Culicoides biting midges are among the most abundant of haematophagous insects, and occur throughout most of the inhabited world. Across this broad range they transmit a great number of assorted pathogens of human, and domestic and wild animals, but it is as vectors of arboviruses, and particularly arboviruses of domestic livestock, that they achieve their prime importance. To date, more than 50 such viruses have been isolated from Culicoides spp. and some of these cause diseases of such international significance that they have been allocated Office International des Epizooties (OIE) List A status. Culicoides are world players in the epidemiology of many important arboviral diseases. In this context this paper deals with those aspects of midge biology facilitating disease transmission, describes the factors controlling insect-virus interactions at the individual insect and population level, and illustrates the far-reaching effects that certain components of climate have upon the midges and, hence, transmission potential.

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A method is described by which Culicoides variipennis (Coq.), C. riethi Kieff. and C. nubeculosus (Mg.) have been maintained as laboratory colonies for over five years. Techniques are given for the maintenance of adults, larvae and pupae; with some modifications these could probably be applied to a range of other species. Adult midges were held in waxed card pill boxes, and provided with sugar solution. Anaesthetised mice provided a blood-meal source. Larvae were reared in pans on a substrate of glass fibre in a medium which contained nutrient broth, grass meal and Bemax. The life-cycle of these species occupies 10–21 days at 23–25°C.

Catches of Culicoides spp. in a Surrey copse in 1967–69 showed the univoltine C. impunctatus Goetgh. to be dominant. This species reached peak numbers from late May to early June, when nulliparous females were commonly seen at rest on tree trunks. Proterandry was marked in catches from light-traps but not apparent in those from emergence traps or taken at rest. Biting was most frequent 1–2 h before sunset; many bites did not result in a blood-meal. Females taken at rest and from emergence traps could complete their ovarian cycle without a blood-meal, most eggs being matured by the former; midges taken biting matured the fewest eggs but could not do so without a blood-meal. Large full fat-bodies were an indication of nulliparity. It was concluded that C. impunctatus can develop the first egg-batch autogenously, and that the resting habits and responses of females to light and human bait change subsequent to the laying of the first batch of eggs.

Surveillance of Culicoides (Diptera: Ceratopogonidae) biting midges was carried out between 2001 and 2003, at 119 sites within a 50 x 50-km grid distributed across Bulgaria, using light trap collections around the time of peak adult midge abundance. Sentinel and ad hoc serum surveillance of hosts susceptible to bluetongue infection was carried out at around 300 sites between 1999 and 2003. Following the initial incursion of bluetongue virus 9 (BTV-9) into Bourgas province in 1999, affecting 85 villages along the southern border, a further 76 villages were affected along the western border in 2001, with outbreaks extending as far north as 43.6 degrees N. The BTV-9 strain in circulation was found to have a low pathogenicity for Bulgarian sheep populations, with less than 2% of susceptible individuals becoming sick and seroconversions detected up to 30 km from recorded outbreaks in the south. The major Old World vector Culicoides imicola Kieffer was not detected among over 70,000 Culicoides identified in summer collections, suggesting that BTV-9 transmission in Bulgaria was primarily carried out by indigenous European vectors. The most likely candidates, the Palaearctic species complexes - the Culicoides obsoletus Meigen and C. pulicaris L. complexes - were widespread and abundant across the whole country. The C. obsoletus complex represented 75% of all individuals trapped in summer and occurred in high catch sizes (up to 15,000 individuals per night) but was not found across all outbreak sites, indicating that both Palearctic complexes probably played a role in transmission. Within the C. pulicaris complex, only C. pulicaris s.s., C. punctatus Meigen and C. newsteadi Austen were sufficiently abundant and prevalent to have been widely involved in transmission, whilst within the C. obsoletus complex most trapped males were C. obsoletus s.s. Adult vectors were found to be largely absent from sites in west Bulgaria for a period of at least 3 months over winter, which, taken along with the spatiotemporal pattern of outbreaks in the region between years, indicates the virus may be overwintering here by an alternative mechanism - either by covert persistence in the vertebrate host or possibly by persistence in larval stages of the vector.

Summary 1.The spread of vector-borne diseases into new areas, commonly attributed to environmental change or increased trade and travel, could be exacerbated if novel vector species in newly invaded areas spread infection beyond the range of traditional vectors. 2. By analysing the differential degree of overlap between the environmental envelopes for bluetongue, a devastating livestock disease, and its traditional (Afro-Asian) and potential new (Palearctic) midge vectors, we have implicated the latter in the recent dramatic northward spread of this disease into Europe. 3. The traditional vector of bluetongue virus, the Afro-Asian midge Culicoides imicola , was found to occur in warm (annual mean 12-20 ° C), thermally stable locations that were dry in summer (< 400 mm precipitation). The Palearctic C. obsoletus and C. pulicaris complexes were both found to occur in cooler (down to 7 ° C annual mean), thermally more variable and wetter (up to 700 mm summer precipitation) locations. 4. Of 501 recorded outbreaks from the 1998-2004 bluetongue epidemic in southern Europe, 40% fall outside the climate envelope of C. imicola , but within the species' envelopes of the C. obsoletus and C. pulicaris complexes. 5. The distribution in multivariate environmental space of bluetongue virus is closer to that of the Palaearctic vectors than it is to that of C. imicola . This suggests that Palearctic vectors now play a substantial role in transmission and have facilitated the spread of bluetongue into cooler, wetter regions of Europe. 6. Synthesis and applications . The risk to Northern Europe now depends on how much of the distributions of the widespread, abundant Palearctic midge vectors (the C. obsoletus and C. pulicaris complexes) bluetongue can occupy, perhaps determined by thermal constraints on viral replication. This was highlighted by the sudden appearance in summer 2006 of bluetongue virus at latitudes of more than 50 ° North -approximately 6 ° further North than previous outbreaks in southern Europe. Future surveillance for bluetongue and for related Culicoides -borne pathogens should include studies to record and explain the distributional patterns of all potential Palearctic vector species.