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WORLD HEALTH ORGANIZATION'

ORGANISATION MONDIALE DE LA SANT6

WHO/VBC/82.8 50 '

ENGLISH ONLY

PRELIMINARY NOTE ON THE APPEARANCE Iti IVORY COAST OF RESISTANCE TO CHLORPllOXIH I N SIMULIUM SOUBRENSE/SANCTIPArlLI LARVAE ALREADY RESISTANT TO TEMEPItOS (ABATE (R))

by

Dan Kurtak,I Moussa Ouedraogo,l Michael Ocran , l Bar ro T616,l and P i e r r e Gu i l l e t*

In t roduct ion

Res is tance t o temephos (OMS 786) i n l a rvae o f Simulium soubrense /sanc t ipaul i (two members o f t h e S. damnosum complex i n West Af r i ca ) was confirmed a t Chutes Gauth ier on the Lower Bandama River i n Ivo ry Coast i n May 1980 (Gu i l l e t e t a l . , 1980). has remained l imi t ed t o S . soubrense j sanc t ipau l i , but has spreod t o o l l of t h e r i v e r b a s i n s i n Ivory Coast normally co lonized by t h a t s p e c i e s (Anon., 1981). Res i s t an t S. soubrense ia rvae now comprise 1OOX of t h e S . damnosum s.1. popula t ion found i n some savanna r i v e r s t r e a t e d wi th temephos. of t he S. damnosum s.1. popula t ion .

The phenomenon

Before t r ea tmen t s , S. soubrense was p resen t on ly a s a smal l component

Chlorphoxim (OMS 1197), t h e only replacement i n s e c t i c i d e then o p e r a t i o n a l , was in t roduced p rov i s iona l ly while t h e t e s t i n g o f o t h e r l a r v i c i d e s cont inued . I n 1980, chlorphoxim was f i r s t used a t Chutes Cauth ier f o r 11 weekly t r ea tmen t s , beginning i n the week of 15 June. was then a pause o f seven weeks due t o a l a c k o f t h e chemical. and continued up t o t h e end of t h e year (10 more weeks). were cons idered s a t i s f a c t o r y (1-2 flies/man/day), and t r ea tmen t s were suspended. not resumed u n t i l t h e week of 10 May 1981. averaga o f f l i e s caught/man/day rose from 24 i n January t o 860 in May. from May u n t i l October 1981, bu t t h e number of f l i e s f e l l very s lowly , reaching two/man/day only i n t h e f i r s t week of September. The ca t ch began t o r i s e aga in i n the second week of September, and reached 2000/man/day i n October at 'which t i m e t rea tments were stopped and a programme of s u s c e p t i b i l i t y t e s t i n g began. Th i s paper p re sen t s r e s u l t s o f those tes ts .

There Treatments were then resumed

The r e s u l t s of t h i s l a s t s e r i e s They were

During t h e suspension pe r iod , t h e monthly Treatments continued

Me tlrods

S u s c e p t i b i l i t y t e s t s were c a r r i e d ou t ) wi th a l coho l s o l u t i o n s of t e c h n i c a l chlorphoxim Th i s method was o f f i c i a l l y (WHO sample) accord ing t o t h e method of Mouchet e t a L ( 1 9 7 7 ) .

adopted by the WdO Expert Committee on Vector Biology and Cont ro l i n 1981 (WHO, 1981) and h a s been used t o ga the r almost a l l of t h e s u s c e p t i b i l i t y d a t a i n the Onchocerc ias i s ControL Programme Area. "mature" l a r v a e (7th i n s t a r ) i n some tests. usc of "young" (4 th and 5 th i n s t a r ) l a rvae . because t h e s u s c e p t i b i l i t y of t h e s e l a rvae was found t o be less than t h a t of young l a rvae (Mouchet e t a l . , 1977). log dose-probi t m o r t a l i t y lines with narrow confidence i n t e r v a l s could no t be prepared .

An important mod i f i ca t ion of t h e method r epo t t ed he re is t he use of "old" o r The o f f i c i a l (Mouchet) teSK method r e q u i r e s t h e The use of mature l a r v a e was no t recommended

Also, t h e m o r t a l i t i e s with mature l a r v a e were very heterogenous and

World Heal th Organiza t ion , Onchocerc ias i s Cont ro l Programme, B.P. 549 Ouagadougou,

I n s t i t u t de Recherches s u r l a Trypanosomiase e t l 'Onchocercose, 1 , B.P. 1500 Bouak6,

Upper Vol ta .

Ivory Coas t . . . The Issue of this document does npt constitute Ca document ne Constitue pas une publication. formal publication. I t should not be reviewed, II ne doit faire I'objet d'aucun compte rendu ou abstracted or quoted withoul the agreement of resume ni d'aucune citation sans i'aulorisation de the World Health Organization. Authors alone I'Organisation Mondiale de la Sante Les opinions are responsible for views expressed in signed I ' exprimees dans les articles signes n'engayenr articles. que leurs auteurs.

OM fonds Documentaire

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Ilowever, t h e r e are s e v e r a l important advantages 50 Ehe use of mature l a r v a e . Under o p c r a t i o n a l c o n d i t i o n s i t is o f t e n d i f f i c u l t t o f i n d l a r g e numbers of young l a r v a e . I n cases of t rea tment f a i l u r e i t is o f t e n the mature l a r v a e which s u r v i v e as s u b j e c t s f o r s u s c e p t i b i l i t y t e s t s . Long t rea tment suspensions t o a l low young l a r v a e t o develop undis turbed a r e not always a c c e p t a b l e f o r ep idemiologica l reasons . l a r v a e and a r e easier t o handle . Most impor tan t , mat$, l a r v a e can be i d e n t i f i e d cy to- taxonomical ly . T h i s permi ts i d e n t i f i c a t i o n OE l 'arvae s u r v i v i n g d i a g n o s t i c d o s e s , which c m n o t be done with young la rvae .

Mature l a r v a e are more robus t than young ' 1

Because of t h e s e advantages, t h e q u e s t i o n of us ing mature l a r v a e was r e s t u d i e d . t o determine i f r e s i s t a n c e could be d e t e c t e d by comparing " r e s i s t a n t " and "suscept ib le" mature la rvae . I

t

As i t 'was be l ieved t h a t some of t h e h e t e r o g e n e i t y of earlier results a rose from t h e i n c l u s t o n of 6 t h - i n s t a r l a r v a e , t h e morphological cr i ter ia for s e l e c t i o n were c a r e f u l l y reviewed. wel l developed and e a s i l y v i s i b l e in t h e h i s t o b l a s t s . These c r i t e r i a select most ly 7 t h - i n s t a r l a r v a e (Grenier & Feraud, 1960; Kurtak, 1980).

Mature l a r v a e were then def ined a s those having t h e f i l a m e n t s of t h e pupal g i l l s The h i s t o b l a s t s may be p a l e o r dark .

P a r a l l e l t e s t s were done with young and mature l a r v a e from t h e saine breeding site t o e s t a b l i s h a d i a g n o s t i c dose f o r mature l a r v a e . The morphological c r i t e r i a were checked by examining groups of dead and s u r v i v i n g mature l a r v a e . among t h e dead l a r v a e , t h e r e were c o n s i s t e n t l y more smaller l a r v a e with less-developed h i s t o b l a s t s than i n t h e s u r v i v i n g group, t h e c r i t e r i a were r e v i s e d t o exclude t h e s e younger l a r v a e . Once t h e c r i t e r i a were f i x e d a s descr ibed i n t h e preceding paragraph, the log dose- p r o b i t m o r t a l i t y l í n e s obta ined p a r a l l e l e d q u i t e c l o s e l y those f o r young l a r v a e and had about t h e same h e t e r o g e n e i t y . The mature l a r v a e were found t o be about 8-10 times less s u s c e p t i b l e than young l a r v a e , bu t t h i s d i f f e r e n c e is cons tan t f o r a given i n s e c t i c i d e .

When i t was i n i t i a l l y found t h a t

The es tab l i shment of d i a g n o s t i c c o n c e n t r a t i o n s for mature l a r v a e h a s g r e a t l y f a c i l i t a t e d surveys f o r r e s i s t a n c e .

It is important t o exclude 6 t h - i n s t a r l a r v a e from test samples , s i n c e it seems t h a t i t is d u r i n g t h i s i n s t a r t h a t s u s c e p t i b i l i t y changes r a p i d l y .

Samples of mature l a r v a e were preserved i n Carnoy's f i x a t i v e a t t h e time of each t e s t , f o r conf i rmat ion of c y t o s p e c i e s by chromosome s t a i n i n g . of mature l a r v a e was taken from t h e same breeding s i t e a t t h e same time. I n tests with mature l a r v a e , s u r v i v i n g test la rvae themselves were preserved.

I n tests with young l a r v a e , a sample

When "d iagnos t ic concent ra t ion" is r e f e r r e d e o , i t is equal t o 0.0125 mg/l of chlorphoxim for young l a r v a e and 0.125 mg/l f o r mature l a r v a e .

P r o b i t a n a l y s i s of t h e d a t a was c a r r i e d o u t by t h e method of Maximum Likel ihood (Finney, 1971) us ing a Texas Ins t ruments TZ 59 C a l c u l a t o r and a programme developed by Dr B. Grab of t h e Heal th S t a t i s t i c a l Methods Uni t , WHO, Geneva.

R e s u l t s

The r e s u l t s of s u s c e p t i b i l i t y tests a t Chutes Gauthier and o t h e r si tes i n t h e Bandama Basin a r e presented i n Table 1 and Figs 1 and 2 . Data from s i tes o u t s i d e t h e Bandama Basin a r e g iven in Table 2 . Comparative d a t a f o r normal mature l a r v a e from o t h e r s i tes a r e pres-nted i n Table 3 and Fig. 3 (no such d a t a e x i s t f o r Chutes C a u t h i e r ) . Due to t h e l a r g e number of l i n e s i n F i g s 1 and 2 , it is not p o s s i b l e t o d i s p l a y i n d i v i d u a l d a t a p o i n t s . However, s t a t i s t i c a l a n a l y s i s of each line i s given i n Table 4.

A map, Fig. 4, g i v e s l o c a t i o n s of t h e tes t si tes.

WIO/VBC/82 .850 Page 3

In l a t e 1981, t h e LC50 f o r chlorphoxim f o r young l a r v a e a t Chutes Gauth ier had become e i g h t t imes h igher than i n January 1977 and 12 t imes h ighe r than i n February 1981 (Table 1 - 'Pests I , 2 and 8 compared wi th 15 and 16). The upper l i m i t f o r t h e LCloo i nc reased from 0.0125 mg/l (d i agnos t i c concen t r a t ion ) t o 0.05 mg/l. " T a l i K y l i n e d i d n o t change s i g n i f i c a n t l y .

The s lope of t he log dose-probi t

For mature l a r v a e , t h e LC50 a t Chutes Gauth ier i n October 1981 i s about e i g h t times t h e f igu ra f o r temephos- res i s tan t l a rvae (Asserekro Tes t No. 1 2 , Table 1). and a t least 10 t imes che f i g u r e s f o r normal popula t ions (Table 3) . w r s u s 0.03125 mg/l o r 0.0625 mg/l for norinal popula t ions . anse-probi t m o r t a l i t y l i n e s a r e (Nos. 17 and 24) s i g n i f i c a n t l y less f o r t h e r e s i s t a n t popula t ions . The abso lu te va lues f o r mature l a r v a e a r e about 10 times those f o r young l a rvae (compare l i n e No. 15 , Fig. 1 v i t h l i n ? No. 14, Fig . 2 ) .

The upper limit o f t h e LCloo i s 0.5 mp/l The s lopes of two of the log

A l l mature l a rvae s u r v i v i n g d i a g n o s t i c concen t r a t ions were i d e n t i f i e d a s S. soubrense/ s n w t i p a u l i , even when o t h e r c y t o s p e c i r s were present i n , t h e gene ra l popula t ion .

Sy January 1982, t h e r e s i s t a n c e t o chlorphoxim covered most of Ivory Coast.

I

A l l S. soubrense popula t ions i n t h e t r e a t e d a r e a a r e now r e s i s t a n t , and the a r e a is t h e same as f d che previous temephos r e s i s t a n c e , --- S . socibranse r e s i s t a n t t o temephos and nu t t o chlorphoxim.

Only; t h e Black Volta River in Ghana con ta ins

Ilowever, f a r t h e r away from t h e lower Bapdama River t h e r e s i s t a n c e is less " in tense" a s i n d i c a t e d by L C 5 0 , LCg5 and LÆ100 va lues and percentage o f s u r v i v a l a t t h e d i a g n o s t i c dose. FOF example, a t Crechaii (Nn. 21, Table 2 ) on ly 4.1% of young l a rvae surv ived a t the d i a g n o s t i c dose ve r sus 70.5% a t Chutes Gauth ier (No. 16, Table 1). An i n t e r e s t i n g except ion t o t h i s t r end was sceii on t h e Uafing River (Ncs. 16 and 17 , Table 2 ) , where loo'% of t h e mature l a rvae surv ived versus 73% a t Chutes Gauth ier (Tes t , 14, ?able 1 ) .

Discuss ion . a ' 11'

The ex i scenc r of well-developed r e s i s t a i c e t o chlorphoxim i n S . soribrense/Ranctipauli , ' s u spec ted because o f t rea tment f a i l u r e , is confirmed by t h e d a t a presented h e r e . These show

The s lopes of the l o g dose-probi t l a rga i n c r e a s e s i n LCs0, LCg5 and t h e l i m i t of t h e LClo0. m o r t a l i t y l i n e s wi th young l a rvae a r e no t , however, reduced a s they were wich cemrphos r e s i s t a n c e ( G u i l l e t et a l . , 1981). Th i s may i n d i c a t e g r e a t e r g e n e t i c homogeneity.

The d a t a f o r n a t u r e l a r v a e show t h e same changes a s chose f o r young l a r v a e , alchough t h c abso lu te va lues a r e h ighe r . d i a g n o s t i c t e s t i n g , but t h a t t he d a t a must be compared with b a s e l i n e d a t a o b t a i a r d us ing mature l a rvae . o p e r a t i o n a l f l e x i b i l i t y will be gained when follow-up tests a r e needed.

Th i s demonstrates t h a t mature l a rvae can be used f o r

I€ tests wi th both young and mature l a rvae a r e inc luded i n base l ine su rveys ,

Opera t iona~ l ly s i g n i f i c a n t r e s i s t a n c e t o chlorphoxim developed i n temephos- res i s tan t l a rvae i n a per iod cif f i v e months, accord ing t o test r e s u l t s . cons idered t o be the f i r s t chlorphoxim t r ea tmen t s in 1980, r e s i s t a n c e took 16 months t o deve lop . F i r s t l y , t h e popula t ion t r e a t e d was a l r e a d y , r e s i s t a n t t o temephos. S ince temephos and chlorphoxim a r e both organophosphorus compounds, i t i s l i k e l y t h a t t h e same enzyme system would d e t o x i f y boch compounds and r e s i s t a n c e t o one would predispose f o r development of r e s i s t a n c e t o t h e o t h e r . As t h e r e was no evidence of c ros s - r e s i s t ance1 e a r l y i n t h e pe r iod of chlorphoxim u s e , add i t iona l s e l e c t i o n was necessary t o develop t$e double r e s i s t a n c e 2 now exh ib i t ed by t h e s e l a rvae . h

Even i f the s t a r t i n g po in t i s

Severa l f a c t o r s may have been involved i n t h i s rap id development of r e s i s t a n c e .

.' Cross - r e s i s t ance is he re de f ined as r e s i s t a n c e t o * a n i n s e c t i c i d e which h a s never been

* Double- res i s tance i s he re de f ined as r e s i s t a n c e t o a replacement i n s e c t i c i d e appear ing

appl ied i n J s t r a i t i a l r eady r e s i s t a n t t o one or seve ra l i n s e c t i c i d e s .

sometime a f t e r i t s in t roduc t ion .

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WHO/VBC/82 .850 page 4

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Secondly, the dosage rate used for chlorphoxim in the field (0.025 mg/l for 10 minutes) was fixed as low as possible since chlorphoxim is'more toxic to non-target organisms than tcmephos (Dejoux 8 Troubat, 1976). This may havb resulted in less than 100% effective treatments which encouraged rapid selection for'iresistance.

Once selection began, the relative isolation of the lower Bandama population (Guillet et al., 1980) would favour fixation of the genes for resistance. It is not possible, Iiuwever, to prove that isolation was essential to development of resistance in this case. Nost neighbouring populations of S. soubrcnse/sanctipauli were also resistant to temephos and being treated with chlorphoxim. to the same selection pressure. Also, since most evidence indicates that flies mate before dispersal, flies arriving from non-treated areas would not "dilute" a resistant population in a successfully treated river. susceptible offspring were killed by the insecticide. treatments (Reiter, 1981). If, however, treatments were notv1007. effective due to poor distribution of the insecticide, then resistant and non-resistant larva! would have equal chances of survival and dilution would be a factor. r i +

Thus flies arriving from other sites would have bean exposed

Their susceptible genes would be eliminated when their

In regard to .the question of the geographic orilgin of the resistance, the data generally indicate a spread by dispersing f l i e s from the lower1 Bandama River. on the Bafing may indicate a secondary source.

However, the results

The data presented here clearly demonstrate resistance to chlorplioxim by S. soubrense/ sanctipauli larvae through most of Ivory Coast. Studies are under way to determine the cross-resistance spectrum of resistant larvae and the effects of synergists of the susceptibility of resistant larvae to chlorphoxim.

ACKNOWLEDGEMENIS

In the Onchocerciasis Control Programe, a report such as this represents the fruits of the efforts of a very large team of staff members at all levels.

First, the support of the Programme Director, Dr E. M. Samba, is gratefully acknowledged.

The authors would like to thank the Chief of Entomological Surveillance, Mr G. Zerbo and the Entomologist for the Western Zone, Dr H . Agoua, for closely watchlng the situation on the lower Bandama River and calling to our attention that the larval population was resisting treatment.

\ b

Mr C. Fiasorgbor and Dr S . E. O. Meredith performed many cytotanomic identifications.

Mr P. Poudiougou and Mr C. AitchedJi, technical officers, organized aerial prospections for the collection of larvae.

tcchnician/entomologists, participated in prospections and/or testing. Mr J. Deh-Deh, Mr J. Nion, Mr J. B. Cbato, Mr H. S. K. Avissey and Mr J. Agyekum,

Finally, one should not forget the faithful service of laboratory assistants and drivers.

RE FERENCES

Anon. (1981) Resistance to temephos in the Simulium-damnosum complex. Current situation (August 1981). Foreseeable entomological and epidemiological consequences. Effects on the vector control strategy o5rthe Programme, mimeographed document OCP/EAC 2 -4 (Report to Expert Advisory Committee)

Dejo,,%, c, & ~roubat, J. J. (1976) Toxicite comparee de deux insecticides organophosphorgs sur la faune aquatique aon-cible en milieu tropical, Rapport NO. 1, Laboratoire d'HYdrObiologie de Bouake, Côte d'Ivoir'e. 0.R.S.T.O.M.

F

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WHQ/VBC/82 .E50 Page 5

Finney, D. J. (1971) Probit analysis, Cambridge University Press, 333 pp.

Grenier, P. C Feraud, L. (1960) Etude biometrique et morphologique de la croissance larvaire chez Simulium damnosum - Theoball, Bull. Soc. Path. exot., 53(3), 563-581

Guillet, P., Escaffre, M., OuBdraogo, M. & Guill&vBrB, D. (1980) Mise en evidence d'une resistance au temephos dans le complexe Simulium damnosum ( S . sanctipauli et S. soubrense) en Côte d'Ivoire. . (Zone du programme de lutte contre l'onchocercose dans la region du bassin de la Volta), Cah. O . R . S . T . O . M . . &rie. Ent. med. et Parasitol., 18(3), 291-299

Kurtak, D. (1980) Notes on the selection of larvae of Simulium damnosum s.1. for insecticide susceptibility tests with particular reference to the cytospecies S . saubrense and S. squamosum, mimeographed document Vector Control Unit

Mouchet, J., QuBlennec, C.. Berl, D., Sechani Y. C ) MBthodologie pour tester la sensibilite aux insecticides des larves de s.1. Cah. O . R . S . T . O . M . , ser. Ent. M6d. et Parasitol., 15(1), 55-66

WHO (1981) Instructions for determining the susceptibility or resistance of black fly larvae to insecticide, WHO unpublished document WHO/VBC/81 .E11

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lo. of I.TY.S

170

671

418

615

194

609

I I 8

652

69

102 I

196

71

14

376

Y

99

51

14

I l l

118

2 19

120 -

- 0.03

0.014

0.014

0.061

0.019

0.11

0.19

0.01

0.21

0.15

0.26

0.13

0.26

Upper I i m l t Ulm 4 1 )

> o 2 5

0.0625

0.5

0.5

>0.0125

0 . 5

>0.0125

0.5

Z0.0125

0.125

0.125

0.125

W.125

0.5

>0.0125

>0.0125

.o.us

> 0.012 5

>0.125

> O . L I S

ao.0125

s0.125

b.1

2.1

3.5

L .O

J .3

-

2.4

10.8

O

24 .4

Y.1

28.8

20.2

4.3

5.3

5.8

O

O

O

17

34.1

44.5

15.6

. 100

3.7

1 .o

1.5

4.1

2 .o

TABLE 3. SUSCEPTIBILITY TESTS WITH CHLORPHOXIM ON NON-RESISTANT MATURE LARVAE OLTSIDE OF M E BANDA!! BASIN

2

5

8

15

16

12

14

1 7

23

24

TABLE 4. STATISTICAL ANALYSIS OF LOG DOSE - PROBIT MORTALITY LINES

Chutes Gauthier 21/1/77

Danangoro 15/9/80

Chutes Gauthier 4/2/81

Chutes Gauthier 23/10/81

Chutes Gauthier 24/10/81

Asserekro 7/10/81

Chutes Gauthier 21/10/81

Chutes Gauthier 27/10/81

Mermis 16/12!81

Latokaha 6/1/62

- Fig.

1

1

1

1

1

1

2

2

2

2

2

Slope

3.15

4 .O1

4.71

6.37

2.63

6.46

4 .O33

4.63

1.73

3.56

1.76

standard :rror o f

slope

0.421

0.348

0.395

tuo

0.417

0.81

0.248

0.427

0.579

0.281

0.568

Chi2 ( n = 1

1.13 (2)

0.066 ( 1 )

3.12 (1)

data

5.32 (2)

1.39 (1)

11.267 (3)

6.85 (1)

0 .52 (2)

0.357 (1)

9.51 (2)

P E

0.568

0.797

0.08

points

0.07

O .24

0.01

o .o09 0.77

0.55

(0.01

Lc50 :95Z confidence interval)

0.0015 (0.0012-0.0018)

0.0016 (0.0015-0.0018)

0.0023 (0.0021-0.0025)

0.0011

0.014 (O .Oll-0.019)

0.019 (0.018-0.021)

0.014 (0.010-0.016)

0.16a

0.15 (0.12-0.19)

O .O47 (0.042 -0.053)

0.0812

E95 (95% confidence In te rva l )

O .O051 (0.0~40‘-O .0075)

0.0042 (0.0036-0.0050)

0.0051 (0.0044-0.0061)

0.0019

O .O60 (0.038 -0.13)

0.035 (0.031-0.043)

0.035 (0.027-0.060)

0.36E

1.3 (0.75-3.4)

0.136 (0.108-0.189)

0.7&

a Heterogeneity does not permit calculat ion.

9% . *,

i .

FIG.l DOSE - MORTALlTY CURVES FOR SUSCEPTIBILITY TESTS WITH CHLORPHOXIM IN THE BANDAMA BASIN 1977 TO 1981 (YOUNG LARVAE OF SIMULIUM DAMNOSUM S. L . )

I

I I l I I I I l I I I l 1 I I l I I I I I 0,0001 mg Il 0,00lmg/ 1 0,Ol mgll Oll mgl I O

I I l I I I 9 Ln

.CONCENTRATION .

FIG. 2 DOSE - MORTALITY CURVES FOR SUSCEPTIBILITY TESTS WITH CHLORPHOXIM IN THE

99

38

95

90

- =

- -

00

>- 7 0 -

2 5 0 -

i- ï 6 0 -

U o 40- 30-

2 20

10

5 -

BANDAMA BASIN (MATURE LARVAE OF SIMULIUM DAMNOSUM S. L.)

-

-

-

2 -

1 -

@ Asserckro 7/10/81 (. resistant to tmcphos p 1 @Chutes Gauthicr 21 /10/8F

@Chutes Gauthiu 27/10/81

@ Mermis 27/ 1 2 / 81

@ Latokaho 6/1 / 82

Numbers refer to table 1

- - .

/@

I I l I I I I I I I I I 1 I I I I I I I l I I I l I I

0,001 m/ I O, O lmg I 1 O,tmg/L 1,Omgll CONCENTRATION MG/ L

I

,’ I

99

98

95

90

80

>- 70

a 5 0 -

k 60- .J

c O

0

40-

30-

2 20-

10

IS

AND NORMAL, MATURE LARVAE OF SIMULIUM DAMNOSUM S.L . A 100~. El IOO*/.

O stope = 5.38 @ Yabo 23/09/81 A Map code 25

- 'b = 0.479 - @ Akakro 17/10/81 Map code 10 chi2 = 1.25(1)

P = .26 Lc50 = 0.0076 (0.0070-0.008 3)

- - LC95 = 0.015 (0.014-0.018)

- - O slope = 4.77 . I

2 N \D

sb =-0.210 chi2 = 32.17 ( 3 ) I

P = co.01 Lc50 = O O14 (0.010-0-027) LX 95 = 0.030 ( 0.022 - 0.23 1

- 5-

2 -

1 - P B " 2 I- .c

m'

m o

r m

I f I I I I I I I I I I I I l I I I I I 1 I l I I N

03 VI O

0,001 0,Ol 0 8 1 1'0

f

I