Mosquitoes and biting midges: data collection, identification, species distribution, and breeding ecology

Renke Lühken
PhD Thesis Culicoides biting midges Mosquitoes


This PhD thesis summarises thirteen stand-alone publications on mosquitoes (Diptera: Culicidae) and Culicoides biting midges (Diptera: Ceratopogonidae). Although species of both groups are potential vectors for a variety of pathogens, these insects received little attention in scientific work in Germany until the beginning of the 21st century. However, with the outbreak of the bluetongue disease and the introduction and spread of exotic mosquito species, the situation changed abruptly and scientific research increased in both research groups.

This thesis, first of all, is focused on different scientific questions regarding the species distribution for two vector groups: biting midges and mosquitoes. Most of the questions this research dealt with, apply to both vectors groups. However, this thesis also gives attention to some aspects specific to one or the other group. The conclusions drawn from this research outlined here not only add to the fundamental research but will directly or indirectly add knowledge to the field of applied vector ecology, which will be also helpful for future monitoring concepts. The work covers the total gain of new scientific insight into vector ecology, starting with new results on sampling (1) and species identification (2), and continuing with correlative studies (3) and experimental studies on species distribution (4). One chapter is devoted to each of these four topics:

The first chapter of this thesis presents two methodological studies. Both investigate the efficacy of (light-)suction traps in dependence of the type of trap for mosquitoes and the position of the trap relative to potential hosts for biting midges. The two studies demonstrated that the different traps or the position of the traps significantly affects the trapping efficacy. Hence, the type of trap and their positioning need to be selected with caution when planning to study the distribution of both vector groups.

In the second chapter, new alternative methods for the identification of cryptic vector species were evaluated. Two studies explored the differentiation of female Culex pipiens s.s. and Cx. torrentium with morphometric wing characters and the four German members of the Anopheles maculipennis group with a real-time PCR assay. Both tools proofed to be useful for a fast and reliable identification, which can be used to understand the environmental factors affecting these species, which is, at least in Germany, a highly neglected field of research.

The studies in the third chapter evaluated the species distribution of immature mosquitoes and biting midges on the local scale (i.e. breeding sites) and analyses the distribution of adults on larger scales (i.e. landscape):

First, breeding sites were studied to evaluate the correlations of physico-chemical data with the presence-absence of Cx. pipiens s.l. and Cx. torrentium, the impact of edaphic factors on the density of dung-breeding Culicoides biting midges, and the biting midge density in relation to the position and substrate temperature in a cattle dung heap. The results of the first study revealed that the Cx. pipiens s.l. and Cx. torrentium often occur in sympatry at the studied breeding sites and there were no substantial differences concerning presence-absence of the two species with regard to the various environmental parameters investigated. The second study highlights the importance of soil moisture, organic substance, and soil texture on the number of emerging Culicoides chiopterus and C. dewulfi. Increasing soil moisture positively correlated with the number of emerging adults for both species of biting midges, while the variable organic substance was only positively correlated with the abundance of C. chiopterus. In contrast, soil textures showed conflicting results, i.e., a positive and negative relationship with the same variables. Furthermore, in the third study, the larval density of C. obsoletus s.s. was negatively correlated with increasing substrate temperature in the dung heap. Secondly, cooperating with the research group Environmental Informatics of the University of Oldenburg, a prototypic software tool was developed to predict the local spread of exotic mosquito species. This tool offers extensive application possibilities, e.g. an exact geographic definition for the design of mosquito surveillance or control programs. Thirdly, the distribution of the various An. maculipennis group members from Germany were studied. Most remarkable was the decline of An. atroparvus compared to historic data, which might be explained by the differences in the overwintering strategy compared the other Anopheles species. An. atroparvus is expected to be more dependent on shelter sites in the immediate vicinity to cattle hosts that were lost as a result of a changed farm building design. Moreover, an elaborated study evaluated which landscape scale, or even multi-scale model, might be suitable to model the distribution of different biting midge species in Germany. The study demonstrated that multi-scale modelling is a promising approach to predict the distribution of Culicoides species. It offers the opportunity to include a diverse set of variables from different scales. This is especially important for hematophagous insects, e.g. when breeding sites, resting sites, or host density have to be taken into account for modelling, which are generally distributed across several scales.

The fourth chapter of this thesis focuses on the breeding ecology of dung-breeding Culicoides (C. chiopterus and C. dewulfi). These experimental studies explicitly evaluated causal relationships between emergence rates and environmental parameters. These studies cover three different topics: 1) Effects of mechanical disturbance of breeding sites. With regard to the development of juvenile biting midges, disturbance should increase the mortality and, thus, was assumed to be an alternative control method. However, no significant reduction of emerging adults was recorded by application of this approach. 2) Effects of disturbance by flooding on the survival of immature biting midges. In this experiments, no individuals emerged from the “permanently flooded” treatment, hence, clearly indicated that long-termed flooding caused a significant increase in mortality. 3) The third study in Chapter 4 is dedicated to seasonal effects and winter development. Effects of temperature and photoperiod on the development of the overwintering population are analysed in order to judge on regulation factors and man-made effects which might uncouple these regulations. The results highlighted the importance of temperature on the spring emergence of dung-breeding Culicoides in Northern Germany.

Any observation has a blind spot. This applies with regard to different aspects of vector ecology, e.g. the knowledge on the advantages and disadvantages of a sampling or identification method, the significance of variables not included in species distribution models, or the ecological interpretation of those variables being included. The different studies in this thesis filled several of these blind spots by a critical evaluation of trapping and identification methods, and the examination of the correlative or causal relationship of the factors affecting the species distribution.