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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

Typology of susceptibilities of aquatic insect larvae to different larvicides in a tropical environment

L. Yaméogo, J.-M. Elouard et M. Simier


Multi-gutter tests are used by the Onchocerciasis Control Programme in West Africa to screen antiblackfly larvicides with reference to non-target aquatic insect larvae. Because the tests are not always conducted under the same environmental conditions, direct comparison of results by the usual methods of data analysis presents some difficulties. The application of reciprocal averaging (correspondence analysis) to data from tests carried out using the same protocol indicates that the test periods have no incidence on the impact in gutter of the larvicides. The analysis also makes it possible to classify the insecticide families according to their degree of general toxicity on fauna. In addition there is no evidence of a uniform effect produced by each insecticide family on the principal taxa.

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1The strategy adopted against Simulium damnosum complex, which transmits human onchocerciasis (Philippon, 1977), by the Onchocerciasis Control Programme (OCP) was the use of larviddes since 1974. Because of the short larval life span of the blackfly, the larviciding was done weekly in fast-flowing river stretches where the vectors’ larvae developed. For many years temephos, an organophosphorus insecticide, in a 20% emulsifiable concentrate formulation, was the only product used until the appearance in 1980 of resistance 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). These findings led to the acceleration of a screening programme in a search for possible alternative larvicides.

2The selection of insecticides is based mainly on their efficacy against the vector (Kurtak, 1986) but also on their toxicity as regards the non-target fauna (Yaméogo et al., 1988, Lévêque, 1989). More than some sixty products from different Chemical families have been tested but less than half of these larvicides have been the subject of studies on the non-target fauna, particularly insects.

3Among the different techniques for studying the short-term impact of larvicides on non-target benthic fauna, that of gutters (Dejoux, 1975 and 1980; Troubat, 1981; Yaméogo, 1984) is usually employed. However, the general comparison of test results has proved difficult just as the general estimation of the toxicity of the Chemicals. Two major reasons account for these difficulties. First, the environmental conditions under which tests are conducted vary considerably from one experiment to another. Secondly, there is a differential susceptibility of the taxa to different insecticides. Besides, from a theoretical and maybe a predictive point of view, it will also be interesting to know whether products belonging to the same Chemical family (organophosphorus, carbamates, organochlorine, pyrethroid and growth regulator compounds, bio-insecticides, etc.) have the same toxicity for the same taxa.

4All these tests, carried out on the non-target fauna as part of the selection of larvicides, have therefore been analysed and the main results are discussed in this document.


5The results, presented and discussed in this paper were obtained during tests conducted using the technique of multi-gutters (Troubat, 1981). A gutter is a plastic experimental apparatus modelled to represent a reduced stretch of river (Fig. 1) with, at one end, a stop screen for natural drift and, at the other end, a net for collection of drift of organisms taken from the river with the substrates (stones, sand, dead leaves and wood, etc.) to colonize the gutter.

Fig. 1. Diagram of two types of gutter (simple A and multiple B) used in situ for the study of the impact of insecticides on benthic fauna.

6The System is installed in rapids’ zones of a river which is not very deep, in such a way that the water runs right through them; the water level reaching about half of their section. Drums containing the pesticide solutions to be tested can be placed at the upstream end of the gutters. Thus, different concentrations of a given pesticide or different pesticides can be tested while maintaining a control gutter.

7Drifting 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 (as all the other samples) to be studied in laboratory.

8It is therefore possible to compare the impact of different larvicides or different concentrations of a larvicide using the detached organisms or those remaining in the gutters.

9The organisms used in the different tests were identified to the family level for all the aquatic insects with the exception of the Chironomidae which were separated to the subfamily or tribe level. Furthermore, only the taxa presenting a wide geographic distribution and which were relatively abundant in almost all the tests (Hydropsychidae, Baetidae, Caenidae, Chironomini, Tanytarsini, Orthocladiinac, Tanypodinae) have been taken into account.

10The percentage of detachment of the organisms in the gutters, considered as a percentage of mortality after correction by the Abbott formula (Finney, 1962) taking the natural mortality in the control gutter into consideration, is the value indicating the toxicity level of the larvicides. Reciprocal averaging analysis (correspondence analysis) were applied to these corrected data by using the "BIOMECO" programme of the Biometrics Group of CEPE-CNRS. Montpellier (France). This method of analysis, introduced by Foucart (1978) and Benzecri (cf. Benzecri & Benzecri, 1986), possesses a good descriptive power as regards tables of positive numbers (without these being tables of probabilities or frequencies). It should also allow a good description of the long-term effects on non-target aquatic insects of the use of pesticides against blackflies (Elouard & Jestin, 1982; Elouard & Fairhurst, 1990; Fairhurst & Curtis, 1988) and of the biotypology of running waters (Culp & Davies, 1980; Dakki, 1985).

11To answer the different questions posed in the introduction, the analyses covered, firstly, the fauna that colonized the gutters before treatment (tested fauna) and then the fauna that remained in the gutters 24 hours after the larviciding was taken into consideration.


1. Typology of the tested fauna

12Analysis of the basic communities that colonized the gutters before treatment and tested using the same protocol at different periods of the year is illustrated in Figure 2. The examination of this factorial plane 1 x 2, whose first axis (Fl) explains 44% of the variance (inertia) out of a total of 79% for the plane, calls for two remarks:

  • the F1 axis is based on the contrast of the flood-subsidence structure of macroinvertebrate communities at Koperagui (October), influenced by the Tricorythidae, with the others (Amou-Oblo and spate period at Koperagui);
  • as regards the second axis (F2), it contrasts the spate period at Koperagui (July-August). influenced by the Tanytarsini and Hydropsychidae, with that of the flood-subsidence at Koperagui and the rise in water level at Amou-Oblo influenced by the other taxa with the exception of the Tanypodinae which did not contribute much to this axis.

13The typologies described above are therefore related mainly to the hydrological seasons.

Fig 2. Typology of fauna in place in gutters during tests conducted under different conditions.
Bae: Bactidae
Cae: Caenidae
Tri: Tricorythidae
Chi: Chironomini
Tat: Tanytarsini:
Tap: Tanypodinae
Ocl: Orthocladiinae
Psy: Hydropsychidae
● Spate at Koperagui
▲ Flood-subsidence at Koperagui
■ Beginning of rise in water level at Amou-Oblo

2. Topology of drift in the control gutters and in the gutters treated

14In order to reveal the impact of the Programme’s operational larvicides on the drift of the organisms, we used the data on the tests carried out at one site but during different hydrological periods and therefore as has been brought out in the previous chapter, with different benthic community structures. The structures of the drift in the gutters treated with operational larvicides and in the control gutters are represented by factorial plane F1 x F2 (Fig. 3). This plane shows different typologies according to the insecticides. The presence of Controls makes it possible to apprehend the extern of the changes caused by the insecticides compared to a "natural" drift. The percentages of inertia of the first two axes of the correspondence analysis arc 39 and 35%. i.e., 74% for the factorial plane.

15Groups corresponding to each of the larvicides tested and to the control gutters are well individualized on the factorial plane. Besides, it is observed that on the F2 axis the Controls project close to temephos and opposite the toxic insecticides (chlorphoxim, permethrin and carbosulfan).

16It indicates, on the one hand, that a different structure in the basic gutter communities does not change the structure reflecting the toxicity of the products. Some reservations are however necessary if the relative abundance of certain taxa is too low in certain tests. On the other hand, this analysis confirms the fact that temephos is a not-very-harmful insecticide compared to the other three tested at the same site using the same methodology.

3. Typology of remaining fauna (in gutter)

17The remaining fauna is that which has escaped from the impact of the larvicides. A comparison of the community structures obtained in this way with the basic ones of the gutters, will make it possible to understand eventual modifications caused by the insecticides. Besides, the typologies of the remaining fauna should be quite close to those of the saxicolous fauna of the watercourses treated with the antiblackfly larvicides; the gutter being considered as a miniature watercourse.

18The objective of this study is to correlate the short-term toxicity of larvicides in gutter Systems with community structures in treated rivers and, later on to establish a model for forecasting the long-term impact of the larvicides on the saxicolous fauna on the basis of their short-term toxicity. Analysis of the typology of the remaining fauna in the gutters seems therefore to be more appropriate than that of the mortality in gutters.

19As seen earlier, the typologies of the fauna in place in the gutters before treatment are related to the hydrological seasons (Fig.2). A correspondence analysis has been made for the fauna remaining in these same gutters 24 hours after treatment (Fig.4). This factorial plane 1 x 2 explains 61% of the inertia. The first axis contrasts the Chironomidae, which are characteristic of the gutters treated with the relatively toxic insecticides (carbosulfan, pyrethroids and chlorphoxim), with the Ephemeroptera which are associated with the most selective doses or larvicides. The second axis contrasts the Chironomini and Orthocladiinae with the Tanytarsini and Tanypodinae. It reflects, therefore, the selectivity of the products as regards the Chironomidae while the F1 axis classifies the larvicides according to their general toxicity to the fauna as a whole. It will be noted that the Hydropsychidae, which present a medium susceptibility to most of the larvicides (Table I). do not contribute much to the axes.

Bae: Bactidae
Cae: Caenidae
Tri: Tricorythidae
Psy: Hydropsychidae
Chi: Chironomini
Tat: Tanytarsini
Tap: Tanypodinae
Ocl: Orthocladiinae
▲ Abate
♦ Chlorphoxim
■ Permethrin
◊ Carbosulfan
○ Control
Fig. 3. Comparison between the typology of fauna that drifted from the gutters treated and from the control during tests conducted at the same site using the same protocol and

20The typologies revealed seem to be stable and do not depend on the basic structure of the gutter communities. They suggest that taxa that are not much affected by an insecticide should remain in the watercourses treated with it, while the population of the susceptible taxa would decrease. However, the complexity of the biological phenomena calls for caution because the immediate impact observed during an isolated treatment could be affected in the long run by factors such as trophic and spatial competition, habituation or resistance or other forms of adaptation, the duration of the larval development cycles, etc.

Az: Azamethifos
Bio: Bioresmethrin
Ca: Carbosulfan
Ct: Permethrin
Cy: Cyphenothrin
De: Deltamethrin
Etf: Etbofenprox
O2: OMS 3002
O4: OMS 3040
O6: OMS 3036
Py: Pyraclofos
Ph: Chlorphoxim
Ta: Talstar
Te: Temephos
Fig. 4. Typology of fauna remaining in multi-gutters after the action of different insecticides tested by OCP using the same protocol.
Abreviations Legend
Bae: Baetidae
Cae: Caenidae
Tri: Tricorythidae
Chi: Chironomini
Tat: Tanytarsini
Tap: Tanypodinae
Ocl: Orthocladiinae
Psy: Hydropsychidae
▲ Pyrethroids
■ Carbamates
* Organophosphorus compounds


21The correspondence analyses applied to the multi-gutter test data show that the impact of the larvicides is greater than the effect of the test period. Besides, although this type of analysis is solely descriptive, it gives a good idea of the toxicity of the larvicides on the main components of the biotic environment. The typologies recorded for the fauna remaining in the gutters contrast the selective insecticides, which are associated with the Ephemeroptera (susceptible organisms), with the relatively toxic insecticides associated with the ubiquitous organisms.

22These typologies do not depend on the insecticide family because some of the pyrethroids present a greater toxicity to the Chironomini (cyphenothrin), and others to the Tanytarsini (deltamethrin, ethofenprox). On the other hand, a certain classification of the toxicities of the products according to insecticide families is observed. Thus, generally speaking, the organophosphorus compounds are less toxic than the pyrethroids which are effective against the onchocerciasis vector.

Table I: Susceptibility of different taxa to various insecticides tested in multi-gutters.

Table I: Susceptibility of different taxa to various insecticides tested in multi-gutters.

+ Detachment less than 30% (low susceptibility)
+ + Detachment between 30 and 60% (moderate susceptibility)
+ + + Detachment between 60 and 80% (high susceptibility)
+ + + + Detachment more than 80% (very high susceptibility)

23Finally, the typologies revealed in this way and analysed, taking into account those established after the long-term use of pesticides in lotic environments (Elouard et al. 1990), should make it possible to develop a model for the prediction of the long-term impact of the larvicides.


24This work, financed entirely by WHO/OCP. was carried out in the field with the technical collaboration of the staff of the OCP Environmental Monitoring Unit (particularly Mr B. Coulibaly and Mr M. Bihoum) and consultants, including Mr B. Wahle, to whom we are grateful. We are also indebted to the Programme Director (Dr E.M. Samba), the Chief of the Vector Control Unit of the Programme (Dr D. Quillévéré), and the members of the Ecological Group of the Programme who encouraged the publication of this paper.



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(Received in Japan May 1992)

Table des illustrations

Légende Fig. 1. Diagram of two types of gutter (simple A and multiple B) used in situ for the study of the impact of insecticides on benthic fauna.
Fichier image/jpeg, 75k
Légende Fig 2. Typology of fauna in place in gutters during tests conducted under different conditions.AbreviationsBae: BactidaeCae: CaenidaeTri: TricorythidaeChi: ChironominiTat: Tanytarsini:Tap: TanypodinaeOcl: OrthocladiinaePsy: HydropsychidaeLegend● Spate at Koperagui▲ Flood-subsidence at Koperagui■ Beginning of rise in water level at Amou-Oblo
Fichier image/jpeg, 44k
Légende AbreviationsBae: BactidaeCae: CaenidaeTri: TricorythidaePsy: HydropsychidaeChi: ChironominiTat: TanytarsiniTap: TanypodinaeOcl: OrthocladiinaeLegend▲ Abate♦ Chlorphoxim■ Permethrin◊ Carbosulfan○ ControlFig. 3. Comparison between the typology of fauna that drifted from the gutters treated and from the control during tests conducted at the same site using the same protocol and
Fichier image/jpeg, 49k
Légende AbreviationsAz: AzamethifosBio: BioresmethrinCa: CarbosulfanCt: PermethrinCy: CyphenothrinDe: DeltamethrinEtf: EtbofenproxO2: OMS 3002O4: OMS 3040O6: OMS 3036Py: PyraclofosPh: ChlorphoximTa: TalstarTe: TemephosFig. 4. Typology of fauna remaining in multi-gutters after the action of different insecticides tested by OCP using the same protocol.Abreviations LegendBae: BaetidaeCae: CaenidaeTri: TricorythidaeChi: ChironominiTat: TanytarsiniTap: TanypodinaeOcl: OrthocladiinaePsy: HydropsychidaeLegend▲ Pyrethroids■ Carbamates* Organophosphorus compounds
Fichier image/jpeg, 60k
Titre Table I: Susceptibility of different taxa to various insecticides tested in multi-gutters.
Légende + Detachment less than 30% (low susceptibility)+ + Detachment between 30 and 60% (moderate susceptibility)+ + + Detachment between 60 and 80% (high susceptibility)+ + + + Detachment more than 80% (very high susceptibility)
Fichier image/jpeg, 157k


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