Beginnings of the problem.

In the last decade, linked to the improvement of the water quality of our rivers and streams for the implementation of wastewater treatment systems, the discomfort caused by black fly bites has appeared and increased (simulidae) in Catalonia.

These insects, relatives of flies, behaving as opportunistic individuals, have colonized the water courses of our internal basins. The partial improvement of the quality provided by the installed purification systems has allowed the colonization of these environments, many times not adequate enough for the installation of a complex biocenosis that can minimize the problem by predation. Although due to the characteristics of the riverbed itself, many times of small size, phenomena such as large avenues linked to equinoctial rainfall regimes cause this channel to undergo major transformations and that in later years the phenomenon of black flies that had been able to reach the character of a plague, radically disappear (the Tech in the south of France, the Francolí in the nineties, and certain tributaries of the Llobregat).

Thus, in recent years those of us who work in mosquito control have seen an increase in the complaints produced by this insect in many places in Catalonia, and this has been echoed many times by the local and regional press. In parallel, the mosquito control services of Girona and that of Llobregat, have come to implement small population controls in order to reduce the populations resulting from certain places in Catalonia.

The case of the Ebro can be considered different, in the first place we are talking about one of the largest rivers in the Iberian Peninsula, where we have evidence of the presence of small populations of black fly linked to permanent water courses near the river. Its characteristics, flow rate, quality and, above all, lack of transparency (due to the predominance of unicellular algae in suspension) gave this final stretch of the Ebro river the characteristic green color that prevented light from reaching the bottom of the channel. This phenomenon, which caused the extinction of the sun's rays beyond 40 cm, did not allow the implantation of macrophytes in the riverbed itself, thus it was not a suitable place for the massive development of simulid larvae. Those were times when the indicator organism that predominated in this lower stretch of the Ebro was the Ephoron virgo, the famous "palometa" that gave us so many problems at the end of summer and autumn.

Starting in the 2000s, these characteristics have gradually changed, unfortunately they have gone unnoticed since these ecological parameters have not been studied by any organism that has recorded, so we can only base these changes on hypotheses.

  • The decrease and regularization of flows (especially the suppression of floods in recent years)
  • The appearance of invasive species that began in the 1980s with the catfish and in 2001 with the appearance of the zebra mussel in the reservoir systems of the lower reaches, a hyperfiltering species.
  • Implementation of wastewater treatment systems in the Ebro basin.
  • Improvement in the rational use of fertigation with a clear decrease in the contributions of various fertilizers that play a fundamental role in the eutrophy of the river.

Surely, one or the sum of all these variables have transformed the ecosystem of the lower section of the Ebro river, where the most apparent characteristic is the clarification of its waters. We currently find that at depths of 5 or 6 meters the solar rays reach the bottom perfectly. This phenomenon has favored the proliferation of aquatic plants (macrophytes), which require the presence of light for their germination and proliferation. In recent years, starting in 2000, we have seen how, on a continuous basis, almost the entire riverbed has been colonized by these macrophytes, especially those of the genus Potamogeton and Myriophyllum, especially the species Potamogeton pectinatus. It is in this new scenario where it has been possible to install, from these nearby relict sites, the larvae of the black fly, and exponentially colonize the entire lower section of the river and thus also progressively their annoyances have appeared to us.

Remembering, CODE had the first contact with this phenomenon in the winter of 2003, when a farmer from the Mora de Ebro area approached us through the DARP delegation in the area and showed us his concern about a coloring insect dark and small in size that caused severe bites on days of full sun when harvesting olives. This specific case, which was unique and implausible in our territory, was unfortunately the announcement of what, as of 2005, has become one of the great problems of this territory.

Search for solutions

In 2005, a Technical Commission was formed at the headquarters of the Government Delegation in the Ebro Lands, made up of technicians from the Department of Agriculture, Livestock and Fisheries (DARP), Department of Health, Department of Environment and Housing, Institute for the Development of the Comarcas del Ebro (IDECE), the Aquatic Ecosystems Unit (UEA-IRTA) and the Consortium of Agro-environmental Services of the Baix Ebre and Montsià regions (CODE), under the coordination of the Territorial Delegation of the Government of the Generalitat in the Ebro Lands, in order to study the characteristics and population distribution of the black fly, seek information on this phenomenon, and propose treatment measures. That same year, the Commission commissioned CODE and UEA-IRTA to carry out a technical study on the black fly problem. Specifically, the work consisted of studying the larvae of the black fly, their habitat, evaluating the dynamics of the larval populations, determining their distribution and determining the effects of the treatments on the species to estimate their effectiveness.

On the part of the DARP, the same week of the constitution of the commission, a series of elements aimed at minimizing the populations of flies present in the cultivation fields is tested. We must remember that it was the time to harvest the fruit and the presence of the black fly caused great hindrances. The adulticide treatment was not very effective due to recolonizations, apart from the problem that it could generate residues in the harvested fruit. The technique of mowing the grass from the terraces was a little more effective since it reduced the areas of refuge for the fly. We must remember that, like all insects, the black fly cannot regulate the temperature, hence, once it has emerged from the waters of the river, it looks for areas with vegetation where the temperature and humidity are favorable, waiting for possible hosts for the that suck the necessary blood by the females to be able to carry out the formation of eggs and give rise to the next generation.

The Department of Health makes a series of recommendations aimed at the affected population, through measures to prevent bites and the corresponding recommendations in the event of a bite.

In the summer of 2005, through IDECE and with the help of COPISA, the company in charge of maintaining the inland waterway, UEA-IRTA and CODE technicians began a series of samplings of several affected sections of the Ebro river.

Operaris en plena tasca de mostreig

The objective of these first surveys (in July and August 05) was to determine the habitat and space colonization preferences of the black fly larvae as well as the identification of the black fly species. Data that were absolutely essential when deciding what type of treatment to carry out. With these outputs, it was found that the larval and pupal forms of the black fly were found mainly on macrophytes, mainly of the genus Potamogeton pectinatus (which dominates a large part of the riverbed), although also in the other major macrophyte at the edges, Myriophyllum sp and with a certain preference for places with current.

The identification of the pupae obtained in the different sections sampled was carried out in the laboratory and with the help of Dr. Gloria González (United Research Services España S.L.). The species was classified as Simulium erythrocephalum.

Once the causal agent of the problem had been classified, an exhaustive bibliographic research could be carried out. At the same time, it contacted other places and control services that had already had to face a similar problem.


On the one hand, the EID Méditerranée Méditerranée (mosquito control service of the southeast Mediterranean of France) studies this phenomenon in the Tec river, a river in the Eastern Pyrenees; and also the Mosquito Control Service of the Bay of Roses and Baix Ter, which works with the problems present in the Ter river and two of its tributaries, the Xunclà and the Terri. In both cases, the flow of these rivers is much lower than that of the Ebro, which determines the sampling and treatment strategy to be followed.

EID Méditerranée

Other information provided are the problems that generate simulid species different from ours, but in rivers with characteristics more similar to ours, fundamentally with a high flow (greater than 100 m3/s). These are the cases of rivers in the United States, such as the Susquehanna in Pennsylvania, or rivers in northern Europe (eg Danube).

Development of a sampling methodology

In this context, a sufficiently agile sampling methodology was developed to estimate with some reliability the density of black fly larvae in the sampled area. Specifically, it was established that:

  • a) for each chosen sampling point, six macrophyte samples will be taken,
  • b) that each one of the macrophyte samples was placed in trays with half water, where it was vigorously shaken so that the black fly larvae attached to the macrophyte were detached. This macrophyte was placed in water drainers so that it lost the highest water content and it was weighed. The larvae count was done directly on the tray and the larvae density was referred to the number of larvae per weight of macrophyte.

Tasques de mostreig

Tasques de mostreig

Tasques de mostreig

Tasques de mostreig

Investigation of tools for its control

Once the prospecting work had been carried out, which provided information on the existence of larvae, it was necessary to face the possibility of launching a type of active fight against these insects by treating the waters of the Ebro river. For reasons Obviously, the treatment of the river had to be done with a biological and highly selective insecticide, characteristics that occur in the spore produced by the bacterium Bacillus thuriengiensis var. israelensis (Bti), which is effective only for dipteran larvae. This fact, although it is true that it has many advantages for the conservation of the environment, reports many inconveniences when applying the treatment, since in order to obtain high effectiveness, the parameters of the treatment area must be controlled very well. (Guarantee that the product reaches all corners of the area to be treated, little time spent in the environment, etc.)

The application of Bti in running water must meet two requirements:

  • The distribution of the product throughout the riverbed must be guaranteed from the point of application. This requirement is guaranteed by the current of the river Ebro.
  • The passage of product where the larvae are must last long enough to allow all the larvae to consume the product. The information collected in other control centers as well as the supply company itself establish this time at 10 minutes.

Finally, the efficacy of this treatment had to be validated in the black fly populations in the Ebro river, so that:

  1. It was necessary to validate the effectiveness of the treatment in the lower part of the Ebro river (of considerable dimensions and flow and different from the rest of the rivers where the treatment with Bti had now been applied).
  2. It was necessary to determine the distance of effectiveness of the treatment.

These questions were basic when it came to quantifying the effectiveness of a treatment in the Ebro and quantifying the economic viability of implementing a control against this insect.

With all these doubts, a pilot test was proposed at a point where we had previously detected the existence of downstream larvae, and that would allow us to draw conclusions quickly, since time was running against us and the insects were beginning to be made. notice in a more annoying way the neighbors of the affected areas.

Pilot test of biological treatment with Bti (April 27, 2006)

In the surveys prior to the pilot treatment, the ideal point was established for carrying out this test downstream from the Ginestar jetty.

To carry out this treatment, a barge from the COPISA company was used, with a large platform where the product could be organized and applied from the river itself.


The necessary dose for the application is obtained from the following formula:

Q = C x ppm x 60 x 10

Where Q represents the amount of product (Bti) needed; C is the flow of water that circulates through the river at the time of application, indicated in m3/s; ppm is the concentration of the product necessary to take effect, which is usually around 4; We must ensure that this dose remains for approximately 10 minutes, enough time for the larvae to be able to ingest enough Bti particles to cause death.

For the application, two tanks of 1000 liters capacity were necessary, where the necessary 400 liters of Bti were mixed with water to allow the application to be prolonged for the necessary 10 minutes.


Of the different factors that appear in the formula, the flow rate is the only one that we cannot control ourselves. This is a very important factor, since the amount of product that will be used to carry out the treatment depends directly on it. Due to the high cost of the product and to guarantee the economic viability of the possible treatments, it is essential to achieve a flow rate at the time of treatment as low as possible in the river, so that with the minimum expenditure of product a high ppm is achieved. For this, they contacted the Ebro Hydrographic Confederation (CHE), who offered their collaboration at all times.

Thus, on April 27 early in the morning, the CODE staff traveled to Ginestar to prepare the treatment, and at approximately 10 in the morning, after checking the river flow, which was set at 135 m3/s, the treatment was carried out, achieving a concentration of 4.9 ppm.

It must be said that we were pleasantly surprised by how easy the product had, once applied, to disperse throughout the river bed, and to pass through the macrophytes, which although they did not occupy this entire bed, formed thick masses of vegetation impossible at first glance, to be penetrated by the product.

Application and dispersion of the product in the pilot test

The objective of this pilot test was to verify the effectiveness of the treatment and at the same time determine the distance of effectiveness in the Ebro river, thus, to estimate the percentage of mortality of black fly larvae, 6 points were chosen downstream from the application point. of the product (Ginestar pier) and two points upstream. The samplings of these points (according to the methodology explained in the previous section) were carried out the day before and the day after the pilot test.

Pilot test. Treatment data

Location map of the treatment and sampling points of the pilot test of 04/27/06


The larval mortality percentages obtained with the pilot test are presented below. It should be mentioned that the table also shows the samplings of two points where there was no product application, which were the points located 1 and 2 km upstream from the treatment point.

  number of larvae / kg macrophyte  
Sampled point before treatment after treatment mortality
- 2km from the treatment point (Ginestar) 12.75 14.5 − 
- 1km from the treatment point (Ginestar) 4.46 17.3 − 
+ 1km from the treatment point (Ginestar) 16.02 0 100 %
+ 3km from the treatment point (Ginestar) 27.05 0 100 %
+ 6km from the treatment point (Ginestar) 15.73 0 100 %
+ 8km from the treatment point (Ginestar) 32.2 0 100 %
+13km from the treatment point (Ginestar) 150.3 0.9 99 %
+18km from the treatment point (Ginestar) 5.32 0.9 82 %

Gràfica de barres de la prova pilot

With this pilot test it was possible to verify the efficacy and viability of the use of Bti to treat black fly larvae from the lower part of the Ebro. It was found that the effectiveness distance of the Bti applied according to the conditions of the pilot test was above of the 15-18 km. Considering that the route of the river from Flix in Amposta is about 85 km, this effective distance implied about 5-6 points of application per treatment, a number of applications much more viable than the one that would have meant having to treat every 5km, such as This is the case of the Ter and Tec rivers.

It should be noted that in the samplings carried out the day after the pilot test, no mortality was observed in other groups of organisms, even in nearby groups with a very similar diet such as Chironomids.

It seemed that we had found an effective, economically and materially viable population control tool.