Influence of biotic and abiotic factors on the distribution and abundance of Culicoides imicola and the Obsoletus Complex in Italy.
ABSTRACT Culicoides imicola Kieffer (Culicoides, Diptera: Ceratopogonidae) is the principal vector of bluetongue virus (BTV) to ruminant livestock in southern Europe. The secondary potential vectors are Culicoides obsoletus (Meigen) and Culicoides scoticus Downes and Kettle of the Obsoletus Complex, Culicoides pulicaris (Linnaeus) of the Pulicaris Complex and Culicoides dewulfi Goetghebuer of the subgenus Avaritia Fox. Between 2000 and 2004 >38,000 light-trap collections were made for Culicoides across Italy including the islands of Sardinia and Sicily. Mapping of the 100 largest collections of C. imicola and of the Obsoletus Complex showed them to be disjunct overlapping in only 2% of the 200 municipalities selected. For each municipality the average values were calculated for minimum temperature, aridity index, altitude, terrain slope, normalised difference vegetation index (NDVI) and percentage forest cover. A factor analysis identified two principal factors ('biotic' and 'abiotic') and explained 84% of the total variability; a discriminant analysis classified correctly 87.5% of the observations. The results indicate adult populations of C. imicola to occur in more sparsely vegetated habitats that are exposed to full sunlight, whereas species of the Obsoletus Complex favour a more shaded habitat, with increased green leaf density. Heliophily and umbrophily, by shortening or lengthening the respective adult life cycles of these two vectors, will likely impact on the ability of each to transmit BTV and is discussed in the light of the current outbreak of BTV across the Mediterranean Basin.
- SourceAvailable from: Niko Verhoest[Show abstract] [Hide abstract]
ABSTRACT: Culicoides imicola is the main vector of the bluetongue virus in the Mediterranean Basin. Spatial distribution models for this species traditionally employ either climatic data or remotely sensed data, or a combination of both. Until now, however, no studies compared the accuracies of C. imicola distribution models based on climatic versus remote sensing data, even though remotely sensed datasets may offer advantages over climatic datasets with respect to spatial and temporal resolution. This study performs such an analysis for datasets over the peninsula of Calabria, Italy. Spatial distribution modelling based on climatic data using the random forests machine learning technique resulted in a percentage of correctly classified C. imicola trapping sites of nearly 88%, thereby outperforming the linear discriminant analysis and logistic regression modelling techniques. When replacing climatic data by remote sensing data, random forests modelling accuracies decreased only slightly. Assessment of the different variables' importance showed that precipitation during late spring was the most important amongst 48 climatic variables. The dominant remotely sensed variables could be linked to climatic variables. Notwithstanding the slight decrease in predictive performance in this study, remotely sensed datasets could be preferred over climatic datasets for the modelling of C. imicola. Unlike climatic observations, remote Remote Sens. 2014, 6 6605 sensing provides an equally high spatial resolution globally. Additionally, its high temporal resolution allows for investigating changes in species' presence and changing environment.Remote Sensing 07/2014; 6:6604-6619. · 2.62 Impact Factor
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ABSTRACT: 1. Culicoides midges (Diptera: Ceratopogonidae) are ubiquitous on farms in the United Kingdom (UK), but little research has explored their abundance, an important determinant of disease risk. Models to explain and predict variation in their abundance are needed for effective targeting of control methods against bluetongue virus (BTV) and other Culicoides-borne diseases. Although models have been attempted at the national scale (e.g. Scotland), no investigations have taken place at a finer spatial scale. 2. Midge abundances were estimated using light traps on 35 farms in Bala, north Wales. Culicoides catches were combined with remotely sensed ecological correlates, and on-farm host and environmental data, within a GLM model. Drivers of local-scale variation were determined at the 1-km resolution. 3. Local-scale variation in abundance exhibited an almost 500-fold difference (74—33 720) between farms in maximum Obsoletus Group catches. The Obsoletus Group model explained 81% of this variance and was dominated by normalized difference vegetation index (NDVI). This is consistent with previous studies suggesting strong impacts of forest cover and vegetation activity on distribution, as well as shaded breeding site requirements. 4. The variance explained was consistently high for the Pulicaris Group, C. pulicaris and C. punctatus (80%, 73% and 74%), the other probable BTV vector species in the United Kingdom. The abundance of all vector species increased with the number of sheep on farms, but this relationship was missing from any of the non-vector models. This is particularly interesting given that none of the species concerned are known to utilize sheep-associated larval development sites. Performance of the non-vector models was also high (65—87% variance explained), but species differed in their associations with satellite variables. 5. Synthesis and application. At a large spatial scale, there is significant variation in Culicoides Obsoletus Group abundance, which undermines attempts to record their nationwide distribution in larger-scale models. Satellite data can be used to explain a high proportion of this variation and, if shown to be generalizable, they may produce effective predictive models of disease vector abundance. We recommend undertaking a prior survey for farms with high Culicoides catches within the sampling area and checking stability in catch size between seasons and years.Journal of Applied Ecology 02/2013; 50(1):232-242. · 4.74 Impact Factor
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ABSTRACT: Culicoides imicola Kieffer and Culicoides bolitinos Meiswinkel (Diptera: Ceratopogonidae) are both of veterinary importance, being vectors of Schmallenberg, bluetongue and African horse sickness (AHS) viruses. Within South Africa, these Culicoides species show a marked difference in their abundances according to altitude, with C. imicola highly abundant in lower altitudes, but being replaced as the dominant species by C. bolitinos in cooler, high-altitude regions.Parasites & Vectors 08/2014; 7(1):384. · 3.25 Impact Factor
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Influence of biotic and abiotic factors on the distribution
and abundance of Culicoides imicola and the
Obsoletus Complex in Italy
A. Contea,*, M. Goffredoa, C. Ippolitia, R. Meiswinkelb
aIstituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise, via Campo Boario, 64100 Teramo, Italy
bCentraal Instituut voor Dierziekte Controle, Posbus 204, 8203 AA Lelystad, The Netherlands
Received 7 June 2007; received in revised form 12 September 2007; accepted 17 September 2007
livestock in southern Europe. The secondary potential vectors are Culicoides obsoletus (Meigen) and Culicoides scoticus Downes
and Kettle of the Obsoletus Complex, Culicoides pulicaris (Linnaeus) of the Pulicaris Complex and Culicoides dewulfi
Goetghebuer of the subgenus Avaritia Fox. Between 2000 and 2004 >38,000 light-trap collections were made for Culicoides
across Italy including the islands of Sardinia and Sicily. Mapping of the 100 largest collections of C. imicola and of the Obsoletus
Complex showed them to be disjunct overlapping in only 2% of the 200 municipalities selected. For each municipality the average
values were calculated for minimum temperature, aridity index, altitude, terrain slope, normalised difference vegetation index
(NDVI) and percentage forest cover. A factor analysis identified two principal factors (‘biotic’ and ‘abiotic’) and explained 84% of
the total variability; a discriminant analysis classified correctly 87.5% of the observations. The results indicate adult populations of
C. imicola to occur in more sparsely vegetated habitats that are exposed to full sunlight, whereas species of the Obsoletus Complex
favour a more shaded habitat, with increased green leaf density. Heliophily and umbrophily, by shortening or lengthening the
respectiveadult lifecyclesofthesetwovectors,will likelyimpact onthe abilityofeach totransmit BTVandis discussedinthe light
of the current outbreak of BTV across the Mediterranean Basin.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Culicoides imicola; Obsoletus Complex; Larval habitats; Forests; Sunlight; Shade
In the past 9 years (1998–2006) the Mediterranean
Basin has been at the centre of the largest outbreak of
bluetongue (BT) in both sheep and cattle, ever to be
recorded. Some 15 countries surrounding the Basin
have been affected including a number of Balkan states
that had not experienced the disease previously.
Culicoides (Avaritia) imicola Kieffer (Culicoides,
Diptera: Ceratopogonidae) is the principal vector of
bluetongue virus (BTV) to ruminants in southern
Europe. Secondary potential vectors are Culicoides
(Avaritia) obsoletus (Meigen) and Culicoides (Avaritia)
scoticus Downes and Kettle of the Obsoletus Complex,
Culicoides (Culicoides) pulicaris (Linnaeus) of the
Pulicaris Complex and Culicoides (Avaritia) dewulfi
Within the Mediterranean Culicoides imicola is the
principal vector accounting for as much as 90% of
disease transmission. However, in the Balkans BT
advanced northwards and eastwards into areas where C.
Available online at www.sciencedirect.com
Veterinary Parasitology 150 (2007) 333–344
* Corresponding author. Tel.: +39 0861 332246;
fax: +39 0861 332251.
E-mail address: firstname.lastname@example.org (A. Conte).
0304-4017/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
Author's personal copy
imicola is absent and also appeared unexpectedly in
northern Europe in August 2006 and again in 2007 in
in areas where C. imicola is not present, leaves no doubt
that species of Culicoides endemic to Europe are
The incriminated and potential palaearctic vectors
are Culicoides obsoletus (Mellor and Pitzolis, 1979;
Savini et al., 2003, 2005; De Liberato et al., 2005),
Culicoides pulicaris (Caracappa et al., 2003) and
Culicoides dewulfi (Meiswinkel et al., 2007). In one of
these studies (Savini et al., 2003) BTV was isolated
from mixed Culicoides species pools which contained
Culicoides scoticus thereby implicating it also in the
transmission of bluetongue disease. Some of these
species occur as far north as the arctic circle, thus
whether they will be able to disseminate BTV to
similarly high latitudes is a serious issue; global
warming could, in future, induce both northward
movement, and increased vectorial capacity, in these,
and in others, of the almost 350 species of Culicoides
known to occur in the palaearctic region.
northward advance of BTVinto continental Europe it is
essential to map accurately their respective geographi-
cal ranges. These ranges are dictated ultimately by the
availability of optimal breeding habitats, which — for
the vector Culicoides of Europe — are not yet well
understood. This gap in knowledge needs to be bridged
because in models, attempting to predict the advance of
vectors under ameliorating climate change, it is
essential to understand not only the climatic but also
the edaphic factors that influence their seasonal and
regional abundance levels.
To understand better the respective niches inhabited
by C. imicola and the Obsoletus Complex we
commenced from the viewpoint that superabundance
could serve as an indicator of ‘biological success’.
Therefore, by mapping only the 200 largest light-trap
collections of the >38,000 made over the first 5 years of
BT in Italy, we hoped to retrieve patterns of prevalence
unique to each taxon, identifying ‘biotic’ and ‘abiotic’
factors promoting (or depressing) the development of
large vector populations.
2. Materials and methods
2.1. Data collection and manipulation
collections were made using the Onderstepoort-type
blacklight trap and according to standardised surveil-
lance procedures (Goffredo and Meiswinkel, 2004).
All the collections were analysed for the presence/
absence and abundance of C. imicola; approximately
3000 of these collections were analysed also for the
Obsoletus species complex and included the largest
catches made and at least one collection/province.
Only collections made during the warmer months of
greatest insect activity (May to November) are
considered. For C. imicola data emanating from
1473 municipalities (of the 8103 comprising Italy)
are used and 602 municipalities for the Obsoletus
Complex. The bulk of the data was obtained from
mobile traps (collection made for one or two nights at
at permanent sites spread almost equidistantly across
the country (at least one trap/1600 km2). Municipa-
lities have been selected as discrete geographic units
(median area of the 200 municipalities = 54 km2); all
the environmental and statistical analyses performed
are defined by the administrative boundaries of these
2.2. Statistical analyses and climate data
For the statistical analyses two groups of 100
municipalities each, i.e. those in which the 100 largest
collections of C. imicola and the 100 largest collections
of the Obsoletus Complex, had been made, were
selected, on the assumption they would reveal areas in
which the vector taxa were ‘biologically successful’.
When a municipality was represented by both vector
groups, only the dominant taxon was selected. The
independent variables included in the statistical
? Mean minimum temperature between May and
November for the years 2000–2004;
? Mean altitude above sea level;
? Average terrain slope (8);
? Percentage area (of a given municipality) covered by
forests (these embracing broad-leaved, coniferous
and mixed forests) (CORINE Land Cover version 12/
? Average of the normalised difference vegetation
index (NDVI) – this being a specific measure of
chlorophyll abundance and light adsorption – in the
period May to November (2000–2002) (Royal
Netherlands Meteorological Institute (KNMI)), and
? Aridity index (rainfall/potential evapotranspiration
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344 334
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Climate data were obtained from the Italian Air
Force Meteorological Service accumulated between
2000 and 2004 from 105 weather stations distributed
almost equidistantly across Italy. These data were
geostatistically interpolated to obtain the average
value for each municipality through ordinary kriging
The remaining variables – available in grid format –
were resampled at a 250 m ? 250 m resolution and
analysed through ArcGis Spatial Analyst; the average
values were then extracted for each municipality and
The non-parametric Mann–Whitney test (Siegel and
Castellan, 1988) was applied to the independent
variables to verify whether a statistically significant
difference existed between the municipalities with the
100 largest captures of the respective vector species. To
identify the variables distinguishing between the
municipalities (and in this way identifying the possible
differences between the respective vector habitats) a
discriminant analysis was performed. A Spearman
correlation matrix was calculated also for the variables
considered. Due to the high correlation amongst
variables, the discriminant analysis was applied to
independent factors, these identified through factor
analysis (Hair et al., 1998), a statistical technique for
determining the degree of relationship amongst inter-
related variables. The principal component method was
used to identify the common factors, while the varimax
method was used for orthogonal rotation. All analyses
were performed in SPSS111.0.
The morphological identification of field material of
C. imicola was based on the redescriptions of
Meiswinkel (1989) and of Dele ´colle and de la Rocque
(2002). A second species complex within Avaritia is the
Obsoletus Complex and differs from the Imicola
Complex in being largely boreal in its distribution; it
comprises six species, which – along with a seventh
undescribed species from Italy – are listed in
Meiswinkel et al. (2004a). Of these C. obsoletus sensu
stricto appears to occur most widely and abundantly
separatewith certainty C. obsoletus from its congeners –
especially C. scoticus – and for this reason nearly all
researchers lump them together under the taxon
Obsoletus Complex or C. obsoletus group. The species
ofwhich can beidentifiedeasilyusing the redescriptions
of Dele ´colle (1985). The widespread sympatric occur-
on molecular evidence (Gomulski et al., 2005).
2.4. Larval habitats
The larval habitat of C. imicola was reviewed
recently (Meiswinkel et al., 2004b). It was first reared at
Onderstepoort in South Africa in 1950 from a swampy
kikuyu grass (Pennisetum clandestinum)—covered area
next to a leaking cement dam; later – also at
Onderstepoort – up to 382 C. imicola/m2were reared
from islands of short kikuyu grass exposed to full
sunlight in effluent-enriched drainage canals (I.T.P.
Pajor, unpublished data, 1983–1985). Nevill (1967) was
the first to show that the pupa of C. imicola is
susceptible to drowning. In the Mediterranean Basin the
predilection of C. imicola for a semi-moist muddy
habitat was confirmed in Israel (Braverman et al., 1974)
where it was reared ‘‘...in and around animal pens in
water trough overflow, at the margins of animal sewage
and drainage canals and puddles created by leakage
from water pipes...’’ (Birley et al., 1984). In Cyprus, C.
imicola was found also to breed where leaks from
irrigation pipes created small seepages with little free
surface water and covered usually by a growth of fresh
grass (Mellor and Pitzolis, 1979); it was noted in both
studies that the habitats favoured by C. imicola were
generally drier than those preferred by other species of
Records on the breeding habitats of the Obsoletus
Complex are scattered throughout the literature in a
number of languages. Furthermore, because the six
species comprising the Obsoletus Complex in the
Palaearctic region are difficult to distinguish from each
other, a significant proportion of the published data may
be conflated. One result is that the respective breeding
habitats of the various taxa are not as clearly defined as
those for the Imicola Complex are (and whose
constituent taxa have radiated into discrete breeding
niches) (Meiswinkel, 1995). Thus, for western Europe,
we are – to a great extent – compelled to assume that
C. obsoletus sensu stricto is the taxon most frequently
referred to in the literature on larval habitats. For
example, Dzhafarov (1964) reported C. obsoletus to
‘‘...develop in large numbers in forest leaf litter’’,
which mirrors the habitat of the taxonomically almost
indistinguishable Culicoides (Avaritia) sanguisuga
Cocquillett in the Fagus grandiflora forests of north-
eastern America (Jamnback and Wirth, 1963) and of
C. sinanoensis in the far eastern Palaearctic (Amosova,
1956). In England Hill (1947) remarked that ‘‘...peaty
soil...would not appear to be characteristic of the
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344335
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breeding-grounds of C. obsoletus’’; herobservation was
supported by Kettle and Lawson (1952) who reared it in
Scotland in only low numbers from ‘‘...wetter areas
of ... moorland where the ground was covered by
moss...’’ and from ‘‘...fresh-water marsh ... sites ...
where the water level was ... not above the surface of
the soil’’. These observations indicate that C. obsoletus,
like C. imicola, favours habitats that do not ever become
waterlogged. In regard to C. scoticus Boorman and
Goddard (1970) reported that in south-east England it
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344336
Fig. 1. (a) Elevation above sea level; (b) aridity index; (c) mean minimum temperature (May to November); (d) normalised difference vegetation
index (May to November).
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was‘‘...acommonspecies... inemergencetraps... set
in open marshy conditions...’’ indicating that breeding
is unlikely to be restricted ‘‘...to rotting fungi as
C. scoticus is far more abundantly and widely
encountered across western Europe than its solely
fungi-dependent larval habitat would infer it to be.
Fig. 1a is a topographic map of Italy combining
the two considered variables of altitude and slope;
Fig. 1b maps the aridity index (ranging between 0.36
temperature between May and November (2000–
2004) whilst Fig. 1d maps the average Normalised
the mean minimum
Difference Vegetation Index (NDVI) for the period
In Fig. 2 the 100 largest captures of C. imicola
(in red) and those of the Obsoletus Complex (in blue)
are mapped. These are superimposed on the combined
distribution of broad-leaved, coniferous and mixed
forests (in green). The U Mann–Whitney test was
statistically significant for all six variables indicating
that a real difference existed between the values calcula-
ted for the two vector groups (Table 1); the correlation
matrix amongst the six climatic and environmental
variables is included (Table 2).
Processing of the six variables using factor analysis
enabled identification of two principal factors: ‘abiotic’
and ‘biotic’ (Table 3). The former includes minimum
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344337
Fig. 2. Geographic distributionof the 200 largest catchesof Culicoides imicola andofthe ObsoletusComplexcollectedbetween theyears2000and
2004; green is the distribution of broad-leaved, coniferous and mixed forests combined.
Author's personal copy
the latter factor includes percentage forest and NDVI.
Together these two factors explained 84% of the total
variability. To show how the observations were
scattered across them, factor scores were calculated
by multiplying standardised values and factor coeffi-
cients (Fig. 3). A scatter diagram (Fig. 4) of the 100
largest collections of the Obsoletus Complex made
across the length of Italy showed no correlation to exist
between latitude and abundance (Spearman’s correla-
tion coefficient 0.018).
A discriminant analysis conducted on the two factors
gave the following standardised coefficients: factor
1 = 0.546and factor 2 = 0.993. Since the two factorsare
independent they offer a reliable index of the
importance of each factor resulting in 87.5% of the
municipalities being classified correctly as to their
‘dominant’ insect vector species (Table 4). The average
values of the six variables in the misclassified groups
are given in Table 5.
4.1. Altitude (Fig. 1a)
The topographical map of Italy shows the steeply
mountainous Alps in the north to arch down towards the
coast in Liguria; from here the Apennines extend along
the spine of the peninsula into the ‘toe’ of Calabria and
onwards into Sicily. C. imicola is totally absent from all
undulate parts of this topography appearing only along
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344338
Mean value and 95% confidence interval of the six variables for the two insect vector groups and U Mann–Whitney test results
VariableMean values and 95% confidence intervals
C. imicolaObsoletus ComplexU Mann–Whitney
aTwo-tailed p < 0.01.
bThe aridity mean value is strongly influenced by a few high extreme values. The median value, in this case more representative of the
municipalities, is 11.47.
Spearman’s correlation matrix of the six independent variables in the two insect vector groups
Min. temperatureAridity index SlopeAltitudeNDVI % Forest
**Two-tailed p < 0.01.
Matrix of factor loadings identified using the varimax method
Factor 1Factor 2
Classification of observed and predicted insect vector groups
C. imicolaObsoletus ComplexTotal
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its more planar margins at lower altitudes (mean value
207.5 m; 95% confidence interval 176.8–238.2 m). To
the contrary large populations of the Obsoletus
Complex are encountered regularly throughout this
mountainous terrain. Although the presented data
indicate its mean altitudinal range to be 587.1 m
(95% confidence interval 488.3–685.9 m), 13% of the
collections considered in this study emanate from
altitudes of >1000 m (and includes a collection of
>30,000 specimens captured at >2100 m in Badia,
4.2. Terrain slope (Fig. 1a)
In an undulate topography, where the slope exceeds
58, water runoff will induce rapid desiccation of the
soils’ surface layer. This will not benefit the larvae of
C. imicola, which require the surface layer (1–2 cm)
to remain moist for at least 7–10 days to be able to
complete their developmental cycle (Meiswinkel,
1995). This likely explains why C. imicola predomi-
nates largely in the extensive plains found along
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344 339
Fig. 3. Scatter plot of each municipality-based observation for the two factors (‘biotic’ and ‘abiotic’) for C. imicola (red) and for the Obsoletus
Fig. 4. A scatter diagram of the 100 largest collections of the Culicoides obsoletus species complex made across the length of Italy; Spearman’s
correlation coefficient of 0.018 showed no correlation to exist between latitude and abundance.
Average values of the six variables in the two misclassified insect vector groups
Observed groupMin. temperatureAridity indexSlopeAltitudeNDVI% Forest
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– where C. imicola is entirely absent – a strongly
undulate topography prevails. In Italy flatter terrains
predominate along coastlines with the result that there
is a significant correlation between high C. imicola
abundances and lower altitudes (Conte et al., 2003,
2004). However, this correlation is somewhat artefac-
tual or, rather, is specific to the particular landscape
beinginventorised.This caveatmustbemadebecause in
more ancient and abraded landscapes – such as occur
inland in South Africa – C. imicola will penetrate into
elevated plains at altitudes approaching 1800 m (pro-
vided the climate remains suitable and the soils are
nutrient-rich) (Meiswinkel, 1998). This may explain
why, in southern Europe, C. imicola is particularly
abundant on the geologically ancient island of Sardinia,
which includes extensive low-lying plains (20%) and
abraded, gently rolling, higher-lying plains. In regard to
the Obsoletus Complex terrain slope appears to affect
little its ability to penetrate a given landscape. Indeed
Jamnback and Wirth (1963) noted that favourable
breeding sites for the closely related C. sanguisuga
‘...were often found on well-drained slopes...’.
4.3. Aridity index (Fig. 1b)
The aridity index is a relative measure based on the
indicate increasing wetness (Lincoln et al., 2003). The
distribution of C. imicola correlates significantly with a
large collections of the Obsoletus Complex came from
Mediterranean region generally, the rains commence in
events and patchy in their occurrence. This patchiness in
linked to the localised and spasmodic ‘eruptions’ of C.
of BTV occur cyclically and after long inter-epidemic
‘silences’ that may last 30 years or more. Although not
treated as a separate variable in this study there is
precipitation are linked to explosive increases in the
numbers of C. imicola in Africa (Nevill, 1971;
Meiswinkel, 1998); indeed, rainfall has for centuries
been linked with extensive outbreaks of Culicoides-
borne diseases both in Africa (Theiler, 1921) and, more
recently, with the North East monsoons in India (Ganesh
in irrigating grazing pastures for livestock – maintains
large foci of C. imicola locally (Meiswinkel, 1995).
4.4. Temperature (Fig. 1c)
Regarding the mean minimum temperatures of May
to November the generalised pattern for Italy is a
graduated southward warming. Within this gradation C.
imicola is restricted to thewarmer southern and western
regions. On the mainland it occurs almost continuously
along the central western coastline for 300 km or more
before petering out in the vicinity of 458N where only
single specimens are captured at a ‘positive’ collection
site and only once or twice per season. Its almost total
absence along the entire eastern Adriatic coastline is
likely due to a strongly undulate topography and lower
average temperatures (the western coastline being
warmer and prevailing topography more planar).
explain the very low presence and abundance of C.
imicola in Sicily and in Puglia where a warm climate
prevails. The former with its more undulate topography,
and the latter with its predominantly calcareous soils
(despite a planar topography) – combined with
excessive aridity – likely suppress C. imicola locally.
distribution ofC. imicola inthewarmest regionsof Italy
indicating that certain edaphic conditions (such as
moisture-retentiveclayey soils) must first prevail before
it is able to establish itself locally. The contrary applies
to the Obsoletus Complex: its widespread latitudinal
and altitudinal occurrence across the Palaearctic region
shows it to persist independently of both soil type and
terrain slope and, in addition, is able to tolerate
significantly lower average temperatures (Table 1).
4.5. Normalised difference vegetation index (NDVI)
(Fig. 1d) and forest (Fig. 2)
The calculation of NDVI for a givenpixelresults in a
lack of green vegetation gives a value approaching 0,
whereas increasingly higher values of 0.8–0.9 indicate
increasing green leaf density. Because NDVI is strongly
seasonal only the values for the period of greatest insect
this period the mean NDVI value for C. imicola was
0.29 (with a 95% confidence interval of 0.27–0.31)
indicating it to favour less vegetated shrub and
grassland areas. This fits multiple and earlier observa-
tions (reviewed above) that C. imicola is a heliophile
breeding predominantly in habitats that are open to
sunlight. For the Obsoletus Complex a mean value of
0.47 (with a 95% confidence interval of 0.45–0.49)
shows it to favour more densely vegetated habitats.
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344340
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Subsequently, we mapped the areas of deciduous
broadleaved and mixed broadleaved/coniferous forest
and overlaidthe 100 largest collections of the Obsoletus
Complex. They were significantly correlated indicating
that the Obsoletus Complex may be an umbrophile,
which would support the observations (reviewed above)
by a number of authors that it breeds preferentially in
forest leaf litter.
Comparative mapping of the 100 largest collections
of C.imicola and ofthe Obsoletus Complexrevealthem
to be strongly disjunct. This disjunction is due to certain
their proliferation. C. imicola is more tightly con-
strained within these factors indicating its optimal
breeding conditions to be less widely available within
the Mediterranean Basin and in particular in Italy. The
wider scattering of the Obsoletus Complex across both
factors demonstrates its tolerance for a wider range of
temperature, altitude and terrain slope. The findings
indicate that C. imicola and the Obsoletus Complex do
not share a ‘‘...common habitat’’, as asserted recently
(Perrin et al., 2006) but remains to be more fully
demonstrated. At present C. imicola is recognised to
occur in muddy and largely unvegetated habitats only
(Braverman et al., 1974; Meiswinkel et al., 2004b)
whilst species of the Obsoletus Complex have been
reported to breed in a wider range of habitats including
marshy areas, water-filled tree holes, forest leaf litter,
old heaps of manure, faeces-contaminated straw and
decaying fungi (Hill, 1947; Kettle and Lawson, 1952;
Amosova, 1956; Buxton, 1960; Weinburgh and Pratt,
1962; Jamnback and Wirth, 1963; Boorman and
Within the Mediterranean Basin C. imicola is
captured only below 458N. This defines also the
northern limit of the warm temperate region of Europe
(Lincoln et al., 2003) and follows, remarkably closely,
the northern limit of the summer-arid sclerophyllous
Mediterranean floral region. However, the occurrence
of C. imicola within this warm temperate zone is
mosaical; for example, in Italy, it is divided into four
isolated metapopulations: Sardinia, Sicily, the north-
western Tyrrhenian coastal plain and the eastern Ionic
coastal plain. We deem this fragmentation to reflect
localised differences in soil type and topography and
that it is not the result of the supposedly recent, and
therefore incomplete, invasion of Europe by C. imicola
(Mellor, 2004; Purse et al., 2005). Almost the exact
converse applies to the Obsoletus Complex: this cold
temperate taxon is ubiquitous throughout much of Italy,
occurs acrossawide range of soil types, can becaptured
excess of 2000 m. There is also no correlation between
latitude and abundance (Fig. 4). Its distribution in
western Europe to as far north as 668N (and beyond)
indicates it to tolerate also extended periods of very low
average mean annual temperatures.
The presence of C. imicola in areas of low NDVI
(values ranging between 0.27 and 0.31) and low forest
cover (percentage ranging between 8.9 and 13.7%)
indicates it to favour more open habitats and resonates
with what is known about its larval habitat (reviewed
above); because these habitats are open to full sunlight
C. imicola can be classed as heliophilic. Conversely, the
occurrence of the Obsoletus Complex in a higher NDVI
range (0.45–0.49) and forest cover (27.6–35.5%)
reveals it to favour breeding habitats that are shielded
from direct solar radiation. Our results show that the
distribution of this umbrophile fits that of deciduous
broadleaved and mixed broadleaved/coniferous forest
and supports earlier – and multiple – observations that it
breeds preferentially in forest leaf litter. It is important
to stress that in a municipality-based study such as ours,
both open and closed habitats can co-occur and likely
explainswhy C. imicola and the Obsoletus Complex are
occasionally found in sympatry, especially when it is
taken into consideration that they are able to fly over
considerable distances (1–5 km) if necessary. Never-
theless, the two species were seldom found co-
dominantly in light-trap collections. This was also
found to be the case in Spain (Ortega et al., 1998).
Heliophily and umbrophily will affect the length of
the respective life cycles of C. imicola and the
Obsoletus Complex. In the Mediterranean Basin – with
its hot, dry summers – 350–400 h of sunlight/month are
not uncommon; intense solar illumination of the larval
habitat of C. imicola – coupled to high night-time
temperatures – will accelerate larval development
leading to multiple generations/season. For example,
C. imicola with 8 during the BT season of July to
December (Braverman and Linley, 1988). This multi-
because escalated bloodfeeding rates will increase the
BTV ingestion rate, which – in warmer climes – will
lead to heightened virogenesis in individual infected
insects, which is optimal at temperatures ranging
between 28 and 29 8C (Van Dijk and Huismans,
1982; Wellby et al., 1996; Paweska et al., 2002). This
chain of linked events likely accounts for C. imicola
being involved in at least 90% of BTV transmission in
A. Conte et al./Veterinary Parasitology 150 (2007) 333–344341
Author's personal copy
the Mediterranean Basin. Much of the converse is
applicable to the Obsoletus Complex: a shaded – and
which has been estimated to range between 42 and 150
days (Hill, 1947; Birley and Boorman, 1982). Such a
retarded breeding rate translates into fewer generations/
season; for example, in the warmer lowlands of
Slovakia, Orsza ´gh and Mas ˇa ´n (1992) calculated three
to four generations over a 5.5–7-month season but only
twogenerations –and inashorter 3.5–4-month season –
in cooler submontane habitats. This slower breeding
rate of the cold temperate Obsoletus Complex should
make it a less efficient disseminator of BTV. However,
the Obsoletus Complex – before 2006 – was implicated
on at least three occasions in the transmission of BTV:
in 1977 on the island of Cyprus (Mellor and Pitzolis,
1979) and in 2002 in mainland Italy, in the regions of
Campania/Puglia (Savini et al., 2003, 2005) and in
Lazio/Tuscany (De Liberato et al., 2005). All these
localities lie south of the ‘‘imicola-line’’, i.e. in areas
experiencing a warm temperate and not a cool
The majority of studies indicate the larvae of the
Obsoletus Complex to not complete their development
in the A horizon of the soil (as C. imicola does) but
rather to do so in the litter layer (or O horizon). Thus –
unlike C. imicola – the Obsoletus Complex can persist
independently of soil type. As indicated by Jamnback
and Wirth (1963) the optimal breeding habitat of the
taxonomically closely related C. sanguisuga in north-
eastern America was ‘...an unusually thick layer of
dead leaves ... which most commonly occurred in
masses of the American beech (Fagus grandiflora)...’.
In Europe the common beech Fagus sylvatica (family
Fagaceae) occurs widely across Italy (and the Balkans)
indicating the larval habitat of C. obsoletus (and of C.
scoticus?) may be similar to that described for C.
sanguisuga in north-eastern America. The beech
extends into the Mediterranean Basin normally at
higher altitudes (up to 1500 m) forming dense stands
over large areas. It dislikes wet ground but tolerates
temperatures as low as ?29 8C. This implies that
diapausing larvae of the Obsoletus Complex may be
equally tolerant enabling them to overwinter in more
northerly areas thatareclimaticallyharsh. Ifleaflitteris
a favoured breeding habitat it would explain why the
Obsoletus Complex is found commonly in urban
gardens. It is possible that the litter fall from a variety
of deciduous trees might be utilised for breeding by
species of the Obsoletus Complex and, if so, would
account for its wide geographic range, which embraces
much of the Holarctic region.
From our broad sketch of the biotic and abiotic
factors impacting upon the distribution of C. imicola
and the Obsoletus Complex it is evident that the
presence of the appropriatebreedinghabitatiscentral to
the issue of vector prevalence. This is because
nourishment and development of the larval stage
remains key to the successful production and persis-
tence of viable adult Culicoides populations in the field.
Although the majority of Culicoides breed in water-
logged habitats (likely as many as 90% of the 1300
species known worldwide) those of the subgenus
Avaritia are unique in that they favour drier, semi-
moist habitats. This is because the pupa cannot float in
water. If this inability of the pupa to rise to the water’s
meniscus proves to be a synapomorphy that defines
Avaritia it would be embedded deeply in the evolu-
tionary history of this subgenus and, therefore, should
provide us with an important stratum of high-quality
information useful for refining predictive risk models in
the future. As discussed by Baylis et al. (2004) the
recently modelled ‘images’ of Culicoides vector
distributions remain inadequate due – almost entirely
– to a still incomplete understanding of vector
ecologies. This likely explains why the distribution
of the Obsoletus Complex as mapped in this study is
almost diametrically opposed to that modelled by Purse
et al. (2004),whose model was based upon data gleaned
in Sicily where we found this taxon to be depauperate
and where forests cover only 8% of the island. As noted
by Grove and Rackham (2001) ‘‘The great forests of the
toe of Italy, with their deceptively Central European
appearance, are utterly unlike the burning hills of
Although the larval habitats of the three commonest
species (C. obsoletus, C. sinanoensis and C. sangui-
suga) of the Obsoletus Complex in Europe and in
northern America, have been described as forest leaf
litter, these three taxa do not all occur equally widely
and, also, are not all found in sympatry. This indicates
them to differ in certain key – but unknown – aspects of
their respective breeding ecologies; their breeding
habitats, along with those of C. scoticus and C. imicola,
remain to be more precisely elucidated and differ-
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