RESEARCH
Insecticide resistance in Anopheles sacharovi Favre in southern Turkey
Résistance d Anopheles sacharovi Favre aux insecticides (sud de la Turquie)
Resistencia de Anopheles sacharovi Favre a los insecticidas en el sur de Turquía
H. KasapI; M. KasapI; D. AlptekinI; Ü. LüleyapI; P.R.J. HerathII
IÇukurova University, Faculty of Medicine, Department of Medical Biology, 01330, Balcali, Adana, Turkey
IIFormerly: Malaria Unit, Division of Control of Tropical Diseases, World Health Organization, Geneva, Switzerland
ABSTRACT
We report the resistance to 12 insecticides of specimens of Anopheles sacharovi, both in laboratory cultures and those collected in the malarious areas of Adana, Adiyaman, Antalya, Aydin, and Mugla in southern Turkey. Mortality was higher 24 h after exposure than immediately after exposure but was unaffected by temperature (24 °C or 29 °C) or the position of the test kit (horizontal or vertical). In Adana, Adiyaman and Antalya, A. sacharovi was susceptible only to malathion and pirimiphos-methyl. In Aydin it was susceptible to both these insecticides as well as to dieldrin, l-cyhalothrin, and etofenprox; and in Mugla it was susceptible to dieldrin, fenitrothion, l-cyhalothrin, cyfluthrin and etofenprox, as well as to malathion and pirimiphos-methyl.
Keywords: Turkey; Anopheles; insecticides; drug resistance; comparative study
RÉSUMÉ
Nous étudions ici la résistance à 12 insecticides de spécimens d Anopheles sacharovi provenant délevages de laboratoire ou capturés dans les régions impaludées dAdana, dAdiyaman, dAntalya, dAydin et de Mugla (sud de la Turquie). On observe une mortalité plus élevée au bout de 24 heures quimmédiatement après lexposition, mais elle nest pas modifiée par la température (24 °C ou 29 °C) ni par la position du nécessaire dépreuve (horizontale ou verticale). A Adana, Adiyaman et Antalya, A. sacharovi na été sensible quau malathion et au pirimiphos-méthyl. A Aydin, il a été sensible à ces deux insecticides ainsi quà la dieldrine, à la lambda-cyhalothrine et à létofenprox ; enfin, à Mugla, il a été sensible à la dieldrine, au fénitrothion, à la lambda-cyhalothrine, à la cyfluthrine et à létofenprox, ainsi quau malathion et au pirimiphos-méthyl.
RESUMEN
Informamos de la resistencia desarrollada contra 12 insecticidas por Anopheles sacharovi, tanto en cultivos de laboratorio como entre los ejemplares capturados en las zonas palúdicas de Adana, Adiyaman, Antalya, Aydin y Mugla, en el sur de Turquía. La mortalidad fue mayor al cabo de 24 horas de la exposición que inmediatamente después de la misma, pero no se vio afectada ni por la temperatura (24 °C o 29 °C) ni por la posición del kit de prueba (horizontal o vertical). En Adana, Adiyaman y Antalya, A. Sacharovi sólo fue sensible al malatión y al pirimifós-metilo. En Aydin fue sensible a esos dos insecticidas, así como a la dieldrina, la lambda-cihalotrina, la ciflutrina y el etofenprox; y en Mugla fue sensible a la dieldrina, el fenitrotión, la lambda-cihalotrina, la ciflutrina y el etofenprox, así como al malatión y el pirimifós-metilo.
Introduction
Malaria is endemic in southern Anatolia. The number of cases has increased in recent years, particularly in south-east Anatolia where the GAP (Southeast Anatolia Project) irrigation and development project is about to be completed. Under the project, irrigation began three years ago, and the area covered by it is increasing annually.
The principal vector of the malaria parasite in Turkey is Anopheles sacharovi Favre; A. superpictus Grassi is also a vector (1, 2). The population densities of both species are highest in malarious areas but even here the distribution of A. superpictus is rather localized. This species occurs in small populations with lower densities in midsummer and is still susceptible to many insecticides.
In Turkey, A. sacharovi has been subjected to continuous field selection pressure by many insecticides that have been used for both agricultural and public health purposes since the introduction of DDT (clofenotane) in 1952. Residuals were introduced throughout the country for malaria control in the following sequence: DDT, g-hexanechlorocyclohexane (g-HCH), dieldrin, malathion, pirimiphosmethyl, bendiocarb. DDT and g-HCH were used during the 1950s and 1960s but resistance developed to them in A. sacharovi in 1959 (3). Dieldrin resistance was detected in A. sacharovi in 1970, and malathion (an organophosphorus insecticide) then replaced the organochlorines (46). Resistance to malathion was first recorded in 1974 (5), and cross-resistance developed to a wide range of organophosphorus and carbamate insecticides, even though they had never been used for house spraying (7, 8). The use of malathion was discontinued in 1984 because A. sacharovi developed resistance to malathion and because householders objected to its unpleasant odour and the stains it left on walls (9). Pirimiphos-methyl replaced malathion in 1984 and was used successfully until 1990 when it was replaced by bendiocarb. A. sacharovi was still susceptible to pirimiphos-methyl in 1992 (10) and was still so even in 1996 (11). In recent years, pirimiphos-methyl, bendiocarb and various residual pyrethroids have been used by the malaria control authorities.
Hemingway et al. (12) reported that DDT resistance in A. sacharovi was being scattered in the population in 1984 despite the replacement of DDT by malathion for malaria control 13 years previously. They also reported that populations of this species in Çukurova had an altered acetylcholinesterase resistance mechanism, conferring broad-spectrum resistance against organophosphates and carbamates. Specimens of A. sacharovi collected in the field in 198990 were still resistant to DDT, organophosphates and carbamates, although at lower frequencies than in 1984 (11, 13).
The present study compares baseline data on susceptibility under WHO test conditions with results obtained on field samples, in order to evaluate current trends in insecticide resistance in southern Turkey.
Materials and methods
Study sites
The study was carried out in five malarial areas in southern Turkey, which varied in insecticide usage, geographical features, social infrastructure, agricultural development and tourism characteristics factors that might have had a bearing on the emergence of insecticide resistance.
Adana is situated in the centre of the Çukurova Plain on the Mediterranean coast, where malaria has long been endemic. Samples of A. sacharovi were collected in Karatas-Tuzla, which had a high population density of mosquitos.
Antalya is located 560 km west of Adana. Orchards and irrigated cotton crops are common in this area. Few cases of malaria occur and there is intensive vector control, conducted by private bodies and the National Malaria Service. Most of the specimens of A. sacharovi were obtained in the village of Küçük Belkis, which was not sprayed before collecting was carried out.
is situated 300 km west of Antalya, with which it shares certain characteristics. However, there are more forests in 
. Specimens of A. sacharovi were obtained in Gökova, Yatagan and Milas.
Aydin, which has relatively few malaria cases, contains much irrigated low country used mainly for cotton and rice crops. Samples were taken in Söke and the surrounding villages of Aydin, where the population density of mosquitos was high. The flow of water was not controlled and consequently many patches of stagnant water provided suitable habitats for mosquito breeding, especially in Söke.
The GAP Project area covers six provinces including Adiyaman, which is about 370 km east of Adana. The irrigation area will be ten times greater than that of Çukurova when the scheme has been completed. At present, only the Harran and Bozova plains, approximately equal in area to Çukurova, are irrigated and have been for three years. Specimens were collected in coastal villages on Lake 
Study design and methods
The field collection of A. sacharovi and susceptibility tests were conducted between late May and September 1998 in Adana and in June and July 1998 in the other provinces, when the populations of the mosquito reached high levels.
Insecticide-impregnated papers prepared in accordance with WHO specifications were purchased from the WHO Collaborating Centre in Malaysia (Vector Control Research Unit, School of Biological Sciences, Universiti Sains Malaysia, Minden, Penang). Pirimiphos-methyl papers were freshly prepared by spreading 2 ml of insecticide solution in acetone on 15 cm x 12 cm rectangles of Whatman No.1 filter-paper. Twelve insecticides approved by WHO for vector control were used:
4% DDT, 0.4% dieldrin (organochlorines);
5% malathion, 1% fenitrothion, 1% pirimiphos-methyl (organophosphates);
0.1% bendiocarb, 0.1% propoxur (carbamates);
0.025% deltamethrin (former WHO discriminating dose), 0.25% permethrin (former WHO discriminating dose), 0.1% l-cyhalothrin, 0.05% and 0.1% cyfluthrin, 1% and 2% etofenprox (pyrethroids).
Tests were conducted on female A. sacharovi of various ages taken from a laboratory culture of the Adana strain maintained at the Medical Biology Department, Çukurova University, and on field samples in accordance with standard WHO procedures (14) but with some modifications. Two WHO test kit positions (vertical and horizontal), two exposure times (1 h and 2 h) and two mean temperatures (24 °C and 29 °C) were used. The temperature and relative humidity in the holding tubes were recorded during exposure and 24 h later. Mortality was noted immediately after exposure (as percentage knockdown) and 24 h later as percentage dead. During the 24-h holding period the test kits were kept in a cardboard box covered with wet towels.
SPSS for MS Windows software was used to calculate t-tests.
Results
The mortality rates 24 h after exposure of culture specimens are shown in Table 1. The mortality rates immediately and 24 h after exposure of field specimens are given in Table 2 and Table 3, respectively, for the vertical and horizontal test kit positions.
The tests on both the culture specimens and the field samples showed that the mortality rates 24 h after exposure were higher than those immediately after exposure for most insecticides (P = 0.000). Mortality rates at 29 °C were not significantly higher than those at 24 °C (P = 0.250, Table 1). Mortality after 2 hours of exposure was significantly higher than that after exposure for 1 hour (P = 0.000). Mortality rates for some insecticides were slightly higher for the horizontal test kit position than for the vertical position, but were lower for some others; the differences, however, were not statistically significant (P = 0.858).
On the basis of the initial results the two combinations shown below were investigated:
vertical test kit position and exposure for 1 h;
horizontal test kit position and exposure for 2 h.
If 100% mortality occurred with the first combination the second was not used.
By applying the WHO criteria (98100% mortality indicates susceptibility, 15), it was found that laboratory culture samples were susceptible to malathion, pirimiphos-methyl and etofenprox; possibly resistant to dieldrin, deltamethrin and l-cyhalothrin; and resistant to the other insecticides (Table 1).
Field samples from Adana were susceptible only to malathion and pirimiphos-methyl (Table 2 and Table 3). In Adiyaman, A. sacharovi was susceptible to malathion, pirimiphos-methyl, l-cyhalothrin and etofenprox, possibly resistant to propoxur and cyfluthrin, and resistant to the other insecticides; in Antalya it was susceptible to malathion, pirimiphos-methyl and 2% etofenprox (horizontal test kit position and 2-h exposure), and resistant to the other insecticides; in Aydin the species was susceptible to dieldrin, malathion, pirimiphos-methyl, fenitrothion, l-cyhalothrin and etofenprox, possibly resistant to permethrin and cyfluthrin, and resistant to the other insecticides; in it was resistant to DDT and bendiocarb, possibly resistant to propoxur, deltamethrin and permethrin, and susceptible to the other insecticides.
Discussion
A. sacharovi demonstrated multiple resistance to all four groups of insecticides. The laboratory culture, established almost 20 years previously, still exhibited resistance to many insecticides, presumably because field specimens had been introduced from time to time during the first 10 years.
Higher mortality occurred at the longer exposure time in all groups but neither test kit position nor temperature had a significant effect. The apparent link between test kit position and knockdown efficacy was possibly attributable to the use of susceptible adults. The increase in mortality associated with the horizontal test kit position was more pronounced in the relatively susceptible Aydin and populations of A. sacharovi than in those of Adiyaman, Adana and Antalya, which had genes for resistance to many insecticides. There is a positive correlation between mortality and temperature for organochlorines and organophosphates (7, 16). It has been shown that for organophosphates mortality increases with time of exposure, whereas for carbamates the insecticide concentration is more important (16). In Çukurova the mortality rates for Culex tritaeniorhynchus and Aedes caspius were positively correlated with temperature and time of exposure to the four groups of insecticides (17).
In Çukurova, carbamates and pyrethroid insecticides are used in about equal proportions in agriculture but mostly aerosol formulations of pyrethroids are used for mosquito control by private bodies and municipalities (17). Insecticide usage in this area has, for many years, selected for resistance (3, 5, 7, 8, 1113). The kdr-type gene responsible for pyrethroid resistance was recently detected in specimens of A. sacharovi from Adana (18). A. sacharovi was resistant to malathion in 1984 but is now fully susceptible.
In the area of Adiyaman the wide range of resistance was attributable to many years of insecticide usage. Although mosquito control efforts with insecticides have not received much emphasis in recently, since it is a nature reserve, aerosol pyrethroid formulations are being applied by fogging. Intensive insecticide usage against agricultural pests and mosquitos has selected resistance in A. sacharovi to a number of compounds, as in Adana and Antalya.
In the Aydin and areas, mosquitos are susceptible to most of the insecticides but caution is needed in order to avoid development of cross-resistance.
The WHO insecticide discriminating concentrations used in this study were reviewed in 1998 (15). The new discriminating values are 0.75% (w/w) permethrin, 0.05% deltamethrin, 0.1% l-cyhalothrin (for A. sacharovi only), 0.5% etofenprox and 0.15% cyfluthrin. The use of these new concentrations may have influenced our results for some insecticides.
A total of 32 agricultural pesticides (1 organochlorine, 15 organophosphates 7 carbamates and 9 pyrethroids) were used in Çukurova between 1992 and 1995 (17). They included potential mosquito control insecticides, i.e. chlorpyriphos, fenithrothion, malathion, l-cyhalothrin, permethrin, deltamethrin, cypermethrin and cyfluthrin. In recent years, municipalities and many other official and private bodies have switched between insecticides, notably pyrethroids as cold aerosols and fogs (17). Such intensive use of insecticides for long periods selects resistance in mosquito populations.
The selection of resistance in A. sacharovi in Çukurova as a consequence of agricultural pesticide usage has been discussed in relation to 11 agricultural organophosphates and carbamates (8) and of 1% propoxur, 0.25% permethrin and 0.025% deltamethrin (11), although these insecticides had been used hardly at all for mosquito control. Mouchet (19), reviewing resistance to insecticides that had never been used for public health purposes, indicated that resistance in A. gambiae in Africa, A. albimanus in Central America, A. culicifacies and A. aconitus in Southeast Asia, A. sacharovi in Turkey and Greece, and C. tritaeniorhynchus in the Western Pacific and South-East Asia Regions was associated with agricultural applications. Such associations were also indicated for A. nigerrimus in Sri Lanka (20) and A. albimanus in Guatemela (21) and Mexico (22). Clearly, vectors are among the non-target organisms affected by agricultural practices. Initially, the destruction of vector populations is a beneficial side-effect but agricultural treatments eventually produce severe selective pressure (19). The marked reductions in the numbers of malaria cases and of mosquitos that have occurred in recent years in Adana and Antalya, where the disease is endemic, may reflect the beneficial stage. The influence of the interaction between agricultural and public health insecticide usage on the selection of resistance is still poorly understood, partly, perhaps, because of the lack of information on insecticides in the environment.
Close cooperation is necessary between health, agriculture and ecosystem analysts in order to understand the influence of insecticide usage. The problem of multiple resistance demands specific and periodic monitoring in each locality. The first step is to assess trends in the frequency of the resistance genes by means of susceptibility tests, and to investigate the efficacy of insecticides in the field with bioassays. The most effective insecticides, possibly giving 100% kill, should then be used in rotation. 
Acknowledgements
The study was supported by the World Health Organization and Çukurova University Research Fund (1998/18), Turkey. We are grateful to Dr P. Trigg, World Health Organization; Dr Coskun Tabuk, Head of the National Malaria Eradication Service, Ministry of Health, Ankara; and to the many health workers in the five provinces involved in the study for making this project possible.
References
1. Kasap H et al. Development of Plasmodium vivax in Anopheles superpictus under experimental conditions. American Journal of Tropical Medicine and Hygiene, 1987, 37: 241245.
2. Kasap H. Comparison of experimental infectivity and development of Plasmodium vivax in Anopheles sacharovi and Anopheles superpictus in Turkey. American Journal of Tropical Medicine and Hygiene, 1990, 42: 111117.
3. Zulueta J de. Insecticide resistance in Anopheles sacharovi. Bulletin of the World Health Organization, 1959, 20: 797 822.
4. Curtis TC. Status of mosquito and fly insecticide susceptibility in Turkey. Mosquito News, 1962, 22: 142146.
5. Ramsdale CD. Insecticide resistance in the Anopheles of Turkey. Transactions of the Royal Society of Tropical Medicine and Hygiene, 1975, 69: 226235.
6. Kasap M. [Insecticides and their use in vector control]. Acta Parasitologica Turcica, 1989,13 (2): 209226 (in Turkish).
7. Ramsdale CD, Herath PRJ, Davidson G. Recent developments of insecticide resistance in some Turkish anophelines. Journal of Tropical Medicine and Hygiene,1980, 83: 1119.
8. Davidson G. The agricultural usage of insecticides in Turkey and resurgence of malaria. In: Proceedings of an International Workshop on Resistance to Insecticides used in Public Health and Agriculture, 2226 February, 1982, Sri Lanka. Colombo, National Science Council of Sri Lanka, 1982: 122129.
9. Ünsal U. [Malaria in Turkey and interventions for its control]. Malaria Eradication Program, Ankara, Turkey, 1984 (unpublished report, in Turkish).
10. Kasap H et al. Residual efficacy of pirimiphos methyl (Actellic) on Anopheles sacharovi in Çukurova, Turkey. Journal of the American Mosquito Control Association, 1992, 8 (1): 4752.
11. Lüleyap Ü. [Physiological resistance to insecticides among mosquitoes in the Çukurova district] (PhD thesis). Institute of Health Sciences, Çukurova University, Adana, Turkey, 1996 (in Turkish).
12. Hemingway J et al. The biochemistry of insecticide resistance in Anopheles sacharovi: comparative studies with a range of insecticide susceptible and resistant Anopheles and Culex species. Pesticide Biochemistry and Physiology, 1985, 24: 68 76.
13. Hemingway J et al. Insecticide resistance gene frequencies of Anopheles sacharovi populations of Çukurova Plain, Adana Province, Turkey. Medical and Veterinary Entomology, 1992, 6: 342348.
14. Instructions on determining the susceptibility or resistance of adult mosquitos to organochlorine, organophosphate and carbamate insecticides diagnostic test. Geneva, World Health Organization, 1981 (unpublished document WHO/ VBC/81.806).
15. Test procedures for insecticide resistance monitoring in malaria vectors, bio-efficacy and persistence of insecticides on treated surfaces. Geneva, World Health Organization, 1998 (unpublished document WHO/CPC/MAL/98.12).
16. Brown AWA, Pal R. Insecticide resistance in arthropods. Geneva, World Health Organization, 1971 (WHO Monograph Series, No. 38).
17. Kasap H et al. [Use of insecticides in Çukurova and development of resistance in mosquitoes] Acta Parasitologica Turcica, 1999, 23 (3): 267272 (in Turkish).
18. Lüleyap Ü et al. [Study of gene mutation affecting voltage-gated sodium channel protein in Anopheles sacharovi: knock-down resistance (kdr)]. Acta Parasitologica Turcica, 1999 (in press) (in Turkish).
19. Mouchet J. Mini-review: agriculture and vector resistance. Insect Science and Application, 1988, 9 (3): 297302.
20. Herath PR, Joshi GP. Factors affecting selection for multiple resistance in Anopheles nigerrimus in Sri Lanka. Transactions of the Royal Society of Tropical Medicine and Hygiene, 1986, 80 (4): 649652.
21. Brogdon WG et al. Independent selection of multiple mechanisms for pyrethroid resistance in Guatemalan Anopheles albimanus (Diptera: Culicidae). Journal of Economic Entomology, 1999, 92 (2): 298302.
22. Penilla RP et al. Resistant management strategies in malaria vector mosquito control. Baseline data for a large-scale field trial against Anopheles albimanus in Mexico. Medical and Veterinary Entomology, 1998, 12 (3): 217233.
Correspondence
H. Kasap; M. Kasap
Çukurova University, Faculty of Medicine, Department of Medical Biology
01330, Balcali, Adana, Turkey