The thesis investigates several aspects of the breeding ecology of the four Culicoides species of the Obsoletus Group, i.e., C. obsoletus s.s., C. scoticus, C. chiopterus and C. dewulfi. These species are among the main vectors of viruses affecting ruminant livestock, as the bluetongue or the Schmallenberg virus. The knowledge on the immature stages of the genus, their breeding sites and development is generally poor, however, essential for the development of effective control measures. The main aims of the thesis included the evaluation and improvement of methods for the research on breeding ecology of Culicoides, and the identification of important breeding habitats of the Obsoletus Group at and around cattle farms in Germany. Moreover, the potential influence of abiotic conditions on the distribution of the immature stages in identified developmental substrates was investigated. The impact of freezing and flooding on the development success was further assessed in experimental studies.
1. In the comparison of emergence traps, Berlese funnel-extraction and sugar-flotation for the assessment of abundances of immature Culicoides in cowpats, significantly more individuals were sampled with the first two methods. The Berlese method provides many benefits, e.g., the rapid availability of data and extraction of viable larvae. However, the choice of method generally depends on the study purpose, the ecology of the investigated species and the type of breeding substrate.
2. Comparing different emergence trap designs, the trap shape (cone-shaped or quadratic) had no impact on the trapping success, while black coloured traps sampled Culicoides emerging from cowpats more effectively than white traps. This difference is presumably related to the stronger light gradient between trap body and collection container in black traps and the positive phototaxis of adult Culicoides. Thus, an ideal trap design should maximise this light gradient, by using an opaque or reflecting trap material, and allow sufficient aeration in order to reduce a potential heat-up within the trap.
3. The most productive breeding habitat of C. obsoletus s.s. in the surroundings of cattle farms are dungheaps, while C. chiopterus and C. dewulfi develop primarily in cowpats. No typical habitat of C. scoticus could be identified.
4. A decreasing substrate pH was associated with a higher abundance or a higher probability of observing C. obsoletus s.s. in dungheaps, as well as C. chiopterus and C. dewulfi in cowpats. Furthermore, the abundance of C. obsoletus s.s. was positively affected by increasing moisture and decreasing trap height on dungheaps.
5. In the exposure of cowpat subsamples to freezing for 48 hours at different sub-zero temperature levels, no immature Culicoides survived freezing at -21 °C. In contrast to C. dewulfi, a small number of C. chiopterus developed after freezing at -18 °C. In combination with a temperature monitoring within cowpats on a pasture, the results indicated, that a significant reduction of the immature populations of these species by low temperatures during winter is unlikely.
6. In experimental floodings of immature stages of C. dewulfi and C. chiopterus, larvae were neither able to swim nor were the pupae able to float. Flooding for 24 hours did hardly affect larvae and pupae, but an exposure for 10 days killed nearly all larvae of C. dewulfi, 50% of C. chiopterus and all pupae of both species. These observations most likely indicate a strong adaption to the development in terrestrial habitats, i.e. cowpats.
7. In consideration of the present results, potential control measures should target the overwintering larval population in breeding habitats, prior to the onset of adult emergence in spring. Dungheaps provide good preconditions for treatments of breeding substrates, as large numbers of immature C. obsoletus s.s. can be found in this habitat in a comparably confined space.