Impact of flooding on the immature stages of dung-breeding Culicoides in Northern Europe

In Northern Europe, dung-breeding Culicoides (e.g., Culicoides chiopterus (Meigen 1830) and Culicoides dewulfi (Goetghebuer 1936)) are considered to be important vectors of the Bluetongue virus and Schmallenberg virus. The interpretation of their distribution is difficult due to the lack of knowledge about their ecology. Previously, soil moisture and especially flooding were identified as important factors that influence the development of several biting-midge species. Therefore, this experimental study addressed the question whether flooding has a negative impact on the development of immature stages of Obsoletus group species. Ten cowpats were collected, and each was divided into four quarters and kept at different moisture regimes in a greenhouse: (1) “dry” (no water added), (2) “control” (regularly moistened), (3) “alternately flooded” and (4) “permanently flooded”, to compare Culicoides emergence. Flooding had a significant negative impact on the emergence of Culicoides. No individuals emerged from the “permanently flooded” treatment and only two individuals were sampled from the “alternately flooded” treatment. In contrast, the total emergence from the non-flooded samples in the “dry” (96 individuals, 38.6% of all Culicoides) and “control” (151 individuals, 60.6% of all biting midges) treatments was considerably higher. Biting midges were predominantly identified as C. dewulfi (161 individuals, 64.7% of all Culicoides) and C. chiopterus (63 individuals, 25.3% of all Culicoides). There were no significant differences in emergence between the “dry” and “control” treatments. Our results highlight the importance of soil moisture on the distribution of C. chiopterus and C. dewulfi. Regarding physiological and behavioural adaptations of other Culicoides species, we argue that pupae of C. chiopterus and C. dewulfi are in danger of drowning when breeding sites are flooded as they cannot float. On the contrary, our results indicate that desiccation might not be harmful to these species.