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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

Risk assessment of etofenprox (vectron®) on non-target aquatic fauna compared with other pesticides used as Simulium larvicide in a tropical environment

L. Yaméogo, K. Traoré, C. Back, J.-M. Hougard et D. Calamari

Résumé

Within the rotational scheme developed by the Programme to fight the resistance of Simulium damnosum to Chemical larvicides, there was an operational gap at discharges between 5 and 70 m3 s-1 for the treatment of rivers where resistance to organophosphates was present. The use of permethrin and carbosulfan was precluded because of risk of environmental impact and, Bacillus thuringiensis ser. H-14 treatments were not envisageable due to cost and logistics constraints. Among the possible complementary groups of larvicides tested, the pseudo-pyrethroids, held promise, because of a mode of action similar to that of pyrethroids, but along with a usually lower toxicity for fish. Etofenprox, one of the pseudo-pyrethroids tested, shows a global detachment of non-target insects in 24 h close to that of pyraclofos, an organo-phosphorus compound (27 against 23%). In laboratory conditions, six times the operational dose which is 0.03 mg l-1 10 min, is needed to cause 50% mortality of Caridina sp. (a small shrimps species) and 30 times this same dose for 95% mortality. For fish species, a safety margin of 400-800 times the operational dose is observed for Oreochromis niloticus and 200-400 times for Tilapia zillii. © 2001 Elsevier Science Ltd. All rights reserved.

Note de l’éditeur

Corresponding author. Tel.: +226-34-29-53; fax: +226-34-28-75.
E-mail address: yameogo@ocp.oms.bf (L. Yaméogo).

Texte intégral

1Received 8 March 2000; accepted 15 March 2000

1. Introduction

2The weekly destruction for 14 years of the aquatic stages of Simulium damnosum s.l. is the principal strategy used since 1974 by the Onchocerciasis Control Programme in West Africa (OCP); the objective being to eliminate onchocerciasis as a disease of public health importance as well as an obstacle to socio-economic development.

3Several activities have been undertaken to minimize the environmental effect of such a Programme and they have been recently reviewed by Calamari et al. (1998). The present paper describes the toxicological studies performed in order to make a risk assessment of etofenprox. For several years, temephos or Abate®, an organophosphorus insecticide formulated with 20% of active material, was the only product used till the appearance in 1980 of a resistance in the vector S. damnosum (Guillet et al., 1980). Chlorphoxim, another organophosphate and Bacillus thuringiensis ser. H-14 were then introduced, but rapidly, a resistance to chlorphoxim was developped by the forest cytotype of the vector (Kurtak et al., 1982). The Programme was then obliged to give priority to the search for new larvicides or new formulations of larvicides. Among the numerous insecticides tested (Yaméogo et al., 1988, 1992), only pyraclofos, which is equally an organophosphate, permethrin (a pyrethroïd) and carbosulfan (a carbamate) were retained for use under well-established conditions in relation to their level of toxicity. Toxicological studies and assessments for pyraclofos (Yaméogo et al., 1993b) and permethrin (Yaméogo et al., 1993a) have been published elsewhere. Pyraclofos treatment was to be carried out at discharges higher than 15 m3 s-1 and, permethrin and carbosulfan spraying concerned discharges higher than 70 m3 s-1 for a maximum of six cycles in the year for the same stretch of river. Therefore, six insecticides could be used in rotation considering the risk of resistance, the operational costs and the effects on the aquatic environment. However, between 15 and 70 m3 s-1, the only usable larvicides, taking into consideration the cost/efficiency ratio, were the organophosphates (temephos, phoxim and pyraclofos), then increasing the risks of resistance of the simulids to these products. It therefore became necessary to continue the research with the view of identifying a non-organophosphorus insecticide that could be used on demand between 15 and 70 m3 s-1.

4It is within this context that the bibliographic search permitted the selection of etofenprox or Vectron®, a pseudopyrethroid described as having weak effects on fish, for an evaluation of its toxicity on the non-target aquatic fauna in the field conditions of the Programme. Some tests were then undertaken on fish and invertebrates in laboratory as well as in the field and under conditions of an operational Programme. This document gives the results obtained during tests conducted in a short-term period.

2. Material and methods

2.1. Characteristics of the product

5Chemical name Ether 3-phenoxybenzylic 2-(4-eth-oxyphenylic)-2-methylpropylic, with popular name (or ISO) etofenprox, the product is a pseudopyrethroid with a molecular formula C25H28O3, with a density of 1.157/23°C and a molecular weight of 376.5 (Udagawa, 1986). It is soluble in a series of ordinary solvents at 25°C and almost insoluble in water (<1 ppb). Its aspect is solid white crystalline; it is without any characteristic odour and is stable in some acids and alkalines. It is an active insecticide through ingestion and contact, having a very low toxicity in mammals and fish (Mitsui Toatsu Chemicals Inc., 1986). Its partition coefficient (log P) in water/n-octanol is 7.05, its half-life in soil is 6 days and the vapour pressure is 2.4x10-4 mm Hg/100°C. The formulation used in the zone of the Programme against the larvae of the S. damnosum s.l. is an emulsifiable concentrate with 20% of active ingredients/litre.

6Other Chemicals tested for comparison were: permethrin (3-phenoxybenzyl-(1 RS)-cis, trans 3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane carboxylate); pyraclofos (RS)-[O-1(4-chlorophenyl)-pyrazol-4-yl-O-ethyl-S-propylphosphorothioate]; OMS 3050 1,1,1,-trifluoro 2-(4-ethoxy-phenyl) 3-(3-(4-chlorophenoxy) benzyloxy) propane. The formulations of the products were diluted taking into account the percentage of active ingredient.

2.2. Gutter tests

7This method makes it possible to compare simultaneously in field conditions the impact of different larvicides or different concentrations of a larvicide. The system is composed of a certain number of plastic experimental apparatus modelled to represent reduced stretches of rivers (gutters) and is directly alimented with river water. The gutter (Troubat, 1981) is equipped, at the upstream end, with a stop screen for natural drift and, at the downstream end, a net for the collection of the drift of organisms taken from the river with the substrates (stones, sand, dead leaves and twigs, etc.) to colonise the gutter. The results were expressed for the total fauna and for the principal taxa present in the gutters as percentage of detachment per insecticide tested. To appreciate the effect of the above-mentioned insecticides on the main species in the gutters, treatments were made every 2 h with increasing doses in two parallel gutters (for each larvicide) having comparable discharges. The drift was collected every 30 min and the results expressed in number of taxa affected and in percentage of detachment for each of the taxa studied.

2.3. Tank test on fishes and shrimps under laboratory conditions

8The acute toxicity tests are performed in 200 1 tanks. The solutions were 150 l for each tank and were oxygenated. The technical products were diluted taking into account the percentage of active ingredient and the results were expressed for each time, e.g., concentration of active principle (mg l-1). Ten to fifteen individuals of fish or 20 shrimps were put into each tank, knowing that 1 g of fish should be reared in 1 l of water. For each of the products, five different doses were prepared and three replicates were done. The larvicide solutions were prepared 1 h before the start of the trials. The tests were performed using the static technique (Ward and Parrish, 1983). The water temperature were 25±1°C. The end point is death. The lethal concentrations are calculated according to probit analysis. The behaviour of the fishes in the tanks was observed and reported three times a day.

9Caridina africana as regards shrimps and two species of fish from the family of Cichlidae (Oreochromis niloticus and Tilapia zillii) were the species tested. They are the most common species encountered in the watercourses of the Programme area. C. africana (Atyidae) is one of the most widespread shrimps species in intertropical Africa. It is a small shrimps (10-30 mm) encountered in small and large rivers, lakes, streams, ponds and pools. O. niloticus Linné, 1758 is well known from many river basins in Africa (Senegal, Gambia, Volta, Niger, Benoue and Tchad basins). T. zillii Boulenger, 1901 is more widespread. In addition to the above-mentioned basins, it is encountered in the Ogun, Oshun, Comoé, Mé, Bandama, Boubo and Sassandra river basins in West Africa. Elsewhere in Africa, T. zillii is known from Oubangui, Uélé, and Ituri (Zaïre), lake Mobutu, the Nile, lake Turkana and from Jourdain basin. These species are of commercial importance in the Programme area and therefore, were considered for testing as an impact on them should be prejudiciable to the activity and life of people in the area. The medium length of the individuals tested was 6.98±1.52 cm for O. niloticus and 7.18±2.5 cm for T. zillii.

2.4. Shrimps monitoring methodology in field conditions

10A test was performed at the operational dose on the Leraba river at 19 m3 s-1 to assess the impact of etofenprox on shrimps. An operational dose is the concentration of insecticide that will kill Simulium larvae with a carry of 6 km at 8 m3 s-1 and of 13 km at 100 m3 s-1 in high flow conditions and it is usually expressed in mg l-1 10 min. Drifting shrimps (C. africana Kingsley 1882) were sampled using a large net (45 cm x 50 cm and 3 mm of mesh) set up for at least a 48 h period of time, 24 h before and 24 h after treatment. The current velocity within the net was measured at least twice a day. The net is placed in a flowing section of the river and emptied every 4 h. The individuals collected just after treatment were kept under observation in cages placed upstream in the untreated section in order to monitor their fate in uncontaminated water. The impact of the larvicide on the shrimps is evaluated therefore by comparing the difference between pre¬ and post-treatment catches for the same stretch.

2.5. Taxonomic level

11The level of identification for insects is the genus and even species for the Hydropsychidae (Trichoptera), Baetidae and Tricorythidae (Ephemeroptera). Chironomidae are identified at the tribe or subfamily levels while most of the other benthic insects are identified at the family level because of lack of detailed taxonomic information on the larvae. Fish and shrimps are normally identified at species level but for the juveniles or individuals in a bad State, the identification is confined to family or order levels. The organisms retained for these tests are those representative of the study area and which adapt themselves well to the working conditions (Dejoux et al., 1983; Durand and Lévêque, 1980; Lévêque et al., 1990).

3. Results and discussions

12The main objective of the tests is to help take a decision on the possible introduction of etofenprox in the larvicides’ rotational scheme of the Programme. The acute toxicity tests as well as the field trials were therefore conducted bearing in mind this objective.

3.1. Gutter tests in the field

13The experiment was carried out on the Oti river, in Togo, at the same time, with etofenprox in comparison with permethrin, OMS 3050 and pyraclofos in the presence of an untreated gutter (see materials and methods for details on the techniques). The percentages of detachment presented below (Table 1) are those corrected using the Abbot formula.

Table 1. Detachment (%) in gutters of some taxonomic groups of invertebrate during the test conducted on the Oti river in Togo with operational doses

Table 1. Detachment (%) in gutters of some taxonomic groups of invertebrate during the test conducted on the Oti river in Togo with operational doses

3.1.1. Control

14Fig. 1 shows the trend and the percentage of detachment for the total fauna. In the control, the detachment is equal to 27% when considering the drift of simuliids but, it becomes less than 20% (when taking into account the non-target fauna alone) which is considered as acceptable in experimental conditions. The drift is dominated by the Baetidae (Centroptilum spp., Baetis spp., Ophelmatostoma kimminsi and Pseudopannota bertrandi).

3.1.2. Etofenprox

15The real detachment of the non-target fauna due only to the use of this product at the operational dose (0.03 mg-1 10 min) is around 27% taking into account the 20% of the natural drift, 24 h after treatment. Ephemeroptera and Simuliidae have a percentage of detachment (61% and 56%, respectively), higher than that of the non-target total fauna (Table 1). Among the Ephemeroptera, Neurocaenis spp. from the Tricorythidae are the most affected, and it is important to note that the Ephemeroptera present a detachment higher than that of the target, the simuliids.

16During the acute toxicity test conducted with five increasing doses of etofenprox (0.015, 0.03, 0.06, 0.15, and 0.3 mg l-1 10 min), the number of taxa entering into the drift did not increase much with the dosages (23 as against 24 at the highest dose). Furthermore, all the taxa affected entered the drift within 30 min of the etofenprox treatment and then the number decreased regularly (Fig. 2).

Fig. 1. Comparison of the detachment of the total fauna in gutter caused by different larvicides.

Fig. 2. Trend of the number of taxa collected in the drift after treatment of two parallel gutters with increased doses of etofenprox: 0.015; 0.03; 0.06; 0.15; 0.3 mg l-1 10 min.

17Instead of the graduai increase in the number of taxa after the dose increase described by Yasuno et al. (1981) for temephos, a decrease was rather recorded at doses 0.03 and 0.06 mg l-1 10 min. Before a new increase occurred at 0.15 mg1-110 min, i.e., at five times the operational dose. For most of the families, the greatest detachments occurred at the dose of 0.03 mg l-1 10 min (0.06l m3 s-1) as shown in Fig. 3. An increase in the dose does not necessarily cause a higher detachment.

3.1.3. Permethrin

18More than 70% of the non-target fauna is affected and here again, the detachment of the Ephemeroptera (Caenidae, Baetidae but especially P. bertrandi, Neurocaenis spp.), is the highest. The less affected organisms are belonging to Diptera from the family of Chironomidae with a detachment of 66%. Even the Trichoptera which present usually a moderate susceptibility to most of the larvicides (Yaméogo et al., 1992), were highly affected (83% of detachment).

19The treatment made at doses 0.015, 0.03, 0,075 and 0.15 mg l-1 10 min did not lead to an increase in the number of taxa in the drift as compare to that made at 0.0075 mg l-1 10 min, i.e., half the operational dose (Fig. 4).

20The Simuliidae, Caenidae and Tricorythidae experienced their greatest drift at half the operational dose. As regards the Chironomidae, permethrin caused the greatest drift at the dose of 0.075 mg l-1 10 min (five times the operational dose) while for the Philopotamidae 0.03 was the most harmful dose (Fig. 5). With regard to the Hydropsychidae, the greatest drift was at 0.015 mg l-1 10 min (operational dose).

3.1.4. OMS 3050

21At the operational dose (0.025 mg l-1 10 min), the detachment of the non-target fauna is about 36%, with the Ephemeroptera being the most affected organisms (66% of detachment).

Fig. 3. Toxicity of etofenprox on the principal benthic invertebrate families tested in two parallel gutters (*Dose in mg l-1 10 min).

Fig. 4. Trend of number of taxa collected in drift after treatment in a gutter with increasing doses of permethrin: 0.0075; 0.015; 0.03; 0.075; 0.15 mg l-1 10 min.

22The Chironomidae are the less affected one (15% of detachment). The toxicity of the product on the main species considered in this study (C. falcifera, C. copiosa, C digitata, A. senegalensis, Neurocaenis sp. and P. bertrandi) is higher than that of etofenprox and permethrin.

3.1.5. Pyraclofos

23The detachment of the non-target fauna due to the effect of pyraclofos alone is close to 23% at the operational dose (0.1 mg l-1 10 min). Apart from the Simuliidae which are highly affected (more than 90% of detachment), more than 50% of the Ephemeroptera become detached 24 h after treatment. Only 8% of the Trichoptera are affected, and the Chironomidae present 30% of detachment.

24Increasing doses (0.04, 0.1, 0.2, 0.5 and 1 mg l-1 10 min) applied induced at 0.1 mg l_l 10 min, a drift of maximum number of taxa (Fig. 6) then, at 1 mg l-1 10 min, i.e., 10 times the operational dose.

25At 0.2 and 0.5 mg l-1 10 min, the maximum number of taxa present in the drift was 20 as against 23 at 0.1 mg l-1 10 min. In considering the principal families present in the gutters treated, Simuliidae and Baetidae had a considerable detachment right from the treatment at half the operational dose (Fig. 7). Despite the dose increases, the detachment percentage was low at 0.1 and 0.2 mg l-1 10 min. Before increasing at 0.5 and 1 mg l-1 10 min. As regards the Hydropsychidae and Philopotamidae (Trichoptera), the dose increases led to drift in creases up to 0.2 mg l-1 10 min (twice the operational dose) at which the detachment was maximal.

Fig. 5. Toxicity of permethrin on the principal benthic invertebrate families tested in two parallel gutters (*Dose in mg l-1 10 min).

Fig. 6. Trend over time of the number of taxa collected in the drift after treatment of the two parallel gutters with increased doses of pyraclofos: 0.05; 0.1; 0.2; 0.5; 1 mg l-1 10 min.

26For the Chironomidae (Diptera), the maximum drift was recorded at the operational dose while for the Caenidae and the Tricorythidae the greatest drifts corresponded to the highest doses. All the benthic invertebrate families react in the same way to pyraclofos. While the Simuliidae and the Baetidae became detached at the dose of 0.05 mg l-1 10 min, the Tricorythidae and Caenidae were affected mainly from 0.5 mg l-1 10 min onwards, i.e., a susceptibility that is 10 times less.

Fig. 7. Toxicity of pyraclofos on the principal benthic invertebrate families tested in two parallel gutters (*Dose in mg l-1 10 min).

27As regards the non-target aquatic insects populations as a whole, the short-term impact of the operational dose of etofenprox is not significantly different from that of pyraclofos at 0.1 mg l-1 10 min using Wilcoxon T-test. By contrast, it is significantly less than that of permethrin.

3.2. Acute toxicity tests in tank on fish

28The results are reported in Table 2 together with confidence limits and slopes for two of the larvicides (permethrin 20% EC and etofenprox 30% EC) on the two species tested (O. niloticus and T. zillii). No mortality was recorded in the control tanks and the behaviour of the individual was normal up to the end of the experiment.

3.2.1. Permethrin

29The 24 h LC50 for O. niloticus is relatively low (0.040 mg l-l) and quite close to that obtained by Yaméogo et al. (1991) with Pollimyrus isidori using static System with periodic replacement of the solution. The same applies to the 48 h LC50. For T. zillii, the lethal concentrations for 24 and 48 h exposure time (0.075 and 0.049 mg l-1, respectively) are almost twice that of O. niloticus.

3.2.2. Etofenprox

30The fishes introduced into the etofenprox solution are a bit more active than those in the control tanks. At the highest doses, some of the individuals demonstrated disorderly movements a few hours after the start of the trial followed by death. Some others corne periodically to the surface of the solution, the mouth open.

31The 24 h LC50 for the two species tested is at least 60 times that obtained with permethrin. As regards the employaient safety margin, it is 400-800 times the operational dose for O. niloticus, and 200-400 times the operational dose for T. zillii. This species is therefore more susceptible to etofenprox than O. niloticus when the opposite is observed with permethrin. Very little data is available on acute toxicity of etofenprox. Kariya et al. (1982), testing the 20% EC formulation of etofenprox on carps at a lower temperature (20°C), reported a medium tolerance limit of 44 mg l-1 for an exposure time of 24 h. At higher temperature in a tropical environment, in comparison to the results of our test on two african characidae species, etofenprox seems to be safer.

Table 2. Median lethal concentration LC50 mg l-1 for two tropical fish species and two Chemical larvicides

Table 2. Median lethal concentration LC50 mg l-1 for two tropical fish species and two Chemical larvicides

a Confidence limits.
b Slopes.

3.3. Acute toxicity tests in tanks on shrimps (C. africana)

32For an exposure time of 2 h in etofenprox solution, the mortalities recorded in tanks made possible the calculation of the following lethal doses:

33LC50-2 h: 0.18 mg l-1 10 min,

34LC95-2 h: 0.88 mg l-1 10 min.

35Six times the operational dose which is 0.03 mg l-1 10 min, and longer exposure time are therefore necessary to cause 50% mortality of this species of shrimps, and 30 times this same dose for 95% mortality. With permethrin, the LC50-2 h is 0.036 mg l-1 10 min (twice the operational dose) and the LC95-2 h is 0.1 mg l-1 10 min, i.e. seven times the operational dose.

3.4. Short-term toxicity tests in river on shrimps

36The catches in the control area were higher than in the treated section. The maximum drift was recorded at night, between 20h00 and 24h00, while the minimum occurred between 12h00 and 16h00. It was only after the treatment at the operational dose (at 16h00) that a difference was observed between the drift in the two zones. The comparative trend of the drift of Caridina (before and after treatment) in the treated section as well as in the control one (Fig. 8(A) and (B)), indicates a similarity of drift in these sections. It was mainly after the treatment (at 16h00) that a difference was recorded between the drift in the two zones. The individuals collected were of small sizes (<1 cm) and it should be noted that none of them was dead 24h00 after the treatment.

Fig. 8. Comparative trend of the drift of caridina in the two zones before (A) and after (B) treatment with etofenprox at Leraba-gare on the Leraba from 21 to 23 October 1993.

37There is therefore a short-lasting impact of etofenprox on the young Caridina at a discharge of 19 m3 s-1. Taking into account the usual susceptibility of these organisms to pyrethroids, this resuit could be considered as exceptional.

4. Conclusion

38Although etofenprox is a pseudopyrethroid, its impact at the operational dose on the benthic insects taken as a whole is quite comparable to that of pyraclofos and far below that of permethrin.

39Almost all the taxa present in the gutters experienced an impact of etofenprox like permethrin at half the operational doses with the result that an increase in the number of taxa in the drift was witnessed with the increases in the insecticide doses in the systems, contrary to what had been reported by some authors for temephos.

40The level of changes to be expected if etofenprox is used in river was estimated using the index proposed by Elouard and Simier (1990) which showed a good correlation between the values observed in the field and those calculated (theoretical) from the gutter test data. The relationship that had been found by the authors from three insecticides (temephos, B.t. H-14 and chlorphoxim) used at the operational dose is

41Y = 0.089x 4-1.86 with r = 1.

42By including pyraclofos, we obtained the following relationship:

43Y = 0.083x + 1.98 with r = 0.99.

44Since the index was established on the basis of eight taxa (Baetidae, Caenidae, Tricorythidae, Hydropsychidae, Chironomini, Tanytarsini, Orthocladiinae and Tanypodinae), we calculated the index for etofenprox and permethrin by using the detachments of the same taxa in gutter. The values obtained for etofenprox (4.7) is comparable to that observed for pyraclofos (4.7) and quite different from that of permethrin (7.5). It could therefore be expected that the changes which the use of etofenprox would cause on the benthic communities would be of the same proportion as those brought about by pyraclofos.

45The level of acute toxicity of etofenprox on fish, and even on shrimps, is far less than that of permethrin. Furthermore, no direct mortality of fish or shrimps was observed in river at low discharge at operational dose even if a short-lasting increase of shrimps in the drift was observed. It is also reported (Mitsui Toatsu Chemicals Inc., 1986; Udagawa, 1988), that administered to rats, the product is excreted into the faeces and urine rapidly. Etofenprox was therefore considered as a serious candidate to fill the operational gap of non-organophosphate insecticide at discharges between 15 and 70 m3 s-1.

Bibliographie

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Laurent Yaméogo Hydrobiologist in the WHO Onchocerciasis control Programme (OCP) since 1982. Environmental monitoring activities’ co-ordinator in OCP from 1984 (acute to medium-terms toxicity tests, long-term monitoring of the larvicides’ impact). Since March 2000, Chief of the vector control unit (VCU) of the Programme, cumulative with the functions of co-ordinator.

Table des illustrations

Titre Table 1. Detachment (%) in gutters of some taxonomic groups of invertebrate during the test conducted on the Oti river in Togo with operational doses
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Légende Fig. 1. Comparison of the detachment of the total fauna in gutter caused by different larvicides.
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Légende Fig. 2. Trend of the number of taxa collected in the drift after treatment of two parallel gutters with increased doses of etofenprox: 0.015; 0.03; 0.06; 0.15; 0.3 mg l-1 10 min.
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-3.jpg
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Légende Fig. 3. Toxicity of etofenprox on the principal benthic invertebrate families tested in two parallel gutters (*Dose in mg l-1 10 min).
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-4.jpg
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Légende Fig. 4. Trend of number of taxa collected in drift after treatment in a gutter with increasing doses of permethrin: 0.0075; 0.015; 0.03; 0.075; 0.15 mg l-1 10 min.
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-5.jpg
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Légende Fig. 5. Toxicity of permethrin on the principal benthic invertebrate families tested in two parallel gutters (*Dose in mg l-1 10 min).
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-6.jpg
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Légende Fig. 6. Trend over time of the number of taxa collected in the drift after treatment of the two parallel gutters with increased doses of pyraclofos: 0.05; 0.1; 0.2; 0.5; 1 mg l-1 10 min.
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-7.jpg
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Légende Fig. 7. Toxicity of pyraclofos on the principal benthic invertebrate families tested in two parallel gutters (*Dose in mg l-1 10 min).
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-8.jpg
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Titre Table 2. Median lethal concentration LC50 mg l-1 for two tropical fish species and two Chemical larvicides
Légende a Confidence limits.b Slopes.
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-9.jpg
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Légende Fig. 8. Comparative trend of the drift of caridina in the two zones before (A) and after (B) treatment with etofenprox at Leraba-gare on the Leraba from 21 to 23 October 1993.
URL http://books.openedition.org/irdeditions/docannexe/image/29343/img-10.jpg
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