This PhD project comprises studies of biting midges (Culicoides) in Denmark with regards to vector-borne diseases such as bluetongue virus (BTV) and Schmallenberg virus (SBV). Both diseases are new in northern Europe. In Denmark there was an outbreak of BTV in 2007 and 2008. BTV infects ruminants, and especially infected sheep and cattle are constitute a problem for farmers. The symptoms of BTV include fever, cyanotic tongue, oedemas and decreased milk production. The last symptom affects the economy and animal welfare in the farming industry. In 2011 and 2012, outbreaks of SBV were also recorded in Denmark. The symptoms of SBV are similar to BTV but also include a high proportion of malformations and stillbirths in lambs. Models of vector-borne diseases can be used to predict an outbreak and evaluate e.g. the optimal control strategy, the economic impact and the number of infected animals. These models need to have proper input regarding the abundance and behavior of the vectors. If no vectors are present in an area, the disease will not spread. Thus the vector abundance is a very important factor for models of vector-borne diseases. This PhD project investigates different key factors important for the abundance and behavior of vectors.