ocP/vcu /HYBroles.z

ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA

PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST

ECOLOGICAL GROUP Sixteenth session

Bouake. 25-27 January 1995

ocP/vcu /HYBroles.z

ORIGINAL: French

MONITORING OF THE ENTOMOFAUNA OF THE GUINEA WATE,RCOURSES TREATED WITH ANTIBI-ACKFLY I-ARVICIDES AS PART OF

ONCHOCERCIASIS CONTROL FROM 1984

TO

¹⁹⁹⁴ SUMMARY

F. KONDE AND K. NABE

1.

General

The annual report on the monitoring of the watercourses covers data from 1984

to t994.

This routine monitoring has been supplemented by several special research prograrnmes concerned with:

- the study of the drift of organisms in pools;

- the study of the vertical distribution of drift;

- the longitudinal zorung of drift in a reach;

- the measurement of the width of the cephalic capsules of some tana.

In

order to get a correct idea of the hydrobiological situation in the vicinity of Kankan, the number of monitoring stations

in

1991 was increased by one, making four (Table

I

and Fig. I), viz., Sassambaya on the Niandan, Tere on the Dion, ^Boussoule on the Milo and Kessane on the Makona.

Insecticides used: (see report).

2.

Monitorins results

Generally speaking, the densities of the taxa (in the drift or on the rock slabs) increase in the dry season; for most of the groups, the maximal abundance takes place between February and

April

(Fig. 5

to

72).

With the exception of some high abundance peaks, it has values obtained during the ten years are of the same order of

is true of the Sassambaya station, whatever the methods or

\

^The collections made with the Surber sampler and

he rrk t

ta\

|( e^

tc"

give more comparable results from 1984 to 7994 than those made with 200-p nets. The difference in the results obtained with the 200-p nets is certainly due to the collection of many young individuals. These abundant collections, which often correspond with localized synchronous hatchings over time, disrupt results on the real evolution of the populations.

As regards the ta:ra,

it

has been noted that some of them show similar seasonal

evolution curves from 1984

to

1994, whatever the sampling method

used:

Caenidae, Orthocladiinae and Hydropsychidae (Fig. 5

to

LZa and 0, ^while, for others, the ^results obtained are practically always different: Tricorythidae (Fig. 5 to l2g),Irptophlebiidae, Chironomini and Elmidae (Tables 7

to

^10).

Generally speaking, modifications of the structure of the communities appeared

on

the treated sites

in

Guinea

but

the overall balances,

in

energy terms, have not changed profoundly. The analyses of the routine monitoring dat show that there is a

marked difference between the treated and untreated rivers, or,

for

the same river, between the pre- and post-larviciding periods (Fig. 5a, 8a, 10a and 11a). Ta:ra that are more susceptible than others suffer greatly from the effects

of

the larviciding. They include Tricorythidae and Euthiplociidae.

We have observed that ta:ra which have

a

short developmental cycle quickly recolonize

the

sites compared

with

those which have

a

long developmental cycle

(Tricorythidae).

This phenomenon

is

confirmed

by the

results

for the

period of December 1989-March 1990, particularly as the larviciding campaign ended late and too much permethrin was used compared

to

the number

of

cycles recommended by the Ecological Group: the consequence was the momentary disappearance of Tricorythidae at Sassambaya on the Niandan.

In conclusion, three ^phases can be distinguished in the evolution of the Guinea watercourses treated

with

antiblacldly larvicides. Firstly, after each sprayrng, great mortalities are seen, which leads to a sudden decrease

in

invertebrate densities. This decrease is all the greater since the first spraying will have taken place at the end of the dry season and there had been an over-dosage (Tere in June 1994). This strong-impact phase may last many months and even

a

year depending on the morphology

of

the treated breeding sites and their geographical situation.

Secondly, there is a slow re-establishment of the situation, favoured, most of the time, by the rainy season. Finally, there is the third phase with densities always less than those present before the treatment. This decrease

in

numbers present can vary from

9.44Vo

to

40.41%o according to Sassambaya on the Niandan or Boussoule on the Milo.

It is hardly thinkable to attribute the variations in seasonal trends obtained during the ^past ten years to the hydrologlr of the sampled period which is the low-water period.

It

has been practically similar from 1.984

to

L994 (Fig.2 to

4).

On the other hand, the hydrology

of

the high-water period could lead

to

variations which would affect the ecological conditions of the flood-subsidence period differently.