Density of adult hoverflies (Dipt., Syrphidae) in sown weed strips and adjacent fields

In 1992 and 1993, studies examined numbers of adult hoverflies in sown weed strips, adjacent fields and a field boundary in the Swiss plateau, near Bern. Hoverflies were observed quantitatively and were divided according to their larval food. In both years, distinctly more aphidophagous and aquatic hoverflies were observed in the weed strips than in the adjacent fields. Copro- and phytophagous hoverflies were observed in very low numbers and they were almost entirely confined to the weed strips and the field boundary. The weed strips contained a high density of flowering plants and therefore proved to be very attractive feeding places for all hoverflies. Numbers of aphidophagous hoverflies declined slightly with increasing distance from the weed strips. Possible impacts of weed strips in encouraging aphidophagous hoverflies and their potential as aphid antagonists are discussed.     

Mobility and resource use influence the occurrence of pollinating insects in restored seminatural grassland fragments

After habitat restoration, species need to recolonize from existing populations. The ability of species to recolonize restored habitats likely depends on their traits. This study aimed to test if species traits and isolation from source habitat can explain the presence of insects in restored grasslands. We surveyed the occurrence of hoverflies and bees in 14 restored seminatural pastures as well as in intact seminatural grasslands in the surrounding landscape. We tested how connectivity, time since restoration, and species traits influence if species that are present in the surrounding landscape also occur in restored pastures. Solitary bee species present in the landscape were less likely to occur in restored pastures compared to bumblebees and hoverflies. The occurrence of bumblebees, but not solitary bees or hoverflies, decreased with time since restoration. The occurrence of solitary bees increased but the occurrence of hoverflies decreased with high connectivity. Migratory hoverflies were more likely to occur in restored pastures than nonmigratory hoverflies, especially in pastures with low connectivity. Among both bumblebees and solitary bees, the occurrence was influenced by nesting traits, with the lowest occurrence of parasitic species and of species digging nests in the ground. The subset of the landscape's species pool that occurs in restored pastures has a contrasting set of traits compared with species in intact source habitats. Both mobility and resource use act as filters that influence the assembly of pollinator communities after restoration. A full recovery of pollinator communities is more likely if source populations are available nearby.     

Smells like aphids: orchid flowers mimic aphid alarm pheromones to attract hoverflies for pollination

Most insects are dependent on chemical communication for activities such as mate finding or host location. Several plants, and especially orchids, mimic insect semiochemicals to attract insects for unrewarded pollination. Here, we present a new case of pheromone mimicry found in the terrestrial orchid Epipactis veratrifolia. Flowers are visited and pollinated by several species of aphidophagous hoverflies, the females of which also often lay eggs in the flowers. The oviposition behaviour of these hoverflies is mainly guided by aphid-derived kairomones. We show that the flowers produce α- and β-pinene, β-myrcene and β-phellandrene, and that these compounds attract and induce oviposition behaviour in female hoverflies. This floral odour profile is remarkably similar to the alarm pheromone released by several aphid species, such as Megoura viciae. We therefore suggest that E. veratrifolia mimics aphid alarm pheromones to attract hoverflies for pollination; this is the first time, to our knowledge, that such a case of mimicry has been demonstrated.     

谁是腊梅的主要传粉者:昆虫行为与传粉作用

访花昆虫不同的访花与飞行行为导致其访花频率小同,进而对植物传粉产生不同的影响.意大利蜜蜂(Apis mellifera ligustica)、苍蝇和食蚜蝇是腊梅(Chimonanthus praecox)最常见的访花类群(或种类),但是它们的访花行为却有很大不同.意大利蜜蜂主要以快速飞行为主,偶见爬行,苍蝇访花时主要以快速飞行、爬行和跳跃为主,而食蚜蝇以长时间单花访问和悬空飞行以及间歇性休息为主.根据访花类群(或种类)在一朵花上的访花时间和花间飞行时间进行推算,每只意大利蜜蜂、苍蝇和食蚜蝇每分钟分别可以访问4.57、2.65和0.53朵花.结合每种(类)昆虫的访花数量推算出意大利蜜蜂、苍蝇和食蚜蝇每分钟分别可以访问498.19、1,089.74和99.78朵花.传粉效力(相同条件下单位时间内相同数目访花者能够授粉的花朵数)实验结果证明苍蝇和意大利蜜蜂分别是93％、100％(n=30),而食蚜蝇只有13％(n=30).苍蝇和意大利蜜蜂可携带大量花粉且具有很高的传粉效力,因此,访花速度的快慢是二者访花能力强弱的主要限制因素.由此,我们认为苍蝇可能是腊梅最主要的传粉者,意大利蜜蜂次之(数量偏少),食蚜蝇再次(访花与飞行行为的影响以及访花效力较低是限制其访花能力的主要因素).     

The role of odour and visual cues in the pan-trap catching of hoverflies (Diptera: Syrphidae)

Coloured pan traps are frequently used to attract and catch insects, such as in the monitoring of populations of beneficial insects in classical or conservation biological control. They are also used in the evaluation of the recovery of insect populations after disturbance and in many other situations where an estimate of relative insect numbers is required. However, the fact that traps may be visible to the insects over a considerable distance can influence the interpretation of catch data. This difficulty may arise, for example, if traps along a transect can attract insects from some or all of the other transect positions. This study compared the effect of different coloured traps on attraction and catch of hoverflies. The hypothesis was that completely yellow traps would attract hoverflies from a distance, while traps that were green outside and yellow inside would catch fewer flies because only those from above or near the trap can see the yellow stimulus. A subsidiary hypothesis was that rose water would enhance hoverfly capture rates. For the two main hoverfly species captured, Melanostoma fasciatum (Macquart) and Melangyna novaezelandiae (Macquart), significantly more individuals were caught in completely yellow traps than in yellow and green or in completely green traps. Moreover, the addition of rose water increased the number of hoverflies caught significantly. It is suggested that if a measure of hoverfly numbers relating to a particular distance along a transect is required, consideration should be given to the ability of hoverflies to see yellow traps from a distance. The use of traps that are green outside would more accurately reflect the local abundance of hoverflies. If higher trap catches of hoverflies are needed for statistical purposes, rose water can enhance catches.     

An exceptionally high diversity of hoverflies (Syrphidae) in the food of the reed warbler (Acrocephalus scirpaceus)

Despite being considered a classical example of protective Batesian mimicry hoverflies (Syrphidae) are known to be preyed upon by various passerines. The aim of the present study was to examine in detail food brought by reed warblers Acrocephalus scirpaceus to their nests to better understand the importance of hoverflies in the diet of small passerines. Using neck collars, 273 food samples containing 8,545 food items delivered to reed warbler and parasitic common cuckoo Cuculus canorus nestlings in warbler nests were recorded. The study was conducted during three breeding seasons in South Moravia, Czech Republic. An unusually high diversity of hoverflies was found — 27 species, including Mesembrius peregrinus (critically endangered species in the Czech Republic) and Mallota cimbiciformis (endangered species) — a new taxon to the Czech Republic. This indicates that nestling diet analyses may provide not only information on avian foraging behaviour but also important faunistic data. Thus, without the detailed identification to species level of material from foraging behaviour studies valuable scientific information may be lost. Overall dominance of Syrphidae was 3.7%, the most common species being Episyrphus balteatus (55.7%, n = 318). However, this number seriously underestimates the importance of hoverflies in the diet of reed warblers as hoverflies are one of the largest prey taken by warblers. Both larvae and pupae were rare, imagines strongly dominating (92.7%). Both specific wasp mimics (e.g., Chrysotoxum verrali) and bee mimics (e.g., Eristalis spp.) were not avoided by foraging reed warblers. The presence of a parasitic cuckoo chick did not affect host foraging behaviour with respect to overall dominance of hoverflies in the diet (warbler 3.3%, cuckoo 3.8%).     

Time since establishment drives bee and hoverfly diversity,abundance of crop-pollinating bees and aphidophagous hoverflies in perennial wildflower strips

Wildflower strips (WFS) are amongst the most commonly applied measures to promote pollinators and natural enemies of crop pests in agroecosystems. Their potential to enhance these functionally important insect groups may vary substantially with time since establishment of WFS. However, knowledge on their temporal dynamics remains scarce, hampering recommendations for optimized design and management. We therefore examined temporal dynamics of taxonomic and functional groups of bees and hoverflies in perennial WFS ranging from one to ≥6 years since sowing with a standardized species-rich seed mixture of flowering plants in 18 agricultural landscapes in Switzerland. The abundance of wild bees, honeybees and hoverflies declined after the second year by 89%, 62% and 72%, respectively. Declines in bee abundance and hoverfly species richness were linear and those of aphidophagous hoverflies exponential, while wild bee species richness peaked in the third year. Declines over time generally paralleled decreases in flower abundance (-83%) and flowering species richness (-61%) and an increase in grass cover (+70%) in WFS. Flowering plant species richness showed strong positive relationships with dominant crop-visiting wild bees and aphidophagous hoverflies. Furthermore, dominant crop-visiting wild bees, but not aphidophagous hoverflies, were positively related to the proportion of (semi-)open semi-natural habitat in the surrounding landscape (500 m radius), but negatively with forest. We conclude that the effectiveness of perennial WFS to promote pollinator diversity, crop-pollinating bees and aphidophagous hoverflies through foraging resources decreases after the first two to three years, probably due to a decline of diverse and abundant floral resources. Although older perennial WFS may still provide valuable nesting and overwintering opportunities for pollinators and natural enemies, our findings indicate that regular re-sowing of perennial WFS may be necessary to maintain adequate floral resource provisioning for effective pollinator conservation and promotion of crop pollination and natural pest control services in agricultural landscapes.     

Hoverfly (Diptera: Syrphidae) community of a cultivated arable field and the adjacent hedgerow near Debrecen,Hungary

A hoverfly (Syrphidae) community was investigated in a cultivated wheat field and the adjacent hedgerow near Debrecen (Hungary). We monitored the change of species richness and abundance of hoverflies along three transects in the hedgerow and in the wheat field in different distances (10 m, 20 m) from the hedgerow. The effect of sampling methods on the number of hoverfly species and individuals was analyzed. Two sampling methods were used to catch hoverflies: netting and pan traps. The whole sampling period was divided into three subperiods, which are early (22^nd April–2^nd June), middle (11^th June–1^st August) and late (6^th August–11^th September). Altogether 1,214 individuals of 22 species were sampled. Fourteen species with 78% of individuals belonged to the aphidophagous group, feeding on aphids as larvae. Altogether 861 individuals of 22 hoverfly species were sampled by netting and 353 individuals of 10 species by pan traps. The total number of hoverfly species was significant lower in the late period than in the early. The total abundance was higher in the middle period compared to the early. The species richness and abundance of aphidophagous species followed a similar pattern as the total species values. The species richness and abundance of hoverflies were significant higher in 10 m and 20 m distance in the wheat field than along the hedgerow. The results suggest that the pan traps were less efficient in the hedgerow than the netting, but in the wheat field they sampled more hoverflies because of visually alluring effect on hoverflies in the absence of flowers.     

Effects of climate change on the distribution of hoverfly species (Diptera: Syrphidae) in Southeast Europe

Climate change presents a serious threat to global biodiversity. Loss of pollinators in particular has major implications, with extirpation of these species potentially leading to severe losses in agriculture and, thus, economic losses. In this study, we forecast the effects of climate change on the distribution of hoverflies in Southeast Europe using species distribution modelling and climate change scenarios for two time-periods. For 2041–2060, 19 analysed species were predicted to increase their areas of occupancy, with the other 25 losing some of their ranges. For 2061–2080, 55% of species were predicted to increase their area of occupancy, while 45% were predicted to experience range decline. In general, range size changes for most species were below 20%, indicating a relatively high resilience of hoverflies to climate change when only environmental variables are considered. Additionally, range-restricted species are not predicted to lose more area proportionally to widespread species. Based on our results, two distributional trends can be established: the predicted gain of species in alpine regions, and future loss of species from lowland areas. Considering that the loss of pollinators from present lowland agricultural areas is predicted and that habitat degradation presents a threat to possible range expansion of hoverflies in the future, developing conservation management strategy for the preservation of these species is crucial. This study represents an important step towards the assessment of the effects of climate changes on hoverflies and can be a valuable asset in creating future conservation plan, thus helping in mitigating potential consequences.     

Oviposition preferences of aphidophagous hoverflies

1. Oviposition preferences of two predatory hoverflies, Episyrphus balteatus and Syrphus ribesii (Diptera, Syrphidae), were studied in the laboratory. 2. There was broad agreement between the two species: two of the top three preferred prey for oviposition in both hoverflies were pea and rose aphids; nettle aphids were consistently the least preferred. 3. Discrimination decreased with age.     

Hoverflies (Diptera: Syrphidae) benefit from a cultivation of the bioenergy crop Silphium perfoliatum L. (Asteraceae) depending on larval feeding type,landscape composition and crop management

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Attractiveness of common insectary and harvestable floral resources to beneficial insects

Resources for natural enemies are often lacking in agricultural fields. The provisioning of floral resources in crop fields can ameliorate this problem by providing nectar and pollen to natural enemies. To select an appropriate floral resource, plants must be screened for their flowering times and attractiveness to natural enemies and pests. We tested the attractiveness of nine species of annual flowering plants to hoverflies, which are important predators of aphids in California lettuce fields. We also sampled arthropods in the foliage of tested plants, to assess the abundances of other natural enemy and pest species that were present. Tested plants included three commonly-used insectary plants and six flowering plant species that showed potential as either harvestable herbs or cut flowers. Harvestable insectary plants may provide additional economic incentive for growers to set aside land for floral resources. The commonly-used insectary plant sweet alyssum consistently attracted the most hoverflies and the least bees, while potentially harvestable plants attracted few hoverflies. Competition with bees may have reduced hoverfly visits to several of the tested plant species. Sweet alyssum also stayed in bloom the longest, and contained the highest numbers of predatory hemipterans. Results suggest that plants should be screened for their attractiveness to not only the target biological control agent, but also to other potential competitors for floral resources. While this initial study focused on a limited selection of harvestable annual plants, a wide variety of other marketable plant species, particularly perennials, remain to be tested for their attractiveness to hoverflies or other beneficial arthropods.     

The selective use of floral resources by the hoverfly Episyrphus balteatus (Diptera: Syrphidae) on farmland

The use of flowers of non-crop plants by the syrphid Episyrphus balteatus in margins of cereal fields was assessed quantitatively. G-tests and the ‘C’ index of Murdoch (1969) were used to compare the use and abundance of twenty-seven plant species by adult flies from June to August in the UK. The flowers of several plant species were used selectively by hoverflies throughout the season while the relative importance of others changed with date. The prospects for managing the field-margin flora to enhance biocontrol by hoverflies is discussed.     

Landscape matrix modifies richness of plants and insects in grassland fragments

There is an increasing awareness that not only area and isolation, but also the characteristics of the landscape surrounding habitat patches influence population persistence and species diversity in fragmented landscapes. In this study, we examine the effects of grassland fragmentation and land use in the landscape matrix (on a 2 km scale) on species richness of plants, butterflies, bees and hoverflies. These organisms were studied in replicated remnant patches of different sizes and isolation, embedded in landscapes dominated either by forest, arable land or a mix of these. We found positive effects of patch area on species richness of the three insect taxa, but not of plants. Isolation had a negative effect only on hoverflies. Matrix type had contrasting effects on the studied taxa. Species richness of plants and butterflies was lowest in patches in landscapes dominated by arable land and highest in forest‐dominated landscapes. For hoverflies, the negative effect of small patch area was strongest in forest‐dominated landscapes, and there was a similar non‐significant trend for bees. Our study shows the importance of considering matrix characteristics when studying responses to habitat fragmentation. Differences in matrix response among organism groups probably impinge on differing mechanisms. A forest matrix is likely to provide additional resources for butterflies but either constitute a barrier to dispersal or deprive resources as compared to an arable matrix for hoverflies. Enhanced plant diversity in grassland patches embedded in forested landscapes can be explained by habitat generalists more easily invading these patches, or by an unpaid extinction debt in these landscapes.     

Pollinator guilds respond contrastingly at different scales to landscape parameters of land‐use intensity

Land‐use intensification is the main factor for the catastrophic decline of insect pollinators. However, land‐use intensification includes multiple processes that act across various scales and should affect pollinator guilds differently depending on their ecology. We aimed to reveal how two main pollinator guilds, wild bees and hoverflies, respond to different land‐use intensification measures, that is, arable field cover (AFC), landscape heterogeneity (LH), and functional flower composition of local plant communities as a measure of habitat quality. We sampled wild bees and hoverflies on 22 dry grassland sites within a highly intensified landscape (NE Germany) within three campaigns using pan traps. We estimated AFC and LH on consecutive radii (60–3000 m) around the dry grassland sites and estimated the local functional flower composition. Wild bee species richness and abundance was positively affected by LH and negatively by AFC at small scales (140–400 m). In contrast, hoverflies were positively affected by AFC and negatively by LH at larger scales (500–3000 m), where both landscape parameters were negatively correlated to each other. At small spatial scales, though, LH had a positive effect on hoverfly abundance. Functional flower diversity had no positive effect on pollinators, but conspicuous flowers seem to attract abundance of hoverflies. In conclusion, landscape parameters contrarily affect two pollinator guilds at different scales. The correlation of landscape parameters may influence the observed relationships between landscape parameters and pollinators. Hence, effects of land‐use intensification seem to be highly landscape‐specific.     

Saproxylic hoverflies benefit by modern forest management (Diptera: Syrphidae)

Saproxylic organisms are regarded as threatened in many European countries. Nevertheless, there have been some signals indicating a recent increase of saproxylic hoverflies in the Netherlands. This paper examines the change in occurrence of saproxylic hoverflies compared to other ecological groups of the same family. The trend analysis is based on the database of an extensive recording programme carried out in the Netherlands. The results show an increase in occupied grid cells for a majority of saproxylic species in relation to other ecological groups. This positive trend occurs among all five ecological subcategories of saproxylic hoverflies recognized in this paper. It is suggested that this increase could be attributed to the changes in the Dutch forests during the past 50 years. Important aspects of these changes are: the increase in forest area (including an increase in area of forest with large and very large trees), the increase in age of forests and the changes in forest management in favour of a more natural approach towards dead and ill trees. In the discussion the possible influence of climate change and the method of trend calculation are discussed.     

Response of pollinators to the tradeoff between resource acquisition and predator avoidance

Although the behaviour of animals facing the conflicting demands of increasing foraging success and decreasing predation risk has been studied in many taxa, the response of pollinators to variations in both factors has only been studied in isolation. We compared visit rates of two pollinator species, hoverflies and honeybees, to 40 Chrysanthemum segetum patches in which we manipulated predation risk (patches with and without crab spiders) and nectar availability (rich and poor patches) using a full factorial design. Pollinators responded differently to the tradeoff between maximising intake rate and minimising predation risk: honeybees preferred rich safe patches and avoided poor risky patches while the number of hoverflies was highest at poor risky patches. Because honeybees were more susceptible to predation than hoverflies, our results suggest that, in the presence of competition for resources, less susceptible pollinators concentrate their foraging effort on riskier resources, where competition is less severe. Crab spiders had a negative effect on the rate at which inflorescences were visited by honeybees. This effect was mediated through changes in the foraging strategy of honeybees, and could, in principle, be reversed by increasing nectar productivity of inflorescences. Our study shows that both pollinator species responded simultaneously and differently to variations in food reward and predation risk, and highlights the importance of studying the foraging strategies of pollinators in order to fully understand how plant–pollinator interactions are established.     

Contrasting responses in community structure and phenology of migratory and non‐migratory pollinators to urbanization

Aim

Anthropogenic landscape change, such as urbanization, can affect community structure and ecological interactions. Furthermore, changes in ambient temperature and resource availability due to urbanization may affect migratory and non‐migratory species differently. However, the response of migratory species to urbanization is poorly investigated, and knowledge for invertebrates in particular is lacking. Our aim was to investigate whether there was a shift in community structure and phenology of hoverflies in urban landscapes, depending on migratory status.

Location

Switzerland.

Methods

Using a paired design, we compared urban and rural landscapes to investigate the impact of urbanization on the abundance, diversity and phenology of hoverflies. Furthermore, we tested whether migratory and non‐migratory species responded differently to urbanization.

Results

We observed a difference in the response of migratory and non‐migratory hoverfly communities. Although the abundance of hoverflies was higher in the rural ecosystem, driven by a high abundance of migratory species, there was no difference in species richness between the land use types. However, the community structure of non‐migratory species was significantly different between urban and rural ecosystems. The phenology of hoverflies differed between the two ecosystems, with an earlier appearance in the year of migratory species in urban landscapes.

Main conclusions

To our knowledge, this is the first study to investigate the response of migratory insect communities to urbanization. We demonstrated that migratory and non‐migratory hoverflies respond differently to urbanization. This highlights the importance of differentiating between trait and mobility groups to understand community assemblage patterns in anthropogenic landscapes. The differences in phenology supports the growing evidence that urbanization not only affects the phenology of vegetation, but also affects the higher trophic levels. Changes in the phenology and community composition of species as a result of anthropogenic landscape change may have important implications for the maintenance of key ecosystem functions, such as pollination.

     

Diurnal activity patterns in hoverflies (Diptera, Syrphidae)

Abstract. 1. Time-budgets are constructed from censuses of hoverflies. Larger species spend less time in flight.
2. Most species are active under similar conditions of light intensity, but a shade-tolerant (Melanostoma scalare Fabr.) and a sun-loving species (Metasyrphus corollae Fabr.) were identified.
3. Large species become active (i.e. move, by flying or feeding) at lower temperatures than small ones, except M.scalare, which is tolerant to cool temperatures. This means that, M.scalare apart, the order of species appearing during the day is size-dependent, largest first.
4. Previous claims about the diel periodicity of syrphids can be reconciled by considering the importance of individual thermal balance.
5. Tolerance to low temperatures in small hoverflies is associated with feeding on anemophilous pollen; this may indicate a proline-fuelled flight.