Episyrphus Balteatus Research Articles

Intraguild interactions and aphid predators: biological efficiency of Harmonia axyridis and Episyrphus balteatus

AbstractIntroductions of the harlequin ladybird Harmonia axyridis into Belgium for aphid biological control have been followed by declines in native aphid natural enemies. We first examined, in laboratory and field conditions, the impact of larval mobility of either H. axyridis or Episyrphus balteatus, the most abundant native hoverfly in central Europe, on aphid suppression. The hoverfly larvae consumed more aphids (Megoura viciae) in a short term, whereas the ladybird ones were more efficient in a long term. We second investigated the intraguild interactions between H. axyridis and E. balteatus larvae and adults. In larva pairings between the two species in laboratory microcosms containing aphid (M. viciae) infested broad bean (Vicia faba) plants, H. axyridis had an intraguild predation (IGP) advantage over the hoverfly. When conspecific larvae were paired together on aphid‐infested plants, no cannibalism between them was detected. The presence of either H. axyridis or E. balteatus larvae on aphid‐infested plants negatively influenced the ovipositional behaviour of H. axyridis and E. balteatus females; lower numbers of laid eggs were recorded compared to control treatment. Moreover, eggs laid by E. balteatus females were also dropped as victims of predation mainly by H. axyridis larvae. Our results suggest that while the exotic ladybird was more efficient in aphid biological control, larvae and eggs of the native hoverfly species face increased IGP by H. axyridis, which would contribute, as a consequence, to the decline in E. balteatus population following invasion.

Assessment of oviposition site quality by aphidophagous hoverflies: reaction to conspecific larvae

Aphidophagous predators adapt their foraging behaviour to the presence of conspecific and heterospecific larvae. We studied the effect of the presence of conspecific larvae and their tracks on the oviposition site selection of an aphid-specific predator, Episyrphus balteatus DeGeer (Diptera: Syrphidae), in two-choice experiments using a leaf disc bioassay. Gas chromatography – mass spectrometry analysis was used to identify the volatile chemicals released from odour extracts of E. balteatus larval tracks. The behavioural effects of these volatile substances on hoverfly females were also evaluated. Our experiments demonstrated that E. balteatus females were deterred from ovipositing when presented with a Vicia faba leaf with aphids and conspecific larvae. The oviposition-deterring stimulus was also active when females were presented with a leaf that contained conspecific larval tracks. A mixture of chemical compounds was found in the volatile pattern of odour extracts of larval tracks. The main volatile chemicals were 3-methylbutanoic acid, 2-methylbutanoic acid, 2-methylpropanoic acid, 3-hydroxy-2-butanone, hexanoic acid and phenol. Females also laid significantly fewer eggs in response to odorant volatiles emitted from larval extracts. These results highlight that predatory hoverfly females avoid ovipositing in aphid colonies in which conspecific larvae or their tracks are already present, suggesting that this behaviour constitutes a strategy that enables females to optimize their oviposition site and reduce competition suffered by their offspring.

Effect of stinging nettle habitats on aphidophagous predators and parasitoids in wheat and green pea fields with special attention to the invader Harmonia axyridis Pallas (Coleoptera: Coccinellidae)

AbstractThe relative occurrence and seasonal abundance of aphids and their natural enemies were visually assessed between May and July 2005–2006 in four types of habitats located in Gembloux (Namur province, Belgium): green pea, wheat and stinging nettle either planted in or naturally growing in woodland adjacent to these crops. Results showed that: (i) Acyrthosiphon pisum Harris, Sitobion avenae F. and Microlophium carnosum Buckton were the most common aphid species, respectively, on green pea, wheat and stinging nettle either in or near field crops; (ii) stinging nettle and field crops shared several important aphidophagous insect species such as the ladybird Coccinella septempunctata L., hoverfly Episyrphus balteatus De Geer and braconid wasp Aphidius ervi Haliday; (iii) the shared beneficial species were typically recorded earlier on stinging nettles than on crops; and (iv) the spatial occurrence of the invasive ladybird Harmonia axyridis Pallas was distinctly associated with stinging nettles, particularly in 2005. Stinging nettles and field crops partially coincide in time, enabling the movement of natural enemies among them. These findings suggest that the presence of stinging nettles in landscapes seems to enhance the local density of aphidophagous insect communities necessary for aphid biocontrol in field crops.

Composition and role of hoverfly (Diptera: Syrphidae) communities in an organic apple orchard

The use of hoverfly (Syrphidae) larvae in biological control is gaining more attention where the damage by their prey, aphids is significant, especially in agricultural fields and orchards, where they lower the number of aphids. The present study focuses on one hand on faunistics, on the other hand on testing the different available collecting methods for later research. Collecting in an organic apple orchard near Debrecen yielded 525 specimens of Syrphidae mostly by netting but also with white tray traps. Collected species are differentiated according to their feeding (94.1% aphidophagous). The three species found in greatest numbers are Sphaerophoria scripta (Linnaeus, 1758), Melanostoma mellinum (Linnaeus, 1758) and Episyrphus balteatus (De Geer, 1776). The available information on their biology is reviewed and their potential value of hoverflies in biological control is discussed.

Pollination biology of Disanthus cercidifolius var. longipes, an endemic and endangered plant in China

AbstractDisanthus cercidifolius Maxim. var. longipes H.T. Chang usually has two inflorescences growing in opposite directions in the axillae, but occasionally three inflorescences grow paratactically. The typical flowering process could be divided into 4 periods: “Pre-dehiscence”, “Initial dehiscence”, “Full dehiscence” and “Withering”. Both the natural population and the planted population had a flowering peak of 15–35 days after the first flower bloomed. There were significant differences between the time courses of flowering of the two populations. Out-crossing is the main breeding system in this species. And autogamy decreases the risk of reproductive failure of this species. The main insect pollinators of D. cercidifolius var. longipes are Episyrphus balteatus de Geer, Scaptodrosophila coracina Kikkawa and Peng, Polistes olivaceus de Geer, Apis cerana Fabricius, Nezara viridula L. and Coccinella septempunctata L., and so on. Among the insects, S. coracina and E. balteatus are the most important and efficient pollinators, but others are inefficient pollinators. Though wind pollination is not efficient, it guarantees reproduction when insect pollinators are not available. “Mass flowering” is an adaptive behavior and reproductive strategy of this species, and “few fruiting” could be caused by the lack of pollinators.

Tomato-aphid-hoverfly: a tritrophic interaction incompatible for pest management

Trichome-based tomato resistance offers the potential to reduce pesticide use, but its compatibility with biological control remains poorly understood. We evaluated Episyrphus balteatus De Geer (Diptera, Syrphidae), an efficient aphidophagous predator, as a potential biological control agent of Myzus persicae Sulzer (Hemiptera, Aphididae) on trichome-bearing tomato cultivars. Episyrphus balteatus’ foraging and oviposition behavior, as well as larval mobility and aphid accessibility, were compared between two tomato cultivars (Lycopersicon esculentum Mill. ‘Moneymaker’ and ‘Roma’) and two other crop plants; broad bean (Vicia faba L.) and potato (Solanum tuberosum L.). Hoverfly adults landed and laid more eggs on broad beans than on three species of Solanaceae. Hoverfly larval movement was drastically reduced on tomato, and a high proportion of hoverfly larvae fell from the plant before reaching aphid prey. After quantifying trichome abundance on each of these four plants, we suggest that proprieties of the plant surface, specifically trichomes, are a key factor contributing to reduced efficacy of E. balteatus as a biological agent for aphid control on tomatoes.

Biology and predatory potential of aphidophagous syrphids on guava aphid, Aphis gossypii Glover (Hemiptera: Aphididae).

The biology of four species of aphidophagous syrphids, namely, Paragus serratus (F.), Episyrphus balteatus (De Geer), Ischiodon scutellaris (F.) and Dideopsis aegrota (F.) was studied on guava aphid, Aphis gossypii Glover, during 2007-08 and all the four species successfully completed their life cycle on A. gossypii. P. serratus had the shortest life cycle on A. gossypii with a total developmental period of 20.7 ± 1.36 days, followed by E. balteatus (21.5 ± 1.17 days), I. scutellaris (21.8 ± 0.91 days) and D. aegrota (26.3 ± 1.08 days). The total larval period of P. serratus, I. scutellaris, E. balteatus and D. aegrota averaged 9.8 ± 0.82, 10.6 ± 0.43, 10.0 ± 0.56 and 11.2 ± 0.64 days, respectively. The pupal period, fecundity and longevity of the syrphids on A. gossypi i were recorded. In order to complete the larval period, each larva of P. serratus, I. scutellaris, E. balteatus and D. aegrota required a total of 333.7 ± 59.12, 380.7 ± 75.85, 410.8 ± 87.60 and 472.5 ± 56.89 numbers of A. gossypii , respectively.

Flower orientation enhances pollen transfer in bilaterally symmetrical flowers

Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators. Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated (horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer.

Intra‐specific variation affects the structure of the natural enemy assemblage attacking pea aphid colonies

Abstract 1. Intra‐specific variation in plant defence traits has been shown to profoundly affect herbivore community structure. Here we describe two experiments designed to test whether similar effects occur at higher trophic levels, by studying pea aphid–natural enemy interactions in a disused pasture in southern England.2. In the first experiment, the numbers and identity of natural enemies attacking different monoclonal pea aphid colonies were recorded in a series of assays throughout the period of pea aphid activity.3. In the summer assay, there was a significant effect of clone on the numbers of aphidophagous hoverfly larvae and the total number of non‐hoverfly natural enemies recruited. Clone also appeared to influence the attack rate suffered by the primary predator in the system, the hoverfly Episyrphus balteatus, by Diplazon laetatorius, an ichneumonid parasitoid. Colonies were generally driven to extinction by hoverfly attack, resulting in the recording of low numbers of parasitoids and entomopathogens, suggesting intense intra‐guild predation.4. To further examine the influence of clonal variation on the recruitment of natural enemies, a second experiment was performed to monitor the temporal dynamics of community development. Colonies were destructively sampled every other day and the numbers of natural enemies attacking aphid colonies were recorded. These data demonstrated that clonal variation influenced the timing, abundance, and identity of natural enemies attacking aphid colonies.5. Taken together, these data suggest that clonal variation may have a significant influence on the patterns of interactions between aphids and their natural enemies, and that such effects are likely to affect our understanding of the ecology and biological control of these insect herbivores.

Improvement of artificial diet for aphidophagous syrphids, Episyrphus balteatus (de Geer) and Eupeodes bucculatus (Rondani) (Diptera: Syrphidae)—Additional effects of fatty acids and preservatives

The addition of fatty acids to drone honeybee brood powder (DP)-containing gel diet was found to improve larval development of aphidophagous syrphids, Episyrphus balteatus and Eupeodes bucculatus (Diptera: Syrphidae). When each fatty acid was added singly to the basic diet, the emergence rate was higher than on unsupplemented diets in E. bucculatus. In particular, the addition of oleic acid and linoleic acid improved the emergence rate and adult body size. The combination of both acids was most effective for the development of E. bucculatus. Two gel diets, consisting of DP, oleic acid, and linoleic acid at specific rates, were designed for young and old larvae, respectively; diet agar concentration was also specified. When these diets were provided to larvae of both species, these emergence rates exceeded 90% and adult body sizes were equivalent to aphid-fed controls. Three preservatives to stabilize diets were tested: propionic acid, sodium benzoate, and methyl paraben. Propionic acid (0.5%) had no effects on E. balteatus, but the emergence rate of E. bucculatus was much inferior to the control diet even at a concentration of just 0.1%.

Étude prospective de la fondation, de l’immigration et des facteurs de mortalité des colonies de pucerons noirs de l’oranger Toxoptera aurantii Boyer de Fonscolombe (Hemiptera : Aphididae) au nord du Gharb

Four hundred and eighty brown citrus aphid T. aurantii colonies from ten cohorts were followed and referred from middle of April to end of May in several citrus orchards in the Gharb area during 2006 et 2007 years. This pest would prefer to colonize the S1 and S2 citrus leaves stages but all other studied stages would be predisposed to be also colonized. The colonies of indeterminate foundation were the most important, followed by those founded by the apterous ones (e. g. summer colonies) and also those resulting from alatae one (e. g. spring colonies). Immigration of alates is also discussed. T. aurantii regulation factors are multiple (17 sources) and may act individually or combined. A significante difference of mortality level of T. aurantii colonies was noted and discussed between regulation's origins. Coccinellidae is the most efficient predators of T. aurantii, with A. decempunctata and i (Coleoptera : Coccinellidae). Impact of beneficial such as A. decempunctata, C. septempunctata, A. decempunctata in association with C. septempunctata on aphid regulation is respectively from 5 to 48.33% ; 5 to 16.7%, and 5 to 18.33%. Impact of other regulation’s origins such as temperature and old leaves on aphid mortality is respectively from 8 to 55% and 3.33 to 40%. Others predators such as Aphidoletes Aphidimyza Rond. (Diptera : Cecidomyiidae), Chrysoperla carnea Stephens (Neuroptera : chrysopidae) and Episyrphus balteatus DeGeer (Diptera : Syrphidae), affect also T. aurantii colonies. The impact of theses indigenous beneficial species, acting individually or with ladybird on colony survival is low, except for A. decempunctata in association with E. balteatus with a maximum of 10%.

Oviposition preference of two hoverfly species in response to risk of intraguild predation

Two experiments were performed in the laboratory to understand the relationships between oviposition preference and larval performance of the two predatory hoverfly species, Episyrphus balteatus de Geer and Metasyrphus corollae Fabricius. We first conducted laboratory experiments to understand the nature and relative strengths of intraguild predation between the larvae of three ladybird species, Harmonia axyridis Pallas, Propylea japonica Thunberg, and Scymnus posticalis Sicard, and two predatory hoverfly species. The interactions between larvae of the three ladybird and two hoverfly species differed depending on the species and developmental stages involved: the relationships between H. axyridis or S. posticalis and the two hoverfly larvae tended to be asymmetric with H. axyridis being a superior species and S. posticalis an inferior species, although the reverse was true for each of these two species when small larvae were paired with large larvae of the hoverflies. The ladybird, P. japonica, had both asymmetric and symmetric relationships depending on the hoverfly species with which it interacted. Secondly, we tested the preference and performance hypothesis for hoverflies, by determining whether hoverfly oviposition preference was related to the risk of intraguild predation posed by the presence of a ladybird larva in an aphid colony. Oviposition preference in relation to larval performance of the two hoverflies differed between the species: E. balteatus females tended to adjust their rate of eggs laying in response to the presence of ladybird larvae, while similar responses of M. corollae females were weak. The relationship between oviposition preference and larval performance in hoverflies is discussed.

Pollination by deceit in Paphiopedilum barbigerum (Orchidaceae): a staminode exploits the innate colour preferences of hoverflies (Syrphidae)

Paphiopedilum barbigerum T. Tang et F. T. Wang, a slipper orchid native to southwest China and northern Vietnam, produces deceptive flowers that are self-compatible but incapable of mechanical self-pollination (autogamy). The flowers are visited by females of Allograpta javana and Episyrphus balteatus (Syrphidae) that disperse the orchid's massulate pollen onto the receptive stigmas. Measurements of insect bodies and floral architecture show that the physical dimensions of these two fly species correlate with the relative positions of the receptive stigma and dehiscent anthers of P. barbigerum. These hoverflies land on the slippery centralised wart located on the shiny yellow staminode and then fall backwards through the labellum entrance. They are temporarily trapped in the inflated chamber composed of the interconnected labellum and column. The attractive staminode of P. barbigerum strongly reflects the colour yellow (500-560 nm), a colour preferred innately by most pollen-eating members of the Syrphidae. No scent molecules were detected using GC mass spectrometry analysis, showing that the primary attractant in this system is visual, not olfactory. Pollination-by-deceit in P. barbigerum is contrasted with its congener, P. dianthum, a brood site mimic that is pollinated by ovipositing females of E. balteatus. As the natural rate of fruit set in P. barbigerum (mean 26.3% pooled over three seasons) is lower than that of P. dianthum (mean 58.5% over two seasons), the evolution of false brood sites in some Paphiopedilum spp. should be selectively advantageous as they may provide an increase in the attention and return rates of dependable pollinators to flowers that always lack a reward.

Evidence for a composite anterior determinant in the hover flyEpisyrphus balteatus(Syrphidae), a cyclorrhaphan fly with an anterodorsal serosa anlage

Most insect embryos develop from a monolayer of cells around the yolk, but only part of this blastoderm forms the embryonic rudiment. Another part forms extra-embryonic serosa. Size and position of the serosa anlage vary between species, and previous work raises the issue of whether such differences co-evolve with the mechanisms that establish anteroposterior (AP) polarity of the embryo. AP polarity of the Drosophila embryo depends on bicoid, which is necessary and sufficient to determine the anterior body plan. Orthologs of bicoid have been identified in various cyclorrhaphan flies and their occurrence seems to correlate with a mid-dorsal serosa or amnioserosa anlage. Here, we introduce with Episyrphus balteatus (Syrphidae) a cyclorrhaphan model for embryonic AP axis specification that features an anterodorsal serosa anlage. Current phylogenies place Episyrphus within the clade that uses bicoid mRNA as anterior determinant, but no bicoid-like sequence could be identified in this species. Using RNA interference (RNAi) and ectopic mRNA injection, we obtained evidence that pattern formation along the entire AP axis of the Episyrphus embryo relies heavily on the precise regulation of caudal, and that anterior pattern formation in particular depends on two localized factors rather than one. Early zygotic activation of orthodenticle is separated from anterior repression of caudal, two distinct functions which in Drosophila are performed jointly by bicoid, whereas hunchback appears to be regulated by both factors. Furthermore, we found that overexpression of orthodenticle is sufficient to confine the serosa anlage of Episyrphus to dorsal blastoderm. We discuss our findings in a phylogenetic context and propose that Episyrphus employs a primitive cyclorrhaphan mechanism of AP axis specification.

Fidelity of Hymenoptera and Diptera pollinators in onion (Allium cepa L.) pollination

AbstractOnion (Allium cepa L.) is protandrous in nature and requires cross‐pollination to avoid inbreeding. The pollination potential of native bees (Hymenoptera) and true flies (Diptera) was assessed in the perspective of finding the best pollinators for onion cross‐pollination and seed multiplication. The community of pollinators was composed of four bee species and twelve true fly species. Episyrphus balteatus, Eupeodes sp., Musca domestica and Eristalinus aeneus were the most abundant pollinators. The maximum pollinator activity was observed from 12 to 24 days after opening of the flowers. The pollination effectiveness of tested bees (Apis dorsata and Apis florea) was greater than true flies (E. balteatus, Eupeodes sp., M. domestica, E. aeneus and Callihoridae sp.) in terms of Spears values.

Emission of alarm pheromone by non‐preyed aphid colonies

AbstractThe sesquiterpene (E)‐β‐farnesene (Eβf) is the primary component of the alarm pheromone of most aphid species. It is released in response to physical stress including attack by natural enemies and causes aphids to cease feeding and disperse. Eβf also acts as a kairomonal cue for aphid natural enemies. In this study, we collected the headspace volatiles released by aphid colonies of different sizes. Gas chromatography‐mass spectrometry analysis demonstrated the presence of Eβf in the absence of predator attack. A quadratic relationship was found between the released (E)‐β‐farnesene amounts and aphid colony size. Behavioural impact of aphid alarm pheromone towards Episyrphus balteatus female oviposition behaviour was also demonstrated in this work. These results highlight the primary role of the small but continuous release of aphid alarm pheromone in mechanisms of decision‐making by aphid predators during their foraging and egg‐laying behaviour.

Cereal aphids (Hemiptera: Aphidoidea) in Serbia: Seasonal dynamics and natural enemies

During 1989-2006 the number of live aphid, mummies, aphid specialist predators and their parasitoids in cereal fields in Serbia were recorded. S. avenae and M. dirhodum were the most common, both on wheat and rye crops. Maximum numbers of cereal aphids were detected between the end of May and the middle of June. A low percentage of mummified aphids was recorded during May and June, but increased rapidly at the beginning of July, when aphids left the wheat crop. Here, we record over 60 species of cereal aphid natural enemies, including primary and secondary parasitoids, aphid specific predators and a wide spectrum of their parasitoids in Serbian cereal crop systems. Aphidius uzbekistanicus Luzhetzki, Aphidius rhopalosiphi De Stefani, Aphidius ervi Haliday and Praon gallicum StarĂ˝ were the most abundant species of primary parasitoids. Of the secondary parasitoids, six species were dominant with Asaphes suspensus (Nees) and Dendrocerus carpenteri (Curtis) generally the most often recorded. Coccinella septempunctata L. was the most abundant coccinellid and syrphid flies were represented by 11 species, two of which, Sphaerophoria scripta (L.) and Episyrphus balteatus (DeGeer), were the most common.

Introducing barley as aphid reservoir in sweet-pepper greenhouses: Effects on native and released hoverflies (Diptera: Syrphidae)

Habitat management is a form of conservation biological control, that includes strategies such as the provision of alternative prey for natural enemies. One example is the strategy, which consists of introducing cereals with aphids in horticultural crops. It is usually combined with parasitoid releases. In this work we evaluated whether aphid-infested barley as enhances native populations of aphidophagous syrphids and extends the residence time of Episyrphus balteatus (De Geer) following release. The work was performed in four replicated sweet-pepper commercial greenhouses in the southeast of Spain. Barley was sown, and infested with the aphid Rhopalosiphum maidis (Fitch). Visual censuses were performed to record released individuals of E. balteatus (marked on the thorax before release) and naturally occurring hoverflies. At the end of the experiment leaf samples were taken to the laboratory, and the immature syrphids were reared for identification. The occurrence of syrphids that come from outside the greenhouse was significantly increased by the presence of the infested barley plants. From a total of 506 observations of adult native syrphids, 81% were Sphaerophoria rueppellii (Wiedemann). We did not find significant differences between control plots and those provided with banker plants in their probabilities to retain released specimens of E. balteatus. Of the syrphids collected at immature stages from the barley leaves, we did not record any E. balteatus, and 100% of the specimens were identified as S. rueppellii. We conclude that the banker plant strategy is effective in attracting natural populations of syrphids into sweet pepper greenhouses, but not in keeping released individuals of E. balteatus in the greenhouses long enough to lay eggs. We suggest that the E. balteatus release method should be amended.

Role of prey–host plant associations on Harmonia axyridis and Episyrphus balteatus reproduction and predatory efficiency

AbstractIn order to predict possible locations of Harmonia axyridis Pallas (Coleoptera: Coccinellidae) and Episyrphus balteatus DeGeer (Diptera: Syrphidae) in the field, we studied their oviposition and prey preferences in relation to several host plant–prey associations under laboratory conditions. Oviposition preference of H. axyridis and E. balteatus females was determined for three aphid (Homoptera: Aphididae)–host plant associations: Microlophium carnosum Buckton on stinging nettle [Urtica dioica L. (Urticaceae)], Acyrthosiphon pisum Harris on green pea [Pisum sativum L. (Fabaceae)], and Sitobion avenae F. on wheat [Triticum aestivum L. (Poaceae)]. Prey preference of H. axyridis and E. balteatus larvae was determined with the aphids M. carnosum, A. pisum, and S. avenae. Harmonia axyridis females showed a strong oviposition preference for the stinging nettle–M. carnosum association. The preferred association for ovipostion by E. balteatus was pea‐hosting A. pisum, on which fecundity was also highest. Feeding behaviour was less restricted in H. axyridis, in which the preferred preys were M. carnosum and S. avenae. A lower specificity of predation was observed in E. balteatus larvae with respect to A. pisum. These laboratory experiments may help us to understand the spatial distribution of H. axyridis and E. balteatus in the field.

Hover flies are efficient pollinators of oilseed rape

Understanding the consequences of declining diversity and abundance of pollinators for crops and floral biodiversity is a major challenge for current conservation ecology. However, most studies on this issue focus on bees, while other invertebrate taxa are largely ignored. We investigated the pollination efficiency of the globally abundant hover fly Episyrphus balteatus on the common crop, oilseed rape (Brassica napus). The study was conducted over a period of 2 consecutive years by means of enclosure experiments at an agricultural site located in Central Hesse (Germany). E. balteatus significantly increased both seed set and yield. This effect was very constant in the 2 years, despite considerable interannual differences in total seed numbers and seed mass. It highlights the important role of hover flies as pollinators of arable crops under varying environmental conditions. In contrast to bees, the effect of E. balteatus was lower at high pollinator densities than at low pollinator densities. This suggests adverse effects of density-dependent factors on pollination efficiency at high densities. Thus, models ignoring the modulating effect of biotic interactions by generally assuming a simple positive relationship between pollinator density and pollination efficiency might not apply to a vital component of the pollinator community.