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Trente ans de lutte contre l’onchocercose en Afrique de l’Ouest. Traitements larvicides et protection de l’environnement

Laurent Yaméogo
Christian Levêque
Jean-Marc Hougard

Références des articles présentés sur le cédérom / Papers references presented on CD-ROM

Short-term toxicity of pyraclofos used as a blackfly larvicide on non-target aquatic fauna in a tropical environment

L. Yaméogo, J.-M. Tapsoba, M. Bihoum et D. Quillévéré


Among the many larvicides tested for the control of Simulium damnosum s.l. larvae, the vector of human onchocerciasis in West Africa, pyraclofos proved to be 100% effective at 100 µg x L-1 for 10 min in river, with a cany of 20 km at 100m3 xsec-1. Tests were then performed both in laboratory and field conditions to evaluate its toxicity on the non-target aquatic fauna. In experimental short-term gutter tests, the detachment of the total benthic insects was 35% al 100 µg x L-1 for 10 min against 17% for temephos at the same dose and 59% for chlorphoxim at 50 µg x L-1 for 10 min. Centroptilum, Baetis and Pseudopannota bertrandi were the most affected organisms. The treatment of a river resulted in a considerable detachment of the same taxonomic groups, plus Orthocladiinae. On the other hand, investigations conducted in tanks showed that the 24-hr LC50 for Chrysichthys nigrodigitatus is 150 µg x L-1 and that for Pollimyrus isidori 170 µg x L-1, values which are not very different from the operational dose of the larvicide (100 µg xL-1 for 10 min.). Nevertheless, in a river, no fish mortality was recorded. Based on fish LC50 and drift of benthic insects, pyraclofos at 100 µg x L-1 was judged to be less toxic to aquatic fauna in the short term than permethrin and carbosulfan.

Texte intégral

1(Received in Japan 27 June 1993; accepted 25 August 1993)


2Human onchocerciasis (or "river blindness") is a filarial disease transmitted in West Africa by the female of the blackfly, Simulium damnosum s.l. (Diptera: Simuliidae), whose larval instars breed in swiftflowing portions of rivers. It is a disease which causes skin reactions and eventually severe ocular lesions followed by blindness. For many years, it was a major public health problem and an obstacle to the socioeconomic development of the fertile valleys in many West African countries.

3In 1974, an Onchocerciasis Control Programme (OCP) was launched in seven of these States, under the auspices of the World Health Organization (WHO), the United Nations Development Programme (UNDP), the Food and Agriculture Organization of the United Nations (FAO), and the World Bank, and with the financial support of many donor countries and institutions, to control the disease.

4Because of the absence of an effective drug for mass treatment of infected humans, it was decided to apply larvicides on the fast-flowing sections of the watercourses where the aquatic larval stages of the vector develop. The very short larval life of the blackfly led to the choosing of weekly larvicide applications to eliminate transmission of the filarial worm by the blackflies. Temephos, an organophosphorus insecticide, in a 20% emulsifiable concentrate formulation, was the only product used until the development in 1980 of a biological resistance in the flies to it (Guillet et. al, 1980). By 1982 resistance to chlorphoxim (another organophosphorus compound) had also developed in certain forest cytotypes of the vector (Kurtak et. al. 1982).

5These findings led to the large-scale use of Bacillus thuringiensis H-14 (Kurtak, 1986) and to the acceleration of a screening programme in a search for possible alternative larvicides. This acceleration became all the more necessary and was strongly recommended by the Ecological Group (an independent advisory body of the Programme) because of a planned extension into four additional countries. With this in mind, permethrin and carbosulfan were selected among many candidate blackfly larvicides (Yaméogo et. al, 1988). Recently, pyraclofos has proved to be an effective insecticide against the blackfly larvae. This paper discusses the results of tests carried out to evaluate the toxicity of pyraclofos for the non-target aquatic fauna as part of the screening of candidate blackfly larvicides undertaken in OCP.


6Pyraclofos (ISO draft), with the Chemical name of (RS)-(0-l-(4-chlorophenyl)-pyrazol-4-yl-O-ethyl S-propyl phosphorothioate), is an organophosphorus compound with a molecular weight of 360.8 (Kono, 1988). It is soluble in water up to 30 ppm at 20°C, and soluble also in alcohol and acetone. Its density is 1.271 (28°C). In water at pH 7 pyraclofos can be hydrolysed with a Chemical half-life in 700h and 1900h respectively at temperatures of 25 and 37° C. It biodegrades at a fast rate, especially in eutrophic waters. Hydrolysis also occurs at basic pH level. Bio-accumulation is unlikely to occur (as with other organophosphorus insecticides), mainly because of metabolic degradation. The formulation used in the trials was an emulsifiable concentrate with 50% of active ingredient, pale yellow in colour. It is an insecticide/acaricide that acts by contact and ingestion. The dose used for the control of S. damnosum s.l. larvae in the Programme area is 0.12 litres of formulation per m3/s of river discharge (0.1 mg x L-1 for 10 min or 60 mg x L-1.s).


7The toxicity study was carried out in the laboratory and in a river using the standard OCP protocol: acute toxicity in tanks for fish (Yaméogo, 1980), gutter tests, sampling of benthic insect drift (Dejoux, 1975 et 1980; Troubat, 1981; Yaméogo, 1984; Yaméogo et. al, 1988) pre- and post-larviciding qualitative observations.

1. Gutter tests

8A gutter is a plastic experimental apparatus modelled to represent a reduced stretch of river with, at the upstream end, a stop screen for natural drift in the river and, at the downstream end, a net for the collection in the gutter of the drift of organisms taken from the river with the substrates (stones, sand, dead leaves and twigs, etc.) to colonize the gutter. The System is installed in rapids’ zones of a river which is not very deep, in such a way that the water runs through the gutters; the water level reaching about half of their section. Drums containing the pesticide solutions to be tested can be placed at the upstream part of the gutters. Thus, different concentrations of a given pesticide or different pesticides can be tested while maintaining a control gutter.

9Drifting fauna is sampled periodically, (every 30 min. for 4 hours, then every hour for 20 hours) for a 24-hr period, then, the remaining organisms in the gutters are collected separately, fixed in alcohol and labelled (watercourse, locality, type of sampling, date, time), like all the other samples, to be studied in laboratory. It is therefore possible to compare simultaneously the impact of different larvicides or different concentrations of a larvicide using the detached organisms and/or those remaining in the gutters.

10In addition to a direct comparison between the percentages of detachment of the organisms tested, factorial correspondence analysis (Benzécri, 1983; Benzécri & Benzécri, 1986) was applied to data collected from several gutter tests with carbosulfan (a carbamate compound), permethrin (a pyrethroid), temephos, chlorphoxim and pyraclofos (organophosphorus compounds) using the "BIOMECO" programme of the Biometrics Group of CEPE-CNRS, Montpellier (France). This method of analysis introduced by Foucart (1978), possesses a good descriptive power as regards tables of positive numbers (without these being tables of probabilities or frequencies). It allows an easy descriptive generalization of the impact of blackfly pesticides on non-target benthic insects (Elouard & Jestin, 1982) and of the biotypology of running waters (Culp & Davies, 1980; Dakki, 1985).

2. Benthic insect drift sampling

11A battery of two nets (ϕ 12 cm) was used to estimate the abundance, in the river, of drifting insects, which fluctuates, as in temperate zones, according to a circadian rhythm (Elouard et Lévêque, 1977). The drift is low during the daytime and maximal during the night-time. Daytime drift is considered as a reflection of the morbidity and mortality of the organisms; it is regarded as passive. On the contrary, night-time drift reflects an active period, a voluntary drift of the organisais, which reflects the density of benthic organisms (Statzer et. al. 1985 et 1987) Since the abundance of benthic insects in the drift is supposed to increase with current speed and other environmental factors (Statzer et. al. 1984), different factors (pH, conductivity, turbidity, human disturbance, etc.) are measured, and the calculated "drift index" is used for the analysis instead of the raw number of organisms collected; the "drift index" is the number of organisms collected per cubic metre of filtrated water. Because of the high number of organisms sampled after larvicide sprayings, drift is collected for 2 min. only, every 30 min, from 6 a.m. to 10 p.m., then every hour. "Drift index" is generally spcaking ten to twenty times higher during night-time than daytime in untreated rivers. Daytime "drift index" increases when a river becomes treated (Yaméogo et. al. 1988).

3. Tank tests on fish species under laboratory conditions

12Acute toxicity tests are performcd in 10-litre glass tanks containing 10 fish each, using the technique with periodic replacement of solutions (Ward & Parrish, 1983). The insecticide solution is changed every 12 hours to avoid the effect of degradation of the insecticide. The water temperatures were 27 ± 2°C. The end point is death. The lethal concentrations are calculated according to probit analysis (Finney, 1952).

13The test concemed mainly Chrysichthys nigrodigitatus Lacépède, 1803 (Bagridae) and Pollimyrus isidori Valenciennes, 1846 (Mormyridae) two fish species quite common in the study area. C. nigrodipitatus is known in the Gambia, Senegal, Niger and Volta basins and in most coastal rivers (Lévêque & Paugy, 1984). The maximum standard length is 475 mm but the average length of the individiuals tested was 65 mm. The species feeds mainly on insect larvae (Chironomidae) but also on small mollusca, zooplankton and Hemiptera (Lévêque et. al. 1988). P. isidori is a small species well known in the Niger, Gambia, Senegal and Volta basins and in coastal rivers (Lévêque & Paugy, 1984). It feeds on insect larvae and on zooplankton (Lévêque et. al. 1988). The average length of the individuals tested is about 47 mm and the maximum standard length of the species is 83 mm.

4. Immediate river toxicity evaluation

14Drift of fish juveniles is collected using a large net (45 x 50 cm and 3 mm of mesh) set up for at least a 48-hr period of time, 24 hrs before treatment and 24 hrs after. The net is placed in a flowing section of the river at 8 a.m., removed at 4 p.m., emptied and replaced immediately after for the whole night up to 8 a.m. the next day. The same protocol is applied for the remaining 24 hrs. The samples collected are preserved in formaldehyde solution and analysed in laboratory. The impact of the larvicide on the fish is made by comparing pre-and post-treatment catches.

5. Taxonomie level

15The level of identification is the genus and even species for the Hydropsychidae (Trichoptera), Baetidae and Tricorythidae (Epheme-roptera). Chironomidae are identified at tribe or subfamily levels while most of the other benthic insects arc identified at the family level because of lack of detailed information. Fish are normally identified at the species level but for juveniles the identification is confined to family or order.

16The organisms retained for the tests are those representative of the study area and which adapt themselves well to the working conditions (Dejoux et. al. 1983; Lévêque et Paugy, 1984).

6. Location of field trial sites

17For the operational trial conducted on the White Bandama in Côte d’Ivoire in 1989, several sampling sites were set up (Fig. 1).

Fig. 1. Location of the operational trial river and the monitoring site in the Onchocerciasis Control Programme in West Africa.

18The total length of the river is about 800 km. The upper half of the watercourse concerned with the trial is in a wooded savanna area. During the operational trial (high-water season), the depth of the stretch of the river studied was between one and 15 metres while the length was about 250 km. The discharge during the first pyraclofos treatment cycle was maximal and around 450 m3 x S-1 (Fig.2).

Fig. 2. Weekly discharge of the White Bandama at station 4 from January to December 1989.

19The sampling point at Station No.2 was situated at some 300 m downstream from the nearest spraying point. It was selected to study the direct and maximum impact of the product on the invertebrate fauna. Station No.3 was located at about 5 km below the spraying point to study the carry of the product. Station No.4 was set up to follow the impact of pyraclofos on fish juveniles. It is also a regular monitoring site for the "Aquatic Monitoring Unit" of the Programme.


1. Gutter tests in the field

20The experiment was carried out on the Sassandra river, in Côte d’ivoire, at the same site, with pyraclofos at 0.1 mg of active ingredient xL-1 for 10 min., in comparison with temephos at 0.1 mg (a.i) x L-1 for 10 min. and chlorphoxim at 0.05 mg (a.i.) x L-1 for 10 min in the presence of an untreated gutter (see materials and methods for details on the techniques).

1.1 Control

21Drift was very low (5% detachment of the total fauna) compared to that usually obtained in this type of experiment (10 to 20% detachment). However, Amphipsyche senegalensis. (Trichoptera: Hydropsychidae) the Simuliidae and Chironomini (Diptera: Chironomidae) presented a percentage of detachment greater than that of the total fauna in the gutter, but it did not exceed 15%.

1.2 Pyraclofos

22The percentage of detachment of the total fauna is around 35%, 24 hours after treatment. Most of the organisms tested presented percentages of detachment less than 40% except Simuliidae, Centroptilum, Baetis and Pseudopannota bertrandi. The detachment of Trichoptera is very low (7%). That of Chironomidae is higher with a mean detachment value of nearly 25%.

1.3 Chlorphoxim

23The total percentage of drift due to chlorphoxim was almost 59%, a value which is within the range of the data reported in Yaméogo et. al 1988. Among the non-target organisms, it was the Ephemeroptera (mainly Centroptilum and Baetis but Pseudopannota bertrandi and Tricorythus also) which were affected most (almost 85% detachment). The effect on the Trichoptera as a whole was moderate but Amphipsyche senegalensis seems to be more susceptible (55% detachment) than the other species tested. The Chironomidae (Diptera) are also not very susceptible to chlorphoxim even though, just as for the other taxonomic groups, this product induced the highest detachment.

1.4 Temephos

24The total detachment of the fauna was about 17%, a value which is within the range of values usually obtained with this pesticide at the dosage of 0.1 mg x L-1 for 10 min. Apart from the Simuliidae which presented a detachment of more than 98% and two ephemeropteran taxa (Centroptilum and Baetis) which were affected up to 85%, most of the other taxa seemed to be little perturbed with a detachment which was less than 20%.

1.5 Comparative impact of the three larvicides

25Since the tests were carried out on the same day and under the same conditions, with a faunal composition similar from one gutter to the other, it is possible to compare directly the results obtained with the different insecticides used (Fig.3).

Fig.3: Percentage of detachment of different taxa after the application of the three larvicides tested

26While at the operational doses Simuliidae, Centroptilum and Baetis were the organisms affected most by the organophosphorus compounds mentioned above (Fig. 3) the classification of the other taxa in terms of their susceptibility varies from one larvicide to another among the organophosphorus compounds. On the other hand, a regrouping of the organisms into Ephemeroptera, Trichoptera, Chironomidae, Simuliidae gives the following classification according to their susceptibility: Trichoptera < Chironomidae < Ephemeroptera < Simuliidae

27Finally, the diagram on the percentage of detachment of the principal taxa reveals two distinct groups, of organisms for pyraclofos and temephos:

  • Simuliidae, Centroptilum and Baetis, on the one hand, which are very susceptible (detachment of more than 80%) and could be affected most in the long terni by the weekly spraying of these larvicides;
  • the Chironomidae, Cheumatopsyche. (Trichoptera), Tricorythus (Ephemeroptera) and Amphipsyche. (Trichoptera), on the other hand, which do not appear to be very susceptible to the effects of pyraclofos and temephos (detachment of less than 40%) and should therefore be able to withstand the impact of the larvicide applications if other phenomena do not corne into play.

28For chlorphoxim, the most susceptible taxa were Tricorythus with Centroptilum, Baetis and Pseudopannota bertrandi. Cheumatopsyche falcifera, C. digitata and the Tanytarsini presented an average susceptibility while Amphipsyche senegalensis is more susceptible than this group of organisms but less than the previous one.

29A factorial correspondence analysis of data collected from gutter tests with carbosulfan, permethrin, temephos, chlorphoxim and pyraclofos was made. Figure 4 shows a clear separation on the second axis (F2) between the most selective larvicides (temephos and pyraclofos) on one side, and permethrin, carbosulfan and chlorphoxim on the other side. The inertia percentages of the first two axes (F1 and F2) of the plane are 37% and 27%, i.e., a total of 64% for this factorial plane F1 x F2.

30The Tanypodinae seem to be particularly affected by temephos, on the First axis (Fl), as against the Caenidae and Tricorythidae which are influenced by carbosulfan, chlorphoxim and permethrin. Among the other Chironomidae, Chironomini and Orthocladiinae do not seem much affected by these larvicides.

31It emerges, therefore, from this experiment that all the organisms do not completely present the same reaction to insecticides belonging to the same Chemical family formulations. Furthermore, generally speaking, a marked impact of the larvicides should be expected on the Ephemeroptera in the long term. However, the monitoring, which has been going on for some fifteen years now in the watercourses treated in the Programme area (Yaméogo et. al, 1988), shows a rarefication of the Tricorythidae and certain Baetidae species in the worst cases. Contrary to all expectations, a marked presence of Pseudopannota bertrandi has been observed in certain hydrobiological monitoring stations (Ano, 1989), due to its short larval life-span (Wuillot, 1990) which allows emergence to occur between two treatment cycles, so that recolonization can take place.

2. Impact of an operational treatment on the insects

32The results presented in this section arc those recorded during the operational trial of pyraclofos on the White Bandama during the high-water period; the operational trial in OCP is the final step of screening before adopting a new insecticide as operational. It covers hundreds of kilometres of a river using a helicopter and operating as in a real larviciding campaign. Drift sampling with double nets and qualitative observations were the only techniques under which the experiment was conducted.

Fig. 4. Typology of the mortality percentages (corrected according to Abbott formula) of the main taxonomic groups in gutters treated with five blackfly larvicides (plane 1 - 2).

2.1 Impact on drift near the spraying point (station 2)

33Before treatment, the trend of the drift index curve was of the classic type (Elouard & Lévêque, 1977; Yaméogo, 1980); low during the day, the drift increased at night to reach 33 individuals per m3 of filtered water, and then decreased regularly (Fig.5a).

34After treatment, the drift collected for 4 hours was comparable to that of the previous day at the same time. The drift index increased only a short time before sunset, then fell to merge with the eve’s biological-activity drift. This increase which occurred only few hours after the application of the product was not due to a delayed effect nor did it correspond to the night drift. It could be due to the larviciding made at 13.4 km upstream, ten minutes before that made at 300 m from the sampling point. Twice greater than the pretreatment night drift, the peak occurred two hours earlier and was influenced by the Chironomidae while the Ephemeroptera made up the greater part of the pre-treatment drift (Fig.5b).

35On the whole, the drift remained low at this station, marked by the abundance of early larval stages and the almost complete absence of surface Hemiptera in the collections.

Fig. 5. Trend in drift of non-target benthic fauna at station 2 (a) and station 3 (c). Relative frequencies of main taxonomic groups in the drift at station 2 (b) and station 3 (d). 13-15/09/89.

2.2 Impact on drift at 5 km downstream spraving point (station 3)

36Just as for the previous station, the pre-treatment drift was of the classic type. It was however more diversified here, indicating different mesological conditions.

37Since the nearest spraying point upstream was located at 5 km from the sampling area, the drift intensity increase occurred 1 hr 30 min. after treatment, and was characterized by the abondance of Orthocladiinae. Of greater magnitude than that recorded at station 2, the drift intensity subsequently decreased but another peak occurred in the night with the peak recorded around 2200h (Fig.5c). The Orthocladiinae, Simuliidae, Baetidae (Centroptilum and Baetis), and Hydropsychidae (Aethaloptera) presented drift curves having more or less the same appearance. The Caenidae and Leptophlebiidae did not seem to have been particularly affected by the treatment. After treatment, the relative composition of the communities underwent a change similar to that of the previous station; the Chironomidae became the most represented, followed by the Ephemeroptera, the Trichoptera and the Simuliidae (Fig.5d).

38The drift here was greater than at 300 m from the spraying point and the second peak recorded around 2100h could be due to a combination of several factors including the natural night drift, probably intensified by a weakening of the organisms, and the effects of a second pyraclofos wave from the treatment point located sorae 20 km upstream.

3. Impact of pyraclofos on the fish fauna

3.1 Acute toxicity test

39The medium lethal concentrations calculated according to the probit analysis (Finney, 1952) for different exposure times are not very different for the two fïsh species tested. However, looking at the slopes of the mortality curves (Table I), it appears that the longer the exposure time, the more small increases in dose resuit in considerable P. isidori mortalities while C. nigrodigitatus becomes less affected by slight dosage increases.

Table I: Median lethal concentrations (LC50) of pyraclofos (TIA-230) for two tropical fish species.

Table I: Median lethal concentrations (LC50) of pyraclofos (TIA-230) for two tropical fish species.

40Apart from the 24-hr LC50, which are higher than the highest dosage used in onchocerciasis vector control (100 µg x L-1 for 10 min.), the other data are between 40 and 100 µg x L-1.

41Compared to other data obtained with permethrin, carbosulfan and cyphenothrin by different authors, particularly Yaméogo et al. 1991, one may note that pyraclofos has the highest 24-hr LC50 (150 µg/1 against 40 µg x L-1, 82 µg x L-1 and 15 µg x L-1 respectively), which means that it is less toxic than the others. But taking into consideration the operational doses, it appears that the 24-hr LC50 of pyraclofos is doser to its highest operational dose (O.D.) than the 24-hr LC50 of permethrin and its O.D. Besides, the 24-hr LC50 of cyphenothrin is equal to its O.D. and the difference between the 24-hr LC50 and the O.D. of carbosulfan is somewhat comparable to that recorded with pyraclofos. It is therefore considered that when used for Simulium control, cyphenothrin presents the highest risk for fish; the risk of permethrin is the lowest, while that of carbosulfan and pyraclofos is medium.

3.2 Immediate river toxicity

42Direct observations made during the larviciding of pyraclofos in a river at the dose of 0.1 mg x L-1 for 10 min did not reveal any fish mortality even though the dose at the spraying point was almost ten times this dose.

43Collections were made with drift nets both before and after the pyraclofos sprayings. The results do not show an increase in the drift intensity but a slight change in the relative composition of the communities (Fig.6). Schilbe mystus (Schilbeidae) dominated in the night drift but the proportion of Characidae increased in the post-treatment night drift.

Fig. 6. Relative composition of juvenile fishes in the night drift before and after pyraclofos spraying at station 4.

44On the other hand, the day drift, solely composed of Characidae, was not influenced by the pyraclofos application. This product does not seem therefore to have a direct impact on fish in rivers. The posttreatment decrease in the drift intensity and the increase in the proportion of Characidae are the main facts which should be mentioned.


45From the above results, it can be seen that the product presents at the operational dose, a toxicity on the aquatic entomofauna which is between that of temephos and that of chlorphoxim. It acts particularly on Centroptilum, Baetis and Pseudopannota bertrandi which detached by more than 80% while the other nontarget organisms presented an average drift of 40%.

46While in the laboratory pyraclofos presents, for fish, a relatively high toxicity which increases with exposure time, this has not been confirmed by the results of the river tests. No fish mortality was recorded and the slight behavioural change observed for Characidae (a particularly fragile fish) was transient. This change did not resuit in an increase in the drift (in number) of Characidae. It should be recalled that in rivers dilution occurs quickly and the fish can flee and avoid areas of high concentrations of the larvicide (Abban & Samman, 1980).

47Short-term toxicity tests are the first stages of the hazard assessement of a candidate antiblackfly larvicide. Since the results presented in this document show that the short-term impact of pyraclofos on the non-target aquatic fauna is not drastic, it has been decided to undertake a large-scale experiment to assess its medium and long-term effects on the aquatic environment. The results will be published when completed.


48This work was fully supported financially by WHO/OCP, and the Programme Director, Dr E. M. Samba, agreed that the results should be published. We are therefore indebted to him.

49Many persons contributed to the realization of this work. We would like to thank, in particular, Mr J. Wuillot of the University of Lyon, Mr Fanfodé Kondé of the University of Kankan and Mr B. Coulibaly, Mr L. Bakoné, Mr B. Dolbézanga and Mr S. Bakayoko of OCP who participated in the data collection during the operational trial of pyraclofos. The fish specimens were fished by Mr Simpore who gave us a highly appreciated help in the laboratory too.

50Finally, we are grateful to Mr T. Mills who undertook the literary revision of the document, and to all those (drivers and technicians) who, in one way or the other, also contributed to the carrying out of these tests.



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Table des illustrations

Légende Fig. 1. Location of the operational trial river and the monitoring site in the Onchocerciasis Control Programme in West Africa.
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Légende Fig. 2. Weekly discharge of the White Bandama at station 4 from January to December 1989.
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Légende Fig.3: Percentage of detachment of different taxa after the application of the three larvicides tested
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Légende Fig. 4. Typology of the mortality percentages (corrected according to Abbott formula) of the main taxonomic groups in gutters treated with five blackfly larvicides (plane 1 - 2).
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Légende Fig. 5. Trend in drift of non-target benthic fauna at station 2 (a) and station 3 (c). Relative frequencies of main taxonomic groups in the drift at station 2 (b) and station 3 (d). 13-15/09/89.
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Titre Table I: Median lethal concentrations (LC50) of pyraclofos (TIA-230) for two tropical fish species.
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Légende Fig. 6. Relative composition of juvenile fishes in the night drift before and after pyraclofos spraying at station 4.
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Onchocerciasis Control Programme
O1 BP 549, Ouagadougou 01
Burkina Faso

Onchocerciasis Control Programme
O1 BP 549, Ouagadougou 01
Burkina Faso

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