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Keywords:

  • bluetongue;
  • dispersal;
  • Rothamsted suction trap;
  • surveillance;
  • vector

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

1.Culicoides biting midges are vectors of internationally important arboviruses including bluetongue virus (BTV). The ecological constraints imposed by the small body size of these insects strongly influence the epidemiology of the diseases they can carry. Bluetongue virus recently emerged in northern Europe, and atmospheric dispersion models have subsequently been employed to simulate vector movement (and hence likely spread of BTV). The data underlying such models, however, have hitherto either been obtained from small-scale studies or from outside the north-western Palaearctic.

2. The effects of seasonality and local meteorological conditions upon the daily presence and abundance of Culicoides vectors were examined using 2760 samples collected across a network of 12 different habitat types in England during 2008. Over 50 000 individuals were estimated to be in the samples with males constituting 62% of the total collection, allowing straightforward comparison between potential vector species in terms of their activity rates and seasonality. Culicoides abundance was linked to livestock density and land use. Farm-associated Culicoides species were recorded at all sites including species thought to be restricted to this ecosystem by larval habitat, suggesting a greater potential for dispersal over land than previously thought.

3.Synthesis and applications. The model developed has already been applied in a functional dispersion model to predict disease risk from wind-borne infected Culicoides incursion into the UK and elsewhere. The study has expounded the long-distance dispersal potential of Culicoides, essential for future prediction of the incursion and spread of Culicoides-borne pathogens. It has additionally contributed to the understanding of the ecology of highly dispersive insect vectors.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

In 2006, a bluetongue virus (BTV) strain of sub-Saharan origin (BTV-8) entered northern Europe, initiating an epidemic that was to prove the most economically damaging outbreak of the virus ever recorded (Maan et al. 2008; Wilson & Mellor 2009). From approximately 2000 cases recorded in the Netherlands, Germany, mainland France and Belgium in 2006, the virus spread rapidly in subsequent years to cause tens of thousands of clinical cases over much of western Europe including the UK (Wilson & Mellor 2009). Outbreaks were eventually brought under control through the use of vaccination campaigns and zoosanitary measures; however, the probability of other strains entering this region remains poorly defined both for BTV and for other pathogens transmitted by Culicoides, including African horse sickness virus (AHSV) (Carpenter, Wilson & Mellor 2009a).

Understanding the ecology and behaviour of vector Culicoides species is a key element in predicting the epidemiology of those pathogens they transmit, and many Culicoides life cycle parameters are strongly related to meteorological conditions (Mellor, Boorman & Baylis 2000; Wittmann & Baylis 2000; Carpenter, Wilson & Mellor 2009b). The dispersal ability and the seasonality of Culicoides adults are of importance in this regard as they largely determine the pathogen spread in the absence of viraemic livestock movement. Culicoides are sensitive to desiccation and have low self-propelled flight speeds in comparison with many other vector groups. Appetitive flight is governed by temperature, humidity, and light intensity thresholds and appears to be inhibited by wind speeds >3 ms−1 (Kettle 1957, 1969; Blackwell 1997; Carpenter et al. 2008b).

Flight distances recorded vary from only a few hundred metres (Kettle 1951; Lillie, Marquard & Jones 1981; Murray 1987) to several kilometres (Kettle 1960; Sellers 1980; Murray & Kirkland 1995) although cues and physiological limitations that restrict range are poorly understood. Many insects use the fast moving nocturnal, low-level jet stream at the top of the atmospheric boundary layer to assist long-distance migration (Chapman et al. 2004; Reynolds, Smith & Chapman 2008), and individual Culicoides have been caught at heights of several hundred metres (Glick 1939; Hardy & Cheng 1986; Johansen et al. 2003; Sanders et al. 2011).

Studies of Culicoides long-distance dispersal are usually inferred from incursions of Culicoides-borne pathogens onto islands and across large stretches of water in the absence of animal movement (Sellers 1980; Calistri et al. 2003; Alba, Casal & Domingo 2004; Gloster et al. 2008). Movement can be inferred over land in a similar way (Sellers 1980; Murray & Kirkland 1995; Braverman & Chechik 1996; Ducheyne et al. 2007); however, in such cases, confounding animal movements and intrinsic difficulties in modelling wind movement over topography at a local scale can be problematic. Nevertheless, models of the probability of movement of Culicoides have proved to be useful tools in the prediction of incursion routes and in the retrospective analyses of outbreaks (Gloster et al. 2008; Hendrickx et al. 2008; Agren et al. 2010).

The seasonal dynamics of adult Culicoides populations are important as they in part determine the timing of outbreaks of Culicoides-borne arbovirus outbreaks (Herniman, Boorman & Taylor 1983; Baylis et al. 1997). Studies of seasonality in European Culicoides have to date largely relied upon the collection of adults in light traps on a weekly sampling basis (Ortega, Holbrook & Lloyd 1999; Conte et al. 2007; Clausen et al. 2009; De Liberato et al. 2010). In the UK, Culicoides have been observed to occur in one, two or three seasonal abundance peaks depending upon species, location and the trapping methodology used (Hill 1947; Service 1971, 1974; Holmes & Boorman 1987; Blackwell et al. 1992; Blackwell 1997).

The Rothamsted Insect Survey routinely monitors aphids and other migrating insects in the UK by means of the Rothamsted suction trap (RST) (Macaulay, Tatchell & Taylor 1988; Harrington & Woiwod 2007) network, and Culicoides represent a part of the by-catch (Shortall et al. 2009). The 12·2 m height of these traps was originally chosen to minimize sampling of insects flying locally whilst sampling a dense layer of migrant insects (Macaulay, Tatchell & Taylor 1988). Whilst a previous, small-scale study of the abundance of Culicoides caught in two RSTs has been carried out in Belgium, the total number caught was too low (n = 761) and the sampling period too short to elucidate more than general trends in the effects of meteorological conditions on monthly activity (Fassotte et al. 2008).

The aim of the present study was to provide a more complete understanding of the effects of seasonal population dynamics and meteorological conditions on the daily flight activity of vector Culicoides as demonstrated from their abundance in RST catches. The data presented provide a significant advance in our understanding of parameters influencing Culicoides ecology, and hence pathogen dispersal and seasonality in the field.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Rothamsted suction traps

Each RST provided a continuous (24 h), standardized sample at a rate of 45 m3 of air per minute through an inlet 12·2 m above-ground level (Macaulay, Tatchell & Taylor 1988). Briefly, the trap consists of an electric fan which pulls air down a vertical tube through an air expansion chamber, to reduce insect velocity, and into a collecting bottle containing methanol. Insect samples were stored in 95% ethanol plus 5% glycerol for identification. Twelve traps at sites across England (Fig. 1) were used in locations representing a variety of habitat types and livestock densities (Table 1). Traps were run continuously in 2008, and samples were collected once every 24 h from 31 March to 16 November. At other times, samples were collected weekly and the numbers divided by seven to create a daily count.

image

Figure 1.  Map showing the location of the twelve Rothamsted suction trap sites. N = Newcastle, AB = Askham Bryan, P = Preston, K=Kirton, BB = Broom’s Barn, We = Wellesbourne, H = Hereford, RT = Rothamsted Tower, Wr = Writtle, SP = Silwood Park, W = Wye, SX = Starcross.

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Table 1.   Rothamsted suction trap site locations with habitat type and cattle density [Cattle density extracted from RADAR (2008)]
SiteLatitudeLongitudeAltitude (m)Habitat typeCattle density (per square km)
Askham Bryan53·992−1·16230Arable21–40
Broom’s Barn52·260·57070Arable<10
Hereford52·125−2·63784Mixed41–80
Kirton52·937−0·0703Arable<10
Newcastle55·213−1·68293Mixed/woodland21–40
Preston53·854−2·76315Pastoral>80
Rothamsted51·807−0·356119Arable<10
Silwood51·408−0·64164Mixed/woodland<10
Starcross50·628−3·45412Pastoral/saltmarsh41–80
Wellesbourne52·201−1·60045Arable21–40
Writtle51·7330·42738Mixed<10
Wye51·1850·93943Mixed10–20

Culicoides recovered from RST samples were separated for further identification. In samples containing more than 50 Culicoides, the total number was estimated by assuming the remaining fraction of the sample volume contained a similar density of Culicoides. Culicoides were identified to species level by wing morphology where possible (Campbell & Pelham-Clinton 1960). Females of the C. obsoletus group (C. obsoletus s.s. (Meigen), C. scoticus (Downes & Kettle), C. dewulfi (Goetghebeur) and C. chiopterus (Meigen)) were not separated to species level because of the unsuitability of the samples for molecular identification techniques. Males of the C. obsoletus group were identified to species level by genital morphology (Campbell & Pelham-Clinton 1960) and were used as a proxy for the relative abundance of females of each species. Hourly meteorological data, including temperature, wind speed, wind direction, cloud cover, rainfall and solar radiation, were provided by the Met Office and were taken from meteorological stations at each trap site or within 20 km.

Statistical analysis and modelling

Summary statistics for the RST catches for each site (total caught, time of first appearance) were analysed using generalized linear models (assuming negative binomial errors and log link or normal errors and identity link, respectively) to identify potential associations with latitude, habitat type and cattle density.

To explore seasonal trends or the effects of meteorological parameters on the numbers of Culicoides caught, however, requires a more detailed analysis because of several features of the RST catch data. First, and most importantly, the RST catches can vary over several orders of magnitude from one day to the next, making it problematic to discern trends on the basis of a descriptive analysis of the data. Secondly, the trap catches are collected over time, so that catches from one day may depend on those for previous days. Thirdly, the trap catches are collected at a number of different sites, with potentially different seasonal dynamics at each. Consequently, to understand the influence of seasonal variation and meteorological conditions on the abundance of Culicoides requires a statistical approach that accounts for these features of the data.

Accordingly, the daily RST catch data were analysed using generalized linear models (assuming Poisson errors and a log link function) that incorporated seasonal components (with different periods) and meteorological parameters. Furthermore, the approach included overdispersion (to allow for the high day-to-day variability in catches), temporal autocorrelation amongst the observations (to allow for dependence between observations) and hierarchical structure in the model parameters (to allow for between-site differences). Parameters in the model were estimated in a Bayesian framework implemented using WinBUGS (Lunn et al. 2000).

Separate models were constructed for eight of the RST catch data sets: total Culicoides; C. obsoletus group females; C. obsoletus s.s. males; C. scoticus males; C. dewulfi males; C. chiopterus males; C. pulicaris males; and C. punctatus males. Full details of the models, their implementation, model checking and inference are provided in Appendix S1, Supporting Information.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Rothamsted suction traps

A total of 20 156 Culicoides were identified from the 2008 samples providing an estimated total catch of 51 552 Culicoides over the 12 sites. Standardization by estimating then counting all Culicoides in 35 of the large samples (>50 Culicoides) demonstrated a good correlation between estimates and the total number of Culicoides in the sample (r = 0·9293; error tended towards underestimation). Trap catches were dominated by the C. obsoletus group (48% of the total catch) and C. pulicaris/C. punctatus (44%) species groups, with the remainder (8%) being other Culicoides species (in order of abundance: C. impunctatus (Goetgh.), C. circumscriptus (Keiffer), C. stigma (Meigen), C. festivipennis (Keiffer), C. achrayi (Kettle and Lawson), C. brunnicans (Edwards), C. nubeculosus (Meigen), C. duddingstoni (Kettle and Lawson), C. segnis (Campbell and Pelham-Clinton), C. vexans (Staeger), C. grisescens (Edwards), C. salinarius (Keiffer), C. maritimus (Keiffer), C. pictipennis (Staeger), C. fagineus (Edwards)). Culicoides impunctatus was present only at Preston, Newcastle and Hereford and represented 1% of the total catch. Males of all four members of the C. obsoletus group were recorded at every trap site (Table 2). Large differences were recorded between trap sites in the abundance and species composition of Culicoides2 = 6013·5, d.f. = 33, < 0·001). There was a male bias at many sites (χ2 = 3311·5, d.f. = 11, < 0·001), with 62% of the total catch being male.

Table 2.   Summary of Culicoides catches from Rothamsted suction traps at 12 sites in 2008. Culicoides obsoletus group males are split into the species (C. obsoletus, C. scoticus, C. dewulfi, C. chiopterus) and individuals too damaged for identification
SiteFirst appearance†C. obsoletus groupC. pulicarisC. punctatusOther CulicoidesTotal
obs ♂scot ♂dew ♂chi ♂dam ♂
  1. obs, obsoletus; scot, scoticus; dew, dewulfi; chi, chiopterus.

  2. †No. days after 1 January 2008 at which >5 individuals first appeared; time shown is for C. obsoletus group, with C. pulicaris shown in brackets if earlier.

Askham Bryan1246741674473483461636217637991745
Broom’s Barn1289717354092601185221
Hereford11382817087178121107019271941071002055
Kirton11712022755486105133117343
Newcastle130 (115)9544493807911717132923085383322434397
Preston121 (90)657289673994274033257579732859675702130532 923
Rothamsted11723323239281009523910445
Silwood1132403998465518151023463289954
Starcross127 (90)77181238738121103955493919123886367
Wellesbourne11742847393322730163519529161113
Writtle1171286323733136730253356
Wye116107405512010172201562635633
Total 11 15220141686216778049110191378577514 7641492221051 552

Total abundance of Culicoides increased significantly (< 0·001) with the density of cattle in the locality of the trap. Higher abundances were also associated (< 0·001) with pastoral compared with mixed or arable habitat types (although habitat type and cattle density are correlated; Table 1). There was no significant association between total abundance and latitude (= 0·09). Culicoides were most abundant at Preston, with 64% of the total estimated catch derived from this trap (Table 2). Culicoides punctatus (Meigen) and C. chiopterus males represented a large proportion of the Preston samples (29·4% and 22·5% respectively) with C. obsoletus group females comprising just 20% at this site (Table 2). The trap at Starcross also exhibited a dominance of C. punctatus males. At this trap, however, C. scoticus represented the largest proportion of the C. obsoletus group males (58%). The abundance of C. punctatus males at Newcastle, in addition to the large number of C. obsoletus s.s., C. scoticus and C. dewulfi males at this site, also skewed the sex ratio towards males. At the remaining sites, where Culicoides were less abundant, C. obsoletus group females were dominant (Table 2).

The first appearance of Culicoides (defined as a daily catch of greater than five individuals) was not significantly correlated with latitude (F1,10 = 2·59, = 0·14), habitat type (F4,7 = 0·82, = 0·56) or cattle density (F4,7 = 0·40, = 0·81) (Table 2). Daily abundance of Culicoides was highly variable at all sites, with day-to-day catches differing by several orders of magnitude (Figs 2 and 3). Even with daily data summarized to a weekly mean catch (Fig. 2), trends in abundance are difficult to ascertain visually. Most sites demonstrated greater Culicoides abundance during spring (April/May) and autumn (September/October) with the exception of Newcastle where abundance was greatest in mid-summer (Fig. 2). Culicoides abundance during the summer at the other sites was typically smaller and less sustained than the peaks earlier and later in the year (Fig. 2). A reduction in Culicoides abundance was noted at all sites in June.

image

Figure 2.  Weekly summary of the numbers of Culicoides biting midges (total male and female) caught at the twelve Rothamsted suction trap sites during 2008: mean daily catch (circles) and minimum and maximum daily catches (error bars) for each week.

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image

Figure 3.  Observed (red circles) and expected (black line: posterior median; blue dotted lines: 2·5th and 97·5th percentiles for the posterior predictive distribution) numbers of Culicoides biting midges (total male and female) caught each day for the twelve Rothamsted suction trap sites during 2008.

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The seasonal activity of Culicoides varied with the abundance of different species at each site (Fig. 4). Culicoides punctatus and C. pulicaris were caught for a longer period than the C. obsoletus group, emerging earlier in the year and staying later than many other species, and this was reflected in the extended Culicoides season length where these species were common (Fig. 4). At the Broom’s Barn and Kirton sites where Culicoides abundance rarely exceeded five per catch, the season of activity was restricted (Fig. 4). C. obsoletus group females and males of each of the four species were present in catches for much of the season at Preston (Fig. 4). At other sites, different species showed shorter periods of activity. At Hereford, C. obsoletus s.s., C. scoticus and C. dewulfi were more abundant earlier in the season than C. chiopterus (Fig. 4); however, at Newcastle and Askham Bryan, C. dewulfi appeared later in the season than C. obsoletus s.s. (Fig. 4).

image

Figure 4.  Seasonal activity of Culicoides biting midges at each of the twelve Rothamsted suction trap sites. Open and closed triangles indicate the first and last days, respectively, on which each species appeared (defined as more than five midges caught); if more than five individuals per day were never caught at the site, no lines or symbols are shown. Colours indicate species: total Culicoides (red); C. obsoletus group females (green); C. obsoletus s.s. males (blue); C. scoticus males (cyan); C. dewulfi males (magenta); C. chiopterus males (black); C. pulicaris males (grey); and C. punctatus males (orange).

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Statistical modelling

Full results for the statistical modelling are presented in the Supporting Information, including parameter estimates for the different models (Tables S1–S3) and comparison of the models and data (Fig. 3; Figs S1–S7). Model checking indicated that the models adequately captured the data (see, for example, Fig. 3) in terms of overall fit, total catch, maximum daily catch and time of first appearance (see Appendix S1 for details).

Seasonality

Statistical modelling did identify seasonal trends in abundance (Fig. 5), but ascribed any apparent peaks at short time-scales to variation in trap catches rather than anything systematic. Seasonal trends with bimodal peaks (in spring and autumn) were identified for total Culicoides, C. obsoletus group females, C. pulicaris males and C. punctatus males (Fig. 5); though, these peaks were only particularly pronounced for C. pulicaris and C. punctatus. By contrast, males of the C. obsoletus group exhibited only a single underlying peak in abundance (Fig. 5). The peaks in abundance for C. dewulfi and C. obsoletus s.s. typically occurred in spring (an exception being the peak for C. obsoletus s.s. at Preston which occurred in autumn), whilst that for C. chiopterus occurred in autumn; however, timing of the peaks for C. scoticus differed much more amongst sites.

image

Figure 5.  Estimated seasonality components (posterior medians) in the species-level models for the number of male Culicoides biting midges caught each day for the twelve sites of the Rothamsted suction traps during 2008. (a) Total Culicoides; (b) C. obsoletus group females; (c) C. obsoletus s.s. males; (d) C. scoticus males; (e) C. dewulfi males; (f) C. chiopterus males; (g) C. pulicaris males; and (h) C. punctatus males. In each figure, sites are identified by the line style: Askham Bryan (solid cyan line); Broom’s Barn (dashed blue line); Hereford (solid magenta line); Kirton (dashed green line); Newcastle (solid blue line); Preston (solid black line); Rothamsted (dashed red line); Silwood (solid red line); Starcross (solid green line); Wellesbourne (dashed cyan line); Writtle (dashed magenta line); and Wye (dashed black line).

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The estimated seasonality in the total Culicoides abundance (Fig. 5a) was compared with that for the species-level abundance of male Culicoides (Fig. 5) by summing the individual species components weighted by their frequency in the trap catches (Fig. S8 Supporting Information). This suggested that the differences amongst sites in total Culicoides abundance reflect differences in the species composition at each site.

Meteorological parameters

The influence of meteorological parameters on trap catches was broadly consistent across species, with larger catches being associated with higher temperatures, lower wind speeds and days on which it was not raining at sunset (Tables S1–S3 Supporting Information); though, the magnitude of the effects did differ between species. There was, however, one exception to this pattern, with higher catches of C. pulicaris being associated with lower temperatures (Table S3 Supporting Information). Moreover, the effect of meteorological parameters was significant only at sites where large numbers of Culicoides were caught. For example, a significant effect of temperature was identified for C. obsoletus s.s. males at Askham Bryan, Hereford, Newcastle and Preston, but not at other sites.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Using daily Culicoides data from 12 suction traps across an entire year, this study examines in unique detail the effects of meteorological conditions and seasonal dynamics on population abundance and dispersal. Although the vector status of Culicoides in northern Europe remains poorly defined (Carpenter et al. 2008a), trap samples were dominated by farm-associated species that have been implicated as potential vectors of BTV (Carpenter et al. 2006; Mehlhorn et al. 2007; Meiswinkel et al. 2007; Dijkstra et al. 2008; Carpenter, Wilson & Mellor 2009a). In contrast to most studies of Culicoides abundance which tend to concentrate on farms in proximity to livestock, the trap sites here were positioned across the UK in a variety of habitats. The abundance of vector species of Culicoides was strongly linked to land use and host density, with those areas surrounded by a greater density of livestock returning greater numbers. The dominance of livestock farm-associated Culicoides species in the RST samples is similar to that observed by Fassotte et al. (2008) and is typical of results using traps of other designs in England (Service 1971, 1974; Holmes & Boorman 1987).

The presence of C. obsoletus group species in traps in drier arable areas where livestock are rare implies a greater dispersal potential of Culicoides over land than previously thought. This was particularly evident from the presence, albeit in low numbers, of C. chiopterus and C. dewulfi which are restricted to cattle dung breeding sites (Kettle & Lawson 1952) in the traps in arable areas. The possibility of alternative breeding sites, such as the dung of horses and wildlife, within the arable areas, however, makes it difficult to suggest a dispersal range for Culicoides from RST sample data. The extent of movement of Culicoides over land may be further investigated with the use of hypervariable microsatellite markers and landscape data to examine the gene flow between populations, possible topographic barriers to dispersal and the use of habitat corridors between suitable areas (Cohuet et al. 2004; Storfer et al. 2007).

The date of first collection of five or more Culicoides at each site was remarkably similar considering the differences in habitat and latitude between sites, but was influenced by the abundance of C. punctatus and C. pulicaris at some sites, which emerged earlier than members of the C. obsoletus group. Analysis of male abundance suggests C. pulicaris and C. punctatus are bivoltine (two emergence peaks each year), whilst the emergence pattern of the C. obsoletus group is less clear. Distinct emergence periods for C. pulicaris and C. punctatus have not previously been recorded (Holmes & Boorman 1987).

Holmes & Boorman (1987) suggested that different species could be responsible for the three emergence peaks that they observed in the C. obsoletus group but they were unable to identify the individuals to species level. Here, C. dewulfi and C. obsoletus s.s. abundance peaked in spring and summer, respectively, whilst peak abundance in C. scoticus appears to be more variable, and following a brief spring flush, C. chiopterus peaked later in the year than the other three species. The abundance of C. chiopterus does not seem to be reflected accurately by UV light traps as this species has previously been found to be abundant at animal-baited traps. Consequently, its importance as a vector may be underestimated (Carpenter et al. 2008b). During the 2006–2008 outbreak of BTV in northern Europe, infection reached a peak in the autumn (Hoffmann et al. 2009). This increase, which correlates with the sustained abundance of C. chiopterus reported here, may be coincidental but may reflect the increased abundance of a more efficient vector at that time of year.

The risk of BTV entering the UK via wind-borne Culicoides from the near continent has been analysed since April 2007 using the NAME (Numerical Atmospheric-dispersion Modelling Environment) dispersion model (Jones et al. 2007) operated daily to predict incursion points and simulate likely spread patterns (Gloster et al. 2008). This model initially relied only upon a few quantitative studies often carried out in other countries to define activity rates, flight range and seasonality. The present Bayesian model for daily Culicoides abundance developed in this study has already been incorporated into the latest IAH-Met Office advisory service for the risk of infectious wind-borne Culicoides reaching the UK from continental Europe, the details of which will be published independently (L. Burgin, pers. comm.). The effect of meteorological conditions on Culicoides abundance was similar for all species at all sites, and the thresholds for temperature, wind speed and precipitation are similar to those seen in Culicoides caught in animal-baited traps (Zhdanova 1975; Olbrich & Liebisch 1987; Carpenter et al. 2008b). This suggests similar wind speed thresholds for flight of the Culicoides species and similar activity responses to temperature variation and precipitation.

In addition to their use in the IAH-Met Office advisory service, the statistical models could, in principle at least, be applied to other sites by making use of the hierarchical structure in the parameters (which allows for between site differences). In this case, we can generate parameter sets for new sites by sampling from the higher-order parameter distributions (see Tables S1–S3 Supporting Information) and use these to simulate Culicoides activity at the site, albeit with considerable uncertainty in any predictions if the models were to be applied in this way.

Conclusion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

This study examined the influence of seasonal population dynamics and meteorological conditions on the daily abundance of potential vector Culicoides species. In addition to acting as a descriptive account of the 2008 season, this model allows the prediction of daily Culicoides abundance according to time of year and weather conditions, insights that are essential to assess the likely activity of Culicoides-borne viruses such as BTV and AHSV during outbreaks. Parameters generated in this study have been integrated into functional risk models of wind-borne infected Culicoides incursion into the UK and other countries, mapping areas of specific risk on a daily basis. The RST sample data suggest the dispersal of Culicoides species over large distances between suitable breeding sites. Understanding the seasonal dynamics and dispersal potential of Culicoides in northern Europe is essential in predicting and managing the continuing threat of incursion of BTV and other Culicoides-borne diseases.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

C.J.S., S.C., S.G. and L.B. are supported by Department for Environment, Food and Rural Affairs (Defra) [grant codes SE4204 and SE4209]. The IAH and RRES receive grant-aided support from the UK BBSRC. The RIS receives additional support from the Lawes Agricultural Trust. Midge identification was funded by Natural England [grant code 21317]. We thank David Sheppard (Natural England), all members of the Rothamsted Insect Survey, the Culicoides Reference Laboratory and suction trap operators.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information
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  • Baylis, M., ElHasnaoui, H., Bouayoune, H., Touti, J. & Mellor, P.S. (1997) The spatial and seasonal distribution of African horse sickness and its potential Culicoides vectors in Morocco. Medical and Veterinary Entomology, 11, 203212.
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusion
  8. Acknowledgements
  9. References
  10. Supporting Information

Appendix S1. Statistical methods.

Table S1. Parameter estimates for the model for total Culicoides biting midges.

Table S2. Parameter estimates for the model for female Culicoides obsoletus group biting midges.

Table S3. Parameter estimates for the species-level models for males of different Culicoides species.

Fig. S1. Observed and expected trap catches for female Culicoides obsoletus group biting midges.

Fig. S2. Observed and expected trap catches for male Culicoides obsoletus s.s.

Fig. S3. Observed and expected trap catches for male Culicoides scoticus.

Fig. S4. Observed and expected trap catches for male Culicoides dewulfi.

Fig. S5. Observed and expected trap catches for male Culicoides chiopterus.

Fig. S6. Observed and expected trap catches for male Culicoides pulicaris.

Fig. S7. Observed and expected trap catches for male Culicoides punctatus.

Fig. S8. Comparison of estimated seasonality components for total Culicoides and the sum of the species-level models for male Culicoides species.

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