WORLD HEALTH ORGANIZATION WHOjVBC/71.264 ORGANISATION MONDIALE DE LA SANTE ENGLISH ONLY • INDEXED SUSCEPTIBILITY OF R. PROLIXUS TO CHLORINATED HYDROCARBON INSECTICIDES IN VENEZUELA
by
Freddy E. Gonzalez-Valdivieso, Bartolome Sanchez Diaz and Felice Nocerino1
1. INTRODUCTION
During 1969 the Vector Control Section carried out an evaluation of the susceptibility of triatomid bugs to insecticides as part of the campaign against Chagas' disease. This campaign has been in operation for a few years. Dieldrin is being used at a dosage of 1 gmjm2 • Susceptibility tests were made on R. prolixus captured in the rural environment by the Service for Biological Evaluation of Insecticides, attached to the Vector Control Section.
2. MATERIALS AND METHODS
The insects which were caught in the field reached the central laboratory in Maracay one to 20 days after capture. These were collected from the States of Cojedes, Portuguesa and Trujillo and were classified according to the locality where they were caught, stage of development and nutritional condition. Those which showed signs of motor instability, mutilation or had not reached the fifth stage of development were eliminated, only those which were apparently normal being employed for the tests.
All these insects were captured in houses which had undergone several spraying cycles and which had been either recently built or repaired.
The technique used was that indicated by WHO in Technical Report No. 265 (1963), as follows:
(a). a strip of impregnated paper measuring 3 x 15 em was introduced into a test tube 20 em long;
(b) 10 specimens (nymphs or adults) were transferred to each tube;
(c) the minimum exposure time was 48 hours;
(d) the tubes were closed with gauze and placed vertically in a metal test tube rack;
(e) this was placed in a box to protect the tubes from direct light, with a relative humidity not greater than 50%;
(f) a minimum of 40 insects were used for each concentration selected;
(g) a control tube was prepared for each test made.
1 Vector Control Section, Division of Rural Endemic Diseases, Department of Malariology and Environmental Sanitation, Ministry of Health and Social Welfare, Maracay, Aragua, Venezuela
The issue of this document does not constitute Ce document ne constitue pas une publication. formal publication. It should not be reviewed, II .ne doit faire !'objet d'aucun compte rendu ou abstracted or quoted without the agreement of resume ni d'aucune citation sans l'autorisation de the World Health Organization. Authors alone !'Organisation Mondiale de Ia Sante. Les opinions are responsible for views expressed in signed exprimees dans les articles signes n'engagent articles. que leurs auteurs. WHOjVBC/71.264 page 2
The dieldrin-impregnated papers employed were supplied by WHO.
3. RESULTS AND DISCUSSION
So as to have a standard of comparison the base level of susceptibility of ~rolixus strains (fifth stage nymphs) was established. These strains were bred in the Zone 1 (Aragua) and Zone 6 (Lara) laboratories. The susceptibility level was the same in both cases (Fig. 1).
The field collected material gave the following results:
(a) Cojedes State: Some of the batches of triatomid bugs sent from this State were not employed, since in many cases most of the insects were in a poor condition as they had been captured up to 20 days beforehand.
In regard to the localities of Chorreron, Canafistola and Guasimal, in the municipality of Cojedes, the adults, numbering 56, all died when exposed to a concentration of 1.6% dieldrin for 48 hours. On the other hand, the mortality among nymphs (F ) from the same localities was only 33% at this concentration. 0
The progeny (Fl) of the surviving nymphs which were bred in the laboratory showed a mortality of 75% when exposed to the same concentration (Fig. 2). On comparing these results with the base susceptibility levels it can be concluded that the material collected in the field shows physiological tolerance to dieldrin.
Nymphs from the settlements of La Peonia and La Danta were exposed to concentrations of 0.8%, 1.6% and 4% (Fig. 3). The susceptibility level observed was less than the base level obtained with laboratory colonies. Consequently the survivors were maintained in the laboratory and bred to obtain fifth stage nymphs of the following generation which were subjected to the same test as the parents. A regression to the base level of susceptibility,was noted.
In the settlements of Cachicamo, Chupulun, Los Bajios, El Blanquito, Aguirre, Buenos Aires and El Perdido (Municipality of Cojedes), R. prolixus nymphs were collected and tested at concentrations of 0.8%, 1.6% and 4% dieldrin (Fig. 4). The results revealed a similar response to that of the laboratory strains.
The specimens collected in the Municipality of Juan A. Bravo were found to be less susceptible (Fig.S).
(b) Portuguesa State: Study of a sample coming from the municipality of Cordova showed tolerance of R. prolixus towards dieldrin, as can be seen from Fig. 6.
(c) Yaracuy State: 70 specimens suitable for testing were obtained from the settlements of La Yuca and Guarataro, although the sample included fourth and fifth stage nymphs some of which had fed and others had not.
Fig 7 shows the results obtained on exposing the field samples and the progeny of the survivors to the doses indicated for a period of 48 hours, as well as the observations made on the same progeny over a total period of 72 hours.
The information obtained indicates that the population tested is res.istant to dieldrin.
(d) Trujillo State: R. prolixus nymphs from the settlements of Los Pajones (Motatan), Cuba (Pampan) and La Vera (Pampan) were tested and showed tolerance in all three cases as indicated in Fig. 8. WHOjVBC/71.264 page 3 ·
In the settlement of Santo Domingo, specimens of ~. prolixus were collected which, after an exposure period of 48 hours, proved to be very little susceptible to dieldrin, as shown in Fig. 9.
In view of the low mortality among field specimens of triatomids, the survivors were used for breeding and the progeny exposed to the highest concentration for periods of 48, 72 and 120 hours. The mortality after the latter period of exposure was only 47%.
These results show that we are here confronted with populations showing physiological resistance to dieldrin.
The evidence obtained from the tests made on·R. prolixus coming from various parts of the country (Tables 1 and 2) where tolerance to dieldrin is seen in some cases and resistance in others, seems to show that spraying with a dose of 1 gjm2 cannot hope to interrupt Chagas' disease transmission in such areas, when it is remembered that a concentration of 4% in impregnated paper represents a dose of 1.5 gm of active material per square metre. Moreover, since human and mechanical errors in spraying may result so that in many cases the actual dose applied will be less than 1 gjm2, the situation is made still worse. All this calls for a review of the insecticides suitable for the control of R. prolixus.
For that purpose, fifth stage nymphs of the second generation of specimens collected at La Yuca and El Guarataro, Yaracuy State, whose parents were the survivors of 72 hours' exposure to dieldrin, were exposed in the laboratory to filter papers impregnated with lindane in concentrations of 0.4%, 0.8%, 1.0% and 1.6%. After 24 hours, 100% mortality was noted at all concentrations. These favourable results showed that lindane was a possible substitute for dieldrin and consequently its effect was studied on the triatomids from the Santo Domingo settlement, Trujillo State.
Fifth stage nymphs, from the first laboratory generation whose parents were the survivors of field specimens originally exposed to dieldrin, were on this occasion exposed to lindane for 48 hours (Fig. 10). Although the mortality was much more satisfactory than with dieldrin, it was not sufficient to consider lindane to be an effective insecticide for the control of the said strain. It will be necessary to make a more prolonged study of the presence of tolerance or resistance towards lindane.
In any case it is felt that organo-phosphorus insecticides are the most suitable for spraying in the area.
4. CONCLUSION
The tests carried out show that the application of dieldrin for the control of R. prolixus over several years has led to the selection of strains which are tolerant or resistant to that insecticide and possibly to all the remaining chlorinated insecticide group. WHO jvBcjn. 264 page 4
TABLE 1. PERCENTAGE MORTALITY OF FIELD-COLLECTED FIFTH STAGE NYMPHS OF R. prolixus EXPOSED TO DIELDRIN TEST PAPERS
Dieldrin concentration, % Source(s) of test sample 0.8 1.6 4.0
La Peonia and La Danta, Cojedes 20 40 60 Cachicamo, etc., Cojedes 30 80 100 Juan Bravo, Cojedes - 66 I 80 Chorreron, etc., Cojedes - 33 - Cordova, Portuguesa 25 56 90
La Yuca and Guarataro, Yaracuy I 0 50 50 Caserio Cuba, Trujillo 2 2 75 La Vera, Trujillo l - 20 80 Los Pajones, Trujillo - 30 92 Santo Domingo, Trujillo 0 I 2 26 I Laboratory Colonies (2) 40 ! 86 100 l I l
TABLE 2. PERCENTAGE MORTALITY OF F1 FIFTH STAGE NYMPHS OBTAINED AS OFFSPRING FROM THE SURVIVORS OF THE TESTS IN TABLE 1 ,
Dieldrin ccncentration, % Original source 0.8 1.6 4.0
La Peonia and La Danta, Cojedes 40 76 97 Chorreron, Cojedes - 75 - La Yuca and Guarataro, Yaracuy - 0 28 Santo Domingo, Trujillo - - 9 Laboratory Colonies (2) 40 86 100 wHojvacjn. 264 page 5 ·
Fig. 1
Susceptibility to dieldrin of two R. prolixus
strains bred in the laboratory (fifth stage nymphs)
1969
100 100 - Aragua State
Lara State 90 - 86
80 -
70 -
,..... 60 - *-....., :>< ....E-< ....:! 50 - ~ 1:<: ~ 40 40 -
30 -
20 -
0 0.8 1.6 4
CONCENTRATION (%) WHOjVBCj71. 264 page 6
Fig. 2
Susceptibility to dieldrin (1.6%)
of field specimens of R. prolixus
Cojedes State, 1969
100 100 J
Adults: field specimens
90 F0 : field specimen nymphs
F1: nymphs bred from the F0 survivors
80 - 75
70
60 J ""ll\?. '-' :>< E-t 50 H :;i! E-t ~ ~ 40
33
30
20
10
0 Adults
POPULATION STUDIED WHO/VBC/71.264 page 7
Fig. 3
Susceptibility to dieldrin of field specimen R. prolixus nymphs and their descendants
Cojedes State, 1969
I 100 .., 97
L__ I Parents (F0 ) 90 - d:::LJ Offspring (Fl)
80 -I 76
70 ,_ I
60 - ""'(>!1. '-' >< E-c ...... 50 .
t:t:~ ~ 40 40 -
30 -
20 -
10 -
0 -~------~--~~~------~~- 0.8 1.6 4
CONCENTRATION (%) WHOjVBC/71. 264 page 8
Fig. 4
Susceptibility to dieldrin of field
specimen R. prolixus nymphs
Cojedes State, 1969
100 100 --
90
80 80
70
40
30
20
10
0 0.8 1.6 4
CONCENTRATION (%) WHojvscjn. 264 page 9
Fig. 5
Susceptibility to dieldrin of field
specimen R. prolixus nymphs
Cojedes State, 1969
100 -
90 -
80 80 ·-
70 - 66
60 -
,...., ci< 50 - '-' >< E-< ..... 40 - 0::~ §§ 30 -
20 -
10 -
0 1.6 4
CONCENTRATION (%) WHOjVBC/71. 264 page 10
Fig. 6
Susceptibility to dieldrin of field
specimen R. prolixus nymphs
Portuguesa State, 1969
100 -
90 90 -
80 -
70 -
60 - ...... 56 *'-" >< E-< H 50 - ...:!
p:::~ ~ 40 -
30 - 25
20 -
10 -
0 0.8 1.6 4
CONCENTRATION (%) WHOjVBC/71.264 page 11 ·
Fig. 7
Susceptibility to dieldrin of field specimen
R. prolixus nymphs and their descendants
Yaracuy State, 1969
100
90
80 80 Descendants of survivors (72 • I hours exposure) 70 I 60 .I
50 I 50 50 ~-7'-~~--·~. Field specimens 40 I I 30 • 27.5
/ ,~Descendants of ' // survivors (48 20 ,.." hours exposure) ;·/ ,...," . ,,.,.. 10 I ,,.. ,,... .,.,, 0 0.8 1.6 2.4 3.2 4.0
CONCENTRATION (%) WHOjVBC/71.264 page 12
Fig. 8
Susceptibility to dieldrin of field
specimen of R. prolixus nymphs
Trujillo State, 1969
100
92 90
Settlement of Los Pajones
80 " "
70 "
,.... 60 ~ '-' :>< E-< H 50
0::~ ~ 40
o.8 1.6 3.2 4.0
CONCENTRATION (%) WHOjVBC/71.264 page 13
Fig. 9
Susceptibility to dieldrin of field specimen
R. prolixus nymphs and their descendants
Trujillo State, 1969
100
90
80 Field specimens (Fo)
0 Offspring (F1 ) 70
60 ...... Ill?. '-' >< 50 .....E-< ..:I 47 ~ ES 0:: (120 hours exposure) ~ 40
30 (72 hours exposure)
20
10 9~ (48 hours exposure) 0
0.8 1.6 2.4 3.2 4.0
CONCENTRATION (%) WHOjVBC/71. 264 page 14
Fig. 10
Susceptibility to lindane of R. prolixus
nymphs bred from field specimens
Trujillo State, 1969
100 J
90
80
70
60 60 58------50 42./. 40 -
30 30
20
10
0
1.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
CONCENTRATION (%)