Multiple vectors and their differing ecologies: observations on two bluetongue and African horse sickness vector Culicoides species in South Africa

Blacklight traps were used to collect Culicoides biting midges weekly between September 1996 and August 1998 at 40 sites distributed equidistantly across South Africa. The seasonal and geographic prevalences of 86 species of Culicoides were elucidated simultaneously, and included C. imicola Kieffer and C. bolitinos Meiswinkel the principal vectors of bluetongue (BT) and African horse sickness (AHS) in the region. These two species were amongst the most prevalent Culicoides to be found and, together, comprised >50% of the more than three million biting midges captured. The data are presented as coloured matrices, and are transformed also into inverse distance weighting (IDW) interpolative maps. The data reveal that the prevalence of each vector is somewhat fractured and it is posited that this is (in part) due to significant differences in their respective breeding habitats. The results illustrate also that the presence of multiple vectors (in any region of the world) will complicate both the epidemiology of the orbiviral diseases they transmit and the formulation of rational livestock movement and disease control strategies. This is especially true for southern Europe where the recent devastating cycle of BT has been shown to involve at least three vectors. Finally, the influence that man has on the development of large foci of vector Culicoides around livestock may be less important than previously suggested but must be investigated further.

Distribution and abundance of Culicoides imicola, Obsoletus Complex and Pulicaris Complex (Diptera: Ceratopogonidae) in Italy

Between 2000 and 2003, thousands of light-trap collections for Culicoides were made throughout Italy and a detailed distribution map of the primary vector of bluetongue (BT) virus (BTV), C. imicola compiled. In some areas, however, where clinical BT occurred and C. imicola could not be captured, the virus was isolated from biting midges belonging to the Obsoletus and/or the Pulicaris Complexes. Thus, the distribution and abundance of these two species complexes in Italy, as determined from about 3,000 collections, are reported here also and compared to that of C. imicola (from about 24,000 collections). The probable spread of the main vector of BT, C. imicola, into the northern third of Italy, and the widespread prevalence of additional vectors of the Obsoletus and Pulicaris Complexes, indicate nearly all regions of Italy to be at some risk to incursions of BTV. However, these complexes comprise at least six and twelve species, respectively, so precisely which species are able to transmit BTV remains incompletely known.

Towards the identification of potential infectious sites for bluetongue in Italy: a spatial analysis approach based on the distribution of Culicoides imicola

A geographic information system (GIS) based on grids was developed by the National Reference Center for Veterinary Epidemiology at the Istituto Zooprofilattico Sperimentale dell'Abruzzo e del Molise 'G. Caporale' (IZS) in Teramo to identify potential infectious sites for bluetongue (BT) disease in Italy. Geographical and climatic variables were used to build a spatial process model (SPM); the different layers were combined by sequential addition. The final grids (with a cell size of 0.0387 decimal degrees) were generated for each season of the year, and the suitability of each cell for the presence of C. imicola given a value ranking from 0 to 10. While this model more accurately predicts the presence of C. imicola in the Basilicata and Sicily regions, it still over-predicted its presence in the Puglia region. This could be due to the occurrence of calcareous soils which dominate the Puglia landscape. The present SPM is an additive model that assigns an equal weight to each variable. However, the results suggest the existence of hitherto unconsidered variables that significantly influence the prevalence of C. imicola. To reflect their importance, these variables should be assigned a higher weighting in future models. However, the decision in regard to precisely what this weighting should be depends on a very thorough knowledge of the ecology of C. imicola.

Christopher Columbus and Culicoides: was C. jamaicensis Edwards, 1922 introduced into the Mediterranean 500 years ago and later re-named C. paolae Boorman 1996?

The biting midge, Culicoides paolae Boorman, described from specimens collected in the extreme south of Italy in 1996, belongs in the subgenus Drymodesmyia. This subgenus was erected by Vargas in 1960 for the so-called Copiosus species group, an assemblage of 22 species endemic to the tropical regions of the New World and, where known, breed in vegetative materials including the decaying leaves (cladodes) and fruits of Central American cacti. The Mexican peoples have utilised these cacti for over 9,000 years; one of these, Opuntia ficus-indica Linnaeus, was brought to Europe by Christopher Columbus following his voyages of discovery. As a taxon C. paolae is very similar to the Central American C. jamaicensis Edwards, 1922 raising the possibility that it (or a closely related species of Drymodesmyia) was introduced into the Mediterranean Region at the time of Columbus, but was (perplexingly) discovered only 500 years later and named C. paolae. The comparison of Sardinian specimens of C. paolae with Panamanian material of C. jamaicensis (housed in the Natural History Museum in London) confirmed the two species to be very similar but unusual differences were noted around the precise distribution of the sensilla coeloconica on the female flagellum. Until it is understood whether these differences represent either intra- or interspecific variation, the question of the possible synonymy of C. paolae must be held in abeyance.

Phylogenetic status and matrilineal structure of the biting midge, Culicoides imicola, in Portugal, Rhodes and Israel

The biting midge Culicoides imicola Kieffer (Diptera: Ceratopogonidae) is the most important Old World vector of African horse sickness (AHS) and bluetongue (BT). Recent increases of BT incidence in the Mediterranean basin are attributed to its increased abundance and distribution. The phylogenetic status and genetic structure of C. imicola in this region are unknown, despite the importance of these aspects for BT epidemiology in the North American BT vector. In this study, analyses of partial mitochondrial cytochrome oxidase subunit I gene (COI) sequences were used to infer phylogenetic relationships among 50 C. imicola from Portugal, Rhodes, Israel, and South Africa and four other species of the Imicola Complex from southern Africa, and to estimate levels of matrilineal subdivision in C. imicola between Portugal and Israel. Eleven haplotypes were detected in C. imicola, and these formed one well-supported clade in maximum likelihood and Bayesian trees implying that the C. imicola samples comprise one phylogenetic species. Molecular variance was distributed mainly between Portugal and Israel, with no haplotypes shared between these countries, suggesting that female-mediated gene flow at this scale has been either limited or non-existent. Our results provide phylogenetic evidence that C. imicola in the study areas are potentially competent AHS and BT vectors. The geographical structure of the C. imicola COI haplotypes was concordant with that of BT virus serotypes in recent BT outbreaks in the Mediterranean basin, suggesting that population subdivision in its vector can impose spatial constraints on BT virus transmission.

Evidence for a new field Culicoides vector of African horse sickness in South Africa

Between February and May 1998, approximately 100 horses died of African horse sickness (AHS) in the cooler, mountainous, central region of South Africa. On 14 affected farms, 156,875 Culicoides of 27 species were captured. C. imicola Kieffer, hitherto considered the only field vector for AHS virus (AHSV), constituted <1% of the total Culicoides captured, and was not found on 29% of the farms. In contrast, 65% of the Culicoides were C. bolitinos Meiswinkel, and was found on all farms. Five isolations of AHSV were made from C. bolitinos, and none from 18 other species of Culicoides (including C. imicola).

The distribution of Culicoides imicola in Italy: application and evaluation of current Mediterranean models based on climate

In August 2000 bluetongue (BT) disease appeared amongst sheep on the island of Sardinia spreading later to Sicily and to mainland Italy. The majority of areas affected by BT were surveyed for Culicoides imicola, the only proven vector of the disease known to occur in the Mediterranean region. The data from 1456 light-trap collections, made in months with a mean temperature of 12.5 degrees C, were used to test the accuracy of current models predicting the prevalence and abundance of C. imicola across the region. For Italy, the distribution of C. imicola was found to be very irregular and did not fit the modelled predictions. The possible reasons for this are discussed, and suggestions made as to which variables may improve this fit in the development of future risk models. In Italy, past surveys failed to reveal the presence of C. imicola, and so could be construed as evidence of its recent invasion, and thus rampant spread northwards. Although equivocal, historical records indicate that C. imicola was overlooked in the past. Six recommendations are made as to the possible future course of Culicoides research in southern Europe.

The effect of climate on the presence of Culicoides imicola in Italy

A model was developed to classify the Italian territories in relation to their suitability to harbour populations of Culicoides imicola and, as a consequence, also able to sustain a bluetongue (BT) epidemic. Italy was subdivided into 3507 10 x 10 km cells. In 546 cells at least one collection was made. The cell was considered the unit for all subsequent analyses. Culicoides were collected using Onderstepoort-type blacklight traps. Some traps were operated weekly at chosen sites; the remainder were moved almost daily to new sites. Only the results obtained during the peak August-November period were used, to exclude bias caused by the seasonality of C. imicola. Climate data for the period 1999-2001 were obtained from 80 weather stations. Multiple logistic regression was performed using the presence or absence of C. imicola in a specific cell as the dependent variable. Annual means of daily values for minimum temperature and minimum relative humidity, and the mean altitude above sea level, were the independent variables. The probability of occurrence of C. imicola in each grid cell was used to create a prediction map for Italy. The model was able to correctly classify 77.5% of the 546 grid cells in which at least one collection had been made. Culicoides imicola was found frequently through much of Sardinia, in parts of southern Italy, and further north along the Tyrrhenian coast, but was absent from along most of the Adriatic coast, and the internal mainland, and from most of Sicily. Six detailed maps are provided. Also mapped are areas where the probability of the occurrence of C. imicola is lower than 5%. This identification of possible mountainous C. imicola-free areas in central Italy could facilitate safer animal trade and transhumance, even if BT infections in traded animals or moving stock, were to go undetected. Needless to say this depends upon no cool-adapted species of Culicoides being involved in the transmission of BT disease.

Phylogenetic analysis of the mitochondrial cytochrome oxidase subunit I gene of five species of the Culicoides imicola species complex

The phylogenetic status of members of the Culicoides imicola Kieffer (Diptera: Ceratopogonidae) species complex of haematophagous midges is unknown, and simple means to identify the members using all life stages are unavailable. In this study, the status of three confirmed (C. imicola s.s., C. bolitinos Meiswinkel and C. loxodontis Meiswinkel) and two provisional (C. tuttifrutti Meiswinkel and C. kwagga Meiswinkel) members of the complex from South Africa was assessed using phylogenetic analysis of partial DNA and amino acid sequences of the mitochondrial cytochrome oxidase subunit I (COI) gene. The four or five individuals of each species analysed contained one or two haplotypes each. Interspecific divergence was significant and characterized by strong A <--> T transversion bias. Phylogenetic trees constructed using neighbour-joining, parsimony and maximum likelihood showed each species to be distinct. Combinations of sites for two restriction enzymes in the COI sequences were species-specific and could form the basis of a diagnostic PCR assay.

Prevalence of Culicoides imicola and other species (Diptera: Ceratopogonidae) ateight sites in Zimbabwe : to the editor

Presents some prevalence data on Culicoides imicola and Culicoides bolitinos in Zimbabwe. Includes data on other common species, as revealed by collections made over 2 years at 8 widely-separated study sites

Molecular differentiation of the Old World Culicoides imicola species complex (Diptera, Ceratopogonidae), inferred using random amplified polymorphic DNA markers

Samples of seven of the 10 morphological species of midges of the Culicoides imicola complex were considered. The importance of this species complex is connected to its vectorial capacity for African horse sickness virus (AHSV) and bluetongue virus (BTV). Consequently, the risk of transmission may vary dramatically, depending upon the particular cryptic species present in a given area. The species complex is confined to the Old World and our samples were collected in Southern Africa, Madagascar and the Ivory Coast. Genomic DNA of 350 randomly sampled individual midges from 19 populations was amplified using four 20-mer primers by the random amplified polymorphic DNA (RAPD) technique. One hundred and ninety-six interpretable polymorphic bands were obtained. Species-specific RAPD profiles were defined and for five species diagnostic RAPD fragments were identified. A high degree of polymorphism was detected in the species complex, most of which was observed within populations (from 64 to 76%). Principal coordinate analysis (PCO) and cluster analysis provided an estimate of the degree of variation between and within populations and species. There was substantial concordance between the taxonomies derived from morphological and molecular data. The amount and the different distributions of genetic (RAPD) variation among the taxa can be associated to their life histories, i.e. the abundance and distribution of the larval breeding sites and their seasonality.

Stabling and the protection of horses from Culicoides bolitinos (Diptera: Ceratopogonidae), a recently identified vector of African horse sickness

The stabling of horses at night reportedly offers protection from African horse sickness and the most significant vector of the disease, Culicoides imicola Kieffer, has been shown to be exophilic. In certain high-lying regions of South Africa, however, C. bolitinos Meiswinkel, may be the major vector of the disease but its entry behaviour into stables is unknown. Accordingly, in the eastern Free State province of South Africa, light trap catches of C. bolitinos inside stables and outside, were compared. Two horse-baited stables, one traditional, and one modern, were used and combinations of stable (old/new), ceiling fans (on/off) and accessibility to Culicoides (stable doors open/closed or windows gauzed/ungauzed) were investigated as treatments. A total of 111,452 Culicoides of 26 species was collected on 60 trap nights; C. bolitinos was dominant (89.1% overall) with C. imicola second in abundance (2.9%). Outside catches were greater on warmer, drier, evenings but were suppressed by high wind speeds. Catches of C. imicola inside stables with doors open, or with windows ungauzed, were less than the numbers captured outside. In contrast, more C. bolitinos were caught in open stables than outside, i.e. open structures may protect horses from the exophilic C. imicola, but may increase attack rates from the endophilic C. bolitinos. The closing of doors and the gauzing of windows, however, led to a 14-fold reduction in numbers of C. bolitinos and C. imicola entering stables. A well-gauzed 'traditional' stable was as effective as a closed 'modern' stable. Ceiling fans had no suppressant effect.

A preliminary attempt to use climate data and satellite imagery to model the abundance and distribution of Culicoides imicola (Diptera: Ceratopogonidae) in southern Africa

Abundances of Culicoides imicola, the insect vector of several livestock viruses, including bluetongue and African horse sickness, were recently published for 34 sites in southern Africa, together with associated climate data. Here, these data are analysed statistically in combination with certain satellite-derived variables, with the aim of developing predictive models of C. imicola abundance. Satellite-derived variables were the land surface temperature (LST, a measure of temperature at the earth's surface) and the normalised difference vegetation index (NDVI, a measure of photosynthetic activity). Two models were developed: (1) climatic variables only and (2) satellite-derived and climatic variables. For model I, the best model used a single predictor variable (the mean daily minimum temperature) only, and accounted for nearly 34% of the variance in C. imicola abundance. Two variable climatic models did not perform significantly better. For model II, the best 1-variable model used the annual minimum LST as a predictor of C. imicola abundance, and accounted for nearly 40% of the variance in C. imicola abundance. The best 2-variable model, which gave a significantly better fit than the 1-variable model, combined the minimum LST and minimum NDVI as predictors of C. imicola abundance, and accounted for nearly 67% of variance. A map of predicted C. imicola abundances is produced on the basis of this 2nd model which, despite some anomalies, agrees largely with what is currently known of the prevalence of C. imicola in the region.

The 1996 outbreak of African horse sickness in South Africa--the entomological perspective

During the 1996 summer season (January-April) in South Africa an estimated 500 horses died of African horse sickness (AHS); 80% of deaths were due to AHS virus serotypes 2 and 4. Nearly all cases occurred in the northern, north-eastern and central parts of South Africa. This study reports the first attempt to verify the involvement of the biting midge Culicoides imicola in a field outbreak of AHS in southern Africa. In light-trap collections made at 47 sites over 12 weeks, C. imicola comprised 94.2% of 4.78 million Culicoides. Culicoides imicola was the most prevalent of 34 species captured and was the only species whose distribution matched that of the disease. Record catches of C. imicola were made, and reveal that in years of above average rainfall its numbers can show a 200-fold increase over those in dry years. Soil type appeared to determine strongly the distribution of C. imicola. The largest populations of C. imicola were found in areas with clayey, moisture-retentive soils whereas the lowest numbers, or none, occurred in areas where the soils were sandy and quick-draining. The deaths of two horses (confirmed AHS) in a sandy area were perplexing as they occurred in a region known to be free of C. imicola. The probable origin of these infections was established.

Some epidemiological and economic aspects of a bluetongue-like disease in cattle in South Africa--1995/96 and 1997

In December 1995 to March 1996 and the early summer of 1997 South Africa experienced above average rainfall which favoured the occurrence of Culicoides transmitted diseases. During this period several outbreaks of an uncommon disease of cattle occurred over a large part of the country. The clinical signs were similar to those of infection with the viruses of bluetongue (BT) and epizootic haemorrhagic disease of deer (EHD). Virus isolation from cattle and Culicoides yielded both viruses. Dual infections occurred on several farms. Typing of BT isolates yielded types 2, 3, 6 and 8. On at least two farms more than one BT virus serotype was involved. On one farm only EHD virus could be isolated from cattle and Culicoides. Serological tests confirmed that on this farm the disease was caused by EHD. In 1932/33, when a similar disease was reported conditions were vastly different. Rainfall figures show that the 1932/33 season was exceptionally dry. Techniques available at that time could not identify EHD and the cause was reported to be BT. The occurrence of BT in a dry season and over a much wider area than the distribution in South Africa of Culicoides imicola, the only proven vector for BT, is a clear indication that other species less dependent on high rainfall are involved. The present isolation of BT virus from three of five pools of parous C. bolitinos is evidence that this species, which breeds in cattle dung, may be an additional vector for BT.

Morphological confirmation of the separate species status of Culicoides (Avaritia) nudipalpis Delfinado, 1961 and C. (A.) imicola Kieffer, 1913 (Diptera: Ceratopogonidae)

Ten character states were used to compare five females and two males of two populations of C. nudipalpis from the Philippines and Timor in south-east Asia with 121 females and 167 males of 11 populations of C. imicola collected throughout its Afro-Asiatic range. It is concluded that they are closely related but good species, and together form a subgroup, or species-pair, in the imicola group; currently this Old World group of the subgenus Avaritia comprises seven described species. Culicoides nudipalpis and C. imicola are most reliably separated on the ratio of the length of the proboscis to the height of the head (P/H ratio: 0.66-0.73 in nudipalpis, 0.82-1.02 in imicola). Compared to C. imicola very little is known of the life habits of C. nudipalpis. The latter's close taxonomic relationship to C. imicola, a proven vector of African horse sickness (AHS) and bluetongue (BT), indicates that the capacity of C. nudipalpis to vector these orbiviruses deserves to be investigated.

Discovery of a Culicoides imicola-free zone in South Africa: preliminary notes and potential significance

In December 1993, a light-trap survey was made of the Culicoides found at eight horse stables and dairies in the sandy dune field west of Port Elizabeth, South Africa. While it was notable that Culicoides numbers were low (4749) and that the diversity was poor (15 species), the most remarkable fact to emerge, was that C. imicola, the only proven vector of the virus of African horse sickness (AHS), was entirely absent. Though not abundant, C. bolitinos, a sister species of C. imicola, was overwhelmingly dominant (91.7%). Its larvae and pupae develop exclusively in the dung of cattle, but it is a species that is not implicated in the transmission of animal viruses. Elsewhere in South Africa, a frost-free climate, good rainfall and a plentiful supply of livestock would normally lead to the development of large foci of C. imicola. That this is not the case in the Port Elizabeth (P.E.) area is most likely owing to the winds inhibiting adult flight and the sandy soils being nutrient-poor and too well-drained to sustain Culicoides larvae. Studies are needed to confirm that sandy soils cannot sustain C. imicola. If so, the sandy coastal areas hold promise for quarantining against AHS.

Culicoides (Diptera:Ceratopogonidae) associated with livestock in the Onderstepoort area, Gauteng, South Africa as determined by light-trap collections

In 54 light-trap collections made at 28 sites in the Onderstepoort area a total of 178,941 Culicoides midges of 35 species was collected in March 1988; the survey was repeated at 26 sites in September and yielded 19,518 Culicoides of 24 species. The number of Culicoides species collected totalled 38. C,imicola was the most abundant species at 27 of the 28 sites sampled, and accounted for 88% and 67% of all midges collected in the two months respectively. This study not only confirms that C. imicola is widespread and abundant in the greater Onderstepoort area, but also that its numbers correlate positively with the historical prevalence of African horse sickness (AHS) and bluetongue (BT) locally. The high numbers of C. imicola make Onderstepoort the ideal site for the study of its laboratory vector capacity. The relatively low numbers of Culicoides spp. other than C. imicola in the Onderstepoort area, will severely limit studies on their roles in the transmission of arboviruses. The origin of the blood-meals of 1338 engorged Culicoides belonging to 13 species was determined by means of cross-over electrophoresis precipitin test; C. imicola fed on cattle, horses, sheep of pigs, Four other Culicoides species showed a similarly wide host range.

Tokunagahelea, a new subgenus of Culicoides Latreille (Diptera: Ceratopogonidae) from the Australasian region with descriptions of two new species

Tokunagahelea is proposed as a new subgenus of the genus Culicoides Latreille to embrace three species from New Guinea and the Solomon Islands. Comparative descriptions of both males and females of Culicoides mikros, sp. nov., and C. geocheloneoides, sp. nov., and a redescription of the female of C. pygmaeus Tokunaga (male unknown), are presented together with a key for differentiation. C. mikros is the designated type species of the subgenus. Systematic relationships of the new subgenus are discussed.

The virtual absence of Culicoides imicola (Diptera: Ceratopogonidae) in a light-trap survey of the colder, high-lying area of the eastern Orange Free State, South Africa, and implications for the transmission of arboviruses

Altogether 52 078 Culicoides biting midges of 35 species were collected during February 1990 and 1993 in 40 light-trap collections made on 17 cattle and/or sheep farms in the Bethlehem and Fouriesburg districts of the colder, high-lying eastern Orange Free State. Culicoides (Avaritia) bolitinos was by far the most abundant species, representing 50.9% of all specimens collected. Culicoides (A.) imicola, considered to be the most common stock-associated species in the summer rainfall areas of southern Africa, and the only proven vector of bluetongue virus (BTV) and African horsesickness virus (AHSV) in the subregion, was uncommon, comprising only 1.4%. While AHS is apparently absent, BT and bovine ephemeral fever (BEF) are endemic in this cooler, high-lying area of South Africa. The virtual absence of C. imicola implies that other Culicoides species, such as C. bolitinos and C. cornutus, may be involved in transmitting BT virus (and perhaps BEF) in the eastern Orange Free State, and possibly elsewhere in Africa. Virus isolation attempts made on 45 single species pools of C. bolitinos, C. pycnostictus, C. milnei, C. leucostictus, C. zuluensis and C. gulbenkiani were, however, negative. Finally, 20 of 28 blood-engorged Culicoides of 11 species, which were tested against cattle, sheep, horse, pig and bird antisera, tested only positive against cattle antisera.

African horsesickness epidemiology: five species of Culicoides (Diptera: Ceratopogonidae) collected live behind the ears and at the dung of the African elephant in the Kruger National Park, South Africa

During the culling of elephants (Loxodonta africana) at five sites in the Kruger National Park, South Africa, a total of 682 Culicoides of five species of the subgenus Avaritia were found live either behind the ears of elephants or attracted to the freshly disembowelled intestinal dung of elephants. The species are Culicoides tororoensis Khamala & Kettle, 1971; C. kanagai Khamala & Kettle, 1971; C. loxodontis Meiswinkel, 1992, and two undescribed species, i.e. Culicoides sp. #50 and Culicoides sp. #54 pale form (p.f.). Of 511 female midges found behind ears, 39.9% were nulliparous, 57.3% empty parous, 2.5% freshly bloodfed and 0.2% gravid. The age composition of this subpopulation indicates that the Culicoides were behind the ears to suck blood and, furthermore, would do so in broad daylight. The age composition of 171 Culicoides of three species attracted to dung was entirely different: 1.8% nulliparous, 14.6% empty parous, and 83.0% gravid, indicating that the great majority of midges captured at dung were about to oviposit or had just oviposited. Immediately after culling, light-traps were operated at two of the sites. Of 4,023 Culicoides of 21 species captured, 93% were of the same five species found on the ears and at the dung of elephants. Using these and other unpublished data pertaining to the rearing of these five Avaritia species from elephant dung over the past seven years, we broadly sketch the life cycle of these Culicoides, the first for any Afrotropical species of the genus. We also discuss the implications the close association between elephant and midge has for the dispersal and geographic distribution of the latter, and how it may influence the involvement of midges in the transmission of diseases such as African horsesickness. Owing to difficulties in identifying species of the subgenus Avaritia in the Afrotropical Region, the taxonomy of each of the five above-mentioned species is briefly appraised. Of the remaining 16 species (7%) captured in light traps 15 (6%) belong to that sector of the genus Culicoides whose immature stages develop in groundwater habitats and include C. imicola, which comprised only 2% of the light-trap collections. The large disparity in the adult abundance patterns of the "dung" and "groundwater" species in the middle of dry bushveld, is probably the result of differences in host and larval habitat preferences, and is briefly discussed. Finally, the few reports extant on the wild-host preferences of Afrotropical Culicoides are reviewed. Five tables and five figures accompany the text.

Subsampling of large light trap catches of Culicoides (Diptera: Ceratopogonidae)

Analyses of 2 light trap catches comprising 6,041 and 1,598 Culicoides showed that the reliability of subsampling such catches increased with subsample size, while the subsampling error decreased with an increase in the number of individuals per species present in a subsample. Subsamples comprising approximately 500 Culicoides are deemed sufficient for comparing population densities between sites and sampling occasions and also give an acceptable indication of relative species abundance at a site. It is recommended that species for which the mean number of specimens in subsamples originating from 3 catches at a site is less than 7, should not be included in biometric analyses aimed at comparisons of population densities. For all other species a test level of 1% rather than 5% should be used for such comparisons. When species ratios obtained from subsampled catches are employed as indicators of abundance, the chi-squared test should be utilized at a 1% level if the ratios originate from 3 catches at a site and at a 0.1% level if only one catch per site is made. Due to poor representativeness of small catches, it is suggested that species for which fewer than 7 individuals are present in a single subsample, be excluded from chi-squared tests. A 5-point procedure for subsampling a large light trap collection of Culicoides is given.