐associated predators shift PSF to favour the host plant when predators colonise live vegetation, but the outcome of PSF will depend upon plant dispersal distance when predators colonise dead vegetation. I apply the model to two spider‐associated invasive plants, finding that spider predators should shift PSF dynamics in a way that inhibits invasion by one forest invader, but exacerbates invasion by another.     

Specialised protagonists in a plant–pollinator interaction: the pollination of Blumenbachia insignis (Loasaceae)

• Analyses of resource presentation, floral morphology and pollinator behaviour are essential for understanding specialised plant‐pollinator systems. We investigated whether foraging by individual bee pollinators fits the floral morphology and functioning of Blumenbachia insignis, whose flowers are characterised by a nectar scale‐staminode complex and pollen release by thigmonastic stamen movements.
• We described pollen and nectar presentation, analysed the breeding system and the foraging strategy of bee pollinators. We determined the nectar production pattern and documented variations in the longevity of floral phases and stigmatic pollen loads of pollinator‐visited and unvisited flowers.
• Bicolletes indigoticus (Colletidae) was the sole pollinator with females revisiting flowers in staminate and pistillate phases at short intervals, guaranteeing cross‐pollen flow. Nectar stored in the nectar scale‐staminode complex had a high sugar concentration and was produced continuously in minute amounts (~0.09 μl·h^?1). Pushing the scales outward, bees took up nectar, triggering stamen movements and accelerating pollen presentation. Experimental simulation of this nectar uptake increased the number of moved stamens per hour by a factor of four. Flowers visited by pollinators received six‐fold more pollen on the stigma than unvisited flowers, had shortened staminate and pistillate phases and increased fruit and seed set.
• Flower handling and foraging by Bicolletes indigoticus were consonant with the complex flower morphology and functioning of Blumenbachia insignis. Continuous nectar production in minute quantities but at high sugar concentration influences the pollen foraging of the bees. Partitioning of resources lead to absolute flower fidelity and stereotyped foraging behaviour by the sole effective oligolectic bee pollinator.

     

Top‐down control by Harmonia axyridis mitigates the impact of elevated atmospheric CO2 on a plant–aphid interaction

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Geographic mosaics of plant–soil microbe interactions in a global plant invasion

Aim Our aim in this study was to document the global biogeographic variation in the effects of soil microbes on the growth of Centaurea solstitialis (yellow starthistle; Asteraceae), a species that has been introduced throughout the world, but has become highly invasive only in some introduced regions. Location  To assess biogeographic variation in plant–soil microbe interactions, we collected seeds and soils from native Eurasian C. solstitialis populations and introduced populations in California, Argentina and Chile. Methods To test whether escape from soil‐borne natural enemies may contribute to the success of C. solstitialis, we compared the performance of plants using seeds and soils collected from each of the biogeographic regions in greenhouse inoculation/sterilization experiments. Results  We found that soil microbes had pervasive negative effects on plants from all regions, but these negative effects were significantly weaker in soils from non‐native ranges in Chile and California than in those from the non‐native range in Argentina and the native range in Eurasia. Main conclusions The biogeographic differences in negative effects of microbes in this study conformed to the enemy‐release hypothesis (ERH) overall, but the strong negative effect of soil biota in Argentina, where C. solstitialis is invasive, and weaker effects in Chile where it is not, indicated that different factors influencing invasion are likely to occur in large scale biogeographic mosaics of interaction strengths.     

The role of plant–mycorrhizal mutualisms in deterring plant invasions: Insights from an individual‐based model

Understanding the factors that determine invasion success for non‐native plants is crucial for maintaining global biodiversity and ecosystem functioning. One hypothesized mechanism by which many exotic plants can become invasive is through the disruption of key plant–mycorrhizal mutualisms, yet few studies have investigated how these disruptions can lead to invader success. We present an individual‐based model to examine how mutualism strengths between a native plant (Impatiens capensis) and mycorrhizal fungus can influence invasion success for a widespread plant invader, Alliaria petiolata (garlic mustard). Two questions were investigated as follows: (a) How does the strength of the mutualism between the native I. capensis and a mycorrhizal fungus affect resistance (i.e., native plant maintaining >60% of final equilibrium plant density) to garlic mustard invasion? (b) Is there a non‐linear relationship between initial garlic mustard density and invasiveness (i.e., garlic mustard representing >60% of final equilibrium plant density)? Our findings indicate that either low (i.e., facultative) or high (i.e., obligate) mutualism strengths between the native plant and mycorrhizal fungus were more likely to lead to garlic mustard invasiveness than intermediate levels, which resulted in higher resistance to garlic mustard invasion. Intermediate mutualism strengths allowed I. capensis to take advantage of increased fitness when the fungus was present but remained competitive enough to sustain high numbers without the fungus. Though strong mutualisms had the highest fitness without the invader, they proved most susceptible to invasion because the loss of the mycorrhizal fungus resulted in a reproductive output too low to compete with garlic mustard. Weak mutualisms were more competitive than strong mutualisms but still led to garlic mustard invasion. Furthermore, we found that under intermediate mutualism strengths, the initial density of garlic mustard (as a proxy for different levels of plant invasion) did not influence its invasion success, as high initial densities of garlic mustard did not lead to it becoming dominant. Our results indicate that plants that form weak or strong mutualisms with mycorrhizal fungi are most vulnerable to invasion, whereas intermediate mutualisms provide the highest resistance to an allelopathic invader.     

Role of social and individual experience in interaction of the meadow ant Formica pratensis (Hymenoptera: Formicidae) with ladybird imagines and hoverfly larvae

The ability to recognize aphidophages is one of the key points in the protection ants provide aphids against their natural enemies. Behavior of honeydew collectors from nature (“field,” control) and laboratory reared “naive” ants of Formica pratensis Retzius, which had never met either “mature” workers or aphids and aphidophages, was observed during their pairwise interactions with ladybird imagines and hoverfly larvae. The majority of the “naive” ants perceived ladybirds as an enemy at their first encounter attacking them immediately without any prior antennation. Ants seem to have a certain innate “enemy image” that lets them react very quickly to protect aphids. Hoverfly larvae were rarely attacked by both “field” and “naive” ants (>15%). During tests with ladybirds ants from nature attacked them and also demonstrated the most aggressive reactions (series of bites and “death grip”) less frequently than the “naive” ants. The percentage of ants avoiding aphidophages after a contact with their chemical defense (reflex bleeding and glue‐like saliva) was significantly higher in the control group. Whereas the “naive” ants did not learn to avoid danger, foragers from nature usually tried to avoid negative experience and used tactics of “short bites.” Overall, experience has been proved to be unimportant for displaying key behavioral reactions underlying ant–ladybird interaction. However, accumulation of experience has been assumed to play an important role in the formation of behavioral strategy that allows honeydew collectors to drive aphidophages away with lower energy costs and avoid or minimize negative consequences of aphidophages’ chemical defense.     

Habitat preferences and diet in the predatory Coccinellidae (Coleoptera): an evolutionary perspective

Coccinellids (ladybird beetles) exhibit considerable diversity in habitat and dietary preference and specificity. This is evident even when comparing species within some coccinellid genera. Resource limitation and competition are suggested as of greatest importance in the evolution of coccinellid habitat preferences. Dietary and habitat specialization has probably occurred in some lineages within broader preferences possessed by generalist ancestors, to avoid the costs associated with migration between habitats and prey switching. Feeding in atypical habitats, on alternative food, when optimal prey are scarce, is likely to have been of great importance in facilitating evolutionary shifts to novel diets and habitats. The broad host ranges of many coccinellid parasitoids and observed interspecific differences in parasitoid prevalence resulting from physiological differences between coccinellid species argue that enemy free space has been of limited importance in habitat and prey shifts in this group. Rapid change may occur in coccinellid foraging patterns, perhaps due to conditioning, and coccinellids may swiftly adapt to new habitats through selection acting on the expression pre-existing traits. Diet, as a determinant of coccinellid migration and gene flow, is likely to affect probable modes of speciation in different coccinellid groups. Parapatric speciation and possibly sympatric speciation are suggested as of possible importance in the genesis of new coccinellid species through prey and habitat shifts.     

Tracking the elusive history of diversification in plant–herbivorous insect–parasitoid food webs: insights from figs and fig wasps

The food webs consisting of plants, herbivorous insects and their insect parasitoids are a major component of terrestrial biodiversity. They play a central role in the functioning of all terrestrial ecosystems, and the number of species involved is mind‐blowing (Nyman et al. 2015 ). Nevertheless, our understanding of the evolutionary and ecological determinants of their diversity is still in its infancy. In this issue of Molecular Ecology, Sutton et al. ( 2016 ) open a window into the comparative analysis of spatial genetic structuring in a set of comparable multitrophic models, involving highly species‐specific interactions: figs and fig wasps. This is the first study to compare genetic structure using population genetics tools in a fig‐pollinating wasp (Pleistodontes imperialis sp1) and its main parasitoid (Sycoscapter sp.A). The fig‐pollinating wasp has a discontinuous spatial distribution that correlates with genetic differentiation, while the parasitoid bridges the discontinuity by parasitizing other pollinator species on the same host fig tree and presents basically no spatial genetic structure. The full implications of these results for our general understanding of plant–herbivorous insect–insect parasitoids diversification become apparent when envisioned within the framework of recent advances in fig and fig wasp biology.     

Hydrophobic tail length plays a pivotal role in amyloid beta (25–35) fibril–surfactant interactions

The amyloid β‐peptide fragment comprising residues 25–35 (Aβ_25‐35) is known to be the most toxic fragment of the full length Aβ peptide which undergoes fibrillation very rapidly. In the present work, we have investigated the effects of the micellar environment (cationic, anionic, and nonionic) on preformed Aβ_25‐35 fibrils. The amyloid fibrils have been prepared and characterized by several biophysical and microscopic techniques. Effects of cationic dodecyl trimethyl ammonium bromide (DTAB), cetyl trimethylammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS), and nonionic polyoxyethyleneoctyl phenyl ether (Triton X‐100 or TX) on fibrils have been studied by Thioflavin T fluorescence, UV–vis spectroscopy based turbidity assay and microscopic analyses. Interestingly, DTAB and SDS micelles were observed to disintegrate prepared fibrils to some extent irrespective of their charges. CTAB micelles were found to break down the fibrillar assembly to a greater extent. On the other hand, the nonionic surfactant TX was found to trigger the fibrillation process. The presence of a longer hydrophobic tail in case of CTAB is assumed to be a reason for its higher fibril disaggregating efficacy, the premise of their formation being largely attributed to hydrophobic interactions. Proteins 2016; 84:1213–1223. © 2016 Wiley Periodicals, Inc.     

PCR primers for polymorphic microsatellite loci in the plant‐ant Azteca ulei cordiae (Formicidae: Dolichoderinae)

Twelve microsatellite loci were isolated from the Peruvian tropical plant‐ant, Azteca ulei cordiae (Hymenoptera: Dolichoderinae). High levels of within‐population variation were observed at most loci, with number of alleles ranging from four to 18, and heterozygosity from 0.118 to 1 per population sample. Cross‐species amplification of these loci was also successfully tested in several other species of the same ant genus Azteca.