Midges
Midges are small (1-5 mm) slender to robust, often dark-colored mosquitoes that are related to lake flies. 167 species occur naturally in Belgium, both stinging and non-stinging. 16 species are observed along the Zeeschelde. The stinging species Culicoides riethi is most common here. This species naturally belongs to mudflats and marsh areas. These are strips of nature that are flooded daily (mudflats) to biweekly (marshes) due to the action of ebb and flow.
Adult female midges lay their eggs in muddy soils that periodically flood and always remain moist, but are not permanently flooded or dry out. The adult animals can then be found near those silt soils, sometimes in large numbers. C. riethi clearly shows an activity peak at dusk and to a lesser extent at sunrise.
History
Midges and midge plagues have always existed along the Schelde. There are reports in Wetteren and Schellebelle in 1906 and along the Durme in Hamme around 1920-1930. However, due to poor water quality at the end of the last century, midges almost completely disappeared from the Zeeschelde. Due to the improvement of water quality, warmer seasons and the expansion of potential habitat (more nature = more chance of midges), midges are becoming more common again, usually without significant nuisance.
Monitoring
Weather conditions cannot be controlled, the improving water quality is also a fact, so only the living environment can be used to prevent mosquito nuisance. Active capture of midges near the source is also being tested. Knowledge about the ecology of midges and the effectiveness of measures taken is in full development. That is why midge monitoring is being carried out on behalf of De Vlaamse Waterweg nv. The aim is to gain understanding of the existing midge populations in the Zeeschelde, both in existing mudflats and marsh areas and in new nature restoration projects. This should also allow to estimate the effects of measures taken.
Midge traps
For the monitoring of adult midges, the so-called Wymeers traps are used, a type of trap that was developed by RBINS (Royal Belgian Institute of Natural Sciences) for monitoring in Wijmeers. The original Wymeers trap is equipped with a LED lamp with UV light that attracts insects. A fine mesh allows to selectively catch midge and prevents other insects from ending up in the trap. In the trap, the midges land on water with a little detergent, causing them to sink and drown. To make identifying and counting the captured midges easier, the water in our Wymeers monitoring trap was replaced by a yellow glue strip. During a measuring session, the UV light in the trap is switched on for 3 days each time. By counting the midges, now captured on the yellow glue strip, in the lab we get a clearer picture of the size of the midge population.
Results
A first observation is that midges are not only active around dusk. Midges were also caught during the day, albeit in much lower numbers. Adult midges do not like full sun. At night the same number of midges were often caught as during twilight. It is difficult to conclude whether they are actually just as active at night. It is possible that the UV light from the Wymeerstraps exerts a greater attraction on the midges at night.
The results also indicate that there is always a certain 'background' of midges along the Zeeschelde and Durme. However, the numbers of observed midges can vary greatly over short distances and over time: midges are mainly carried passively by the wind, which means that numbers in places with different air circulation can differ greatly. The distance to the Schelde nature, wind strength and wind direction will largely determine the distribution of midges.
On top of the dike at Lippenbroek, a FCA-CRT (flood control area with controlled reduced tide) that was already constructed in 2006 and where the ecosystem is now well developed, an average of 8 midges was caught in 2022-2023 during the 3 measurement periods. On top of the dike, the nuisance was very limited. In 2023, only a limited number of midges were caught in June at the ferry in Driegoten and the pontoon at the district house in Dendermonde. There was no nuisance.
In Sint-Amands near the mudflats at the grave of Emiel Verhaeren, up to 702 stinging midges were caught during 1 measurement session. In 2023 the numbers were clearly lower, with an average of 63 stinging midges. Nuisance was also reported in 2023. The lower numbers in 2023 are attributed to the less favorable weather. The sunny month of June still had a maximum of 226 midges, but in the wet and gloomy month of July the number dropped to 2. The dry autumn led to slightly higher numbers (34 midges). The natural mudflats in Sint-Amands can therefore clearly be a source of midges. Where the nuisance manifests itself largely depends on the spread of the insects by the wind.
Many midges also occur in the depoldered area of Groot Schoor. On average, 117 midges were caught in the area in 2023 per measurement session, with a maximum of 760 midges during the measurement period in September 2023. In 2022, the maximum was even higher: 1050 midges were caught in August.
In the wider area surrounding the Groot Schoor and Sint-Amands, the numbers of mosquitoes were also monitored in several gardens to estimate the spread and nuisance. The numbers caught can vary greatly depending on the prevailing weather conditions (wind direction, sun, precipitation). The distance to mudflats and the presence of barriers such as dense vegetation can also play a role. On average, 9 stinging midges were caught in the gardens in 2023 per measurement session at twilight, with a peak of up to 161 midges in June in a garden northwest of Groot Schoor. In 2022, the numbers were even higher: an average of 51 midges with a maximum of 222 midges.
Conclusion
In summary, we can say that midges mainly occur in higher elevated but very wet mudflats, both along the natural banks of the Schelde (e.g. in Sint-Amands) and its tributaries and along created nature reserves (e.g. Groot Schoor Hamme). Weather conditions have a major influence on the presence of midges; the wind direction determines where any nuisance might occur. 2023 had fewer midges than 2022, perhaps due to the bad summer of 2023. Specifically around Groot Schoor, the further development of the area with the gradual formation of a well-branched creek system and therefore better drainage and vegetation development will also have played a role. To map this evolution, midge monitoring will continue in 2024.