Black flies: vicious winners of change

They resemble harmless houseflies – but some species of black flies bite people. © Dorian Dörge

Bloodsuckers that scratch small wounds in the skin: The medically relevant species of black flies in Germany could benefit from climate and land use change, researchers report. For the first time, they analyzed the distribution patterns and ecological requirements of various native species of this group of insects and derived their development potential from this. According to the scientists, the problematic representatives of black flies in particular could be increasingly appearing in our country and possibly transmit diseases.

The focus is usually on mosquitoes or horseflies - but in addition to these pests, there is a previously less well-known group of blood-sucking flying insects in Germany. The black flies (Simuliidae) are represented very diversely here, with a total of 57 different species. However, many only specialize in infesting certain species of birds and mammals. But some species are flexible - they can affect various farm animals and also humans. They often go unnoticed because at first glance they resemble harmless houseflies. However, black flies are sharply equipped: females use special mouthparts to create small wounds in the skin of their victims and then ingest the drop of blood that forms there.

Potentially dangerous “pool vacuums”

With this concept, known as “pool sucking,” black flies can cause specific problems in animals and humans: “Anticoagulant and anesthetic substances introduced into the wound can trigger serious allergic reactions or secondary bacterial infections can occur. Black flies are also vector-competent, meaning they are able to transmit pathogens that cause certain infectious diseases through their bite,” says senior author Sven Klimpel from the Goethe University Frankfurt. Tropical black fly species are particularly notorious for spreading the pathogen that causes so-called river blindness (onchocerciasis) in Africa and South America.

Although our native black flies cannot transmit this disease, the possible significance of the various species as disease vectors has so far been little researched. Klimpel and his team have now devoted themselves to a more detailed study of the distribution patterns and ecological requirements of some of the black fly species common in Germany. The basis of their study was 1,526 data sets with information on twelve of the most common species in Hesse, North Rhine-Westphalia, Rhineland-Palatinate and Saxony. A central aspect was the demands on characteristics of the breeding waters. Because black flies lay their eggs in rivers and streams. The larvae then develop there, from which the winged insects later emerge.

The analysis of the data led to the division of black flies into three biogeographical groups: species adapted to the upper reaches of rivers, species widespread across different landscapes and lowland species. In addition to their distribution, these groups of black flies have different requirements and tolerance levels for environmental conditions. Based on these principles, the team then developed forecasts of how the populations of different black flies could develop given current trends in the context of climate and land use change.

Problematic representatives on the rise

As the team reports, the models show that populations of headwater-adapted species will decline due to rising temperatures and changes in water quality. But lowland species in particular could benefit significantly from anthropogenic change, spreading and becoming more common. The problem is that this third group includes, in particular, the black fly species that are relevant to veterinary and human medicine. They are characterized by a particularly aggressive stinging behavior towards mammals and humans and often occur in very large numbers. “Future higher temperatures could lead to shortened development times, to more generations per year and thus to a more frequent occurrence of black flies overall,” summarizes first author Sarah Cunze from the Goethe University Frankfurt.

The researchers now plan to support the results with direct evidence of development trends. They also want to use laboratory tests to determine the extent to which different blackfly species are able to transmit certain pathogens under the conditions prevailing in Europe. “The development trends for the medically relevant black fly species derived from the results of our study are an example of how vector-borne infectious diseases can be promoted by global change,” says Klimpel. “Our modeling approaches and results can now help to efficiently design monitoring and action programs for vector-competent species and to derive predictions about future developments,” says the scientist.

Source: Senckenberg Society for Natural Research, specialist article: Science of The Total Environment, doi: 10.1016/j.scitotenv.2024.170454

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