Evaluation of organophosphorus and synthetic pyrethroid insecticides against six vector mosquitoe species

DORTA, D.M. et al. Evaluation of organophosphorus and synthetic pyrethroid insecticides against six vector mosquitoe species. Rev. Saúde Pública, 27: 391-7, 1993. Three organophosphorus compoundsmalathion, folithion and temephosand two synthetic pyrethroidsalphamethrin and deltamethrinwere used for monitoring the susceptibility status of larvae and adults of six vector mosquitoe species: Culex quinquefasciatus (Filariasis) and Aedes albopictus (Dengue) (both laboratory and field strains); laboratory strains of Aedes aegypti (Dengue), Anopheles slephensi and Anopheles culicifacies (Malaria), and Culex tritaeniorhynchus (Japanese encephalitis) in India. From the LC50 values obtained for these insecticides, it was found that all mosquito species including the field strains of Cx. quinquefasciatus and Ae. albopictus were highly susceptible Except for Cx. quinquefasciatus (field strain) against malathion, 100% mortality was observed at the discriminating dosages recommended by World Health Organization. The residual effect of alphamethrin, deltamethrin, malathion and folithion at 25 mg (ai)/m on different surfaces against six species of vector mosquitoes showed that alphamethrin was the most effective on all four treated surfaces (mud, plywood, cement and thatch). Nevertheless, residual efficacy lasted longer on thatch than on the other surfaces. Therefore, synthetic pyrethroids such as alphamethrin can be effectively employed in integrated vector control operations.


Introduction
The use of chemicals to control insects possibly dates back to classical Greece and Rome.In the twentieth century, after the insecticidal potential of DDT was discovered and insecticidal organophosphorous compounds were developed, a new era of insect and pest control began (Hassall 14 ,  1982).Unfortunately problems relating to the use of insecticides have arisen, some of them serious and causing considerable concern in scientific as well as in lay circles (Beroza 4 , 1970).The development of resistance in mosquitoes to a wide variety of conventional insecticides has posed a serious problem for vector control program (Georghiou et al. 12,13 , 1966; 1973; Das and Rajagopalan 8 , 1980; Ariaratnan and Georghiou 3 ,  1975).To overcome these problems, the search for new compounds with insecticidal effect has been undertaken throughout the world.A new group of compounds known as synthetic pyrethroids, less toxic and comparatively safer, are promising tools for future vector control program (Elliot et al 11 ,  1978; Darwazeh et al. 6 , 1978; Poister et al. 18 , 1981; Kottkump et al. 16 ,1981).
The present study compares the susceptibility status of larvae and adults of six vector mosquito species viz.Culex quinquefasciatus (Filariasis), Anopheles stephensi and Anopheles culicifacies (Malaria), Aedes aegypti and Aeds albopictus (Dengue) and Culex tritaeniorhynchus (Japanese encephalitis) in India (Manson-Bahr and Bell 17 ,  1982) to three organophosphorus compounds and two synthetic pyrethroids, using both laboratory and field strains of mosquito.

Material and Method
Malathion (50% EC), folithion (50% EC), temephos (50% EC), deltamethrin (Technical grade 99.9%) and alphamethrin (10% EC) were used for monitoring the susceptibility status of adults and larvae of Cx. quinquefasciatus, An. stephensi, Ae. aegypti, Ae. albopictus, An. culicifacies and Cx.tritaeniorhynchus, maintained at the insectories, Vector Control Research Centre, Pondicherry.In the cases of Cx. quinquefasciatus and Ae.albopictus field collected strains were also used.Insecticide impregnated papers for monitoring the adult susceptibility (Busvine 5 , 1958) and standard solutions for assessing the larval susceptibility (WHO 23 , 1981) were prepared in different concentrations at the VCRC.The procedures followed for determining the susceptibility status of larva and adult are the same as described by WHO 23,24 ,  1981).From the percentage mortalities resulting from different concentrations, LC 50 and LC 90 values were obtained by probit analysis (Sokal and Rohlf 21 , 1981).Control mortality, if any, was corrected by using Abbott's formula (Abbott 1 ,1925).
To evaluate residual activity, a bioassay was earned out in accordance with WHO methodology (WHO 25 , 1981), modified at VCRC (Rajavel et al 19 , 1987).Malathion (5% WP), alphamethrin (5% WP), deltamethrin (2.5% WP) and folithion (50% EC) were sprayed at the rate of 25 mg (ai)/ m 2 on different surfaces viz.cement, mud, thatch and plywood.The mosquito were exposed to treated surfaces for 1 hour and after 24 hours mortality was recorded at weekly intervals until 50% mortality was observed.All the experiments were carried out at 27±2°C and 70-80% relative humidity.

Results and Discussion
Larvicidal efficacy of deltamethrin, alphamethrin, temephos and folithion against six species of mosquito is shown in tables 1-4.The LC 50 values given in Table 1 showed that deltamethrin was more effective against Cx.quinquefasciatus (lab) than the other mosquito species tested.An. stephensi was found to be the least susceptible.The field strain of Cx. quinquefasciatus was found to be about 260 times less susceptible than the laboratory strain, when tested under similar condictions, using decamethrin against Cx.quinquefasciatus (lab.) and Ae.aegypti, it was found to be 24 and 12 times less susceptible respectively than the same strains tested on this occasion.On the other hand, Cx. quinquefasciatus (field) and An.stephensi where found to be twice as susceptible as had been reported earlier for the same insecticide (Das and Kalyanasundaram 10 ,1984).
Larval LC 50 and LC 90 values of alphamethrin are presented in Table 2 and the results indicate that Ae. aegypti was the most and Cx.quinquefasciatus (field) the least susceptible species.In an earlier study when alphamethrin was tested against Cx.quinquefasciatus, Ae. aegypti and An.stephensi (Amalraj et al. 2 , 1987) it showed a potency 600, 83 and 40 times higher respectively than that here observed.
Larval LC 50 and LC 90 values presented in Table 3 reveal that Ae. albopictus from laboratory was the most susceptible, and An.stephensi the least susceptible species to temephos.The susceptibility status of Cx. quinquefasciatus, An. stephensi, Ae. aegypti and An.culicifacies observed in the presented study was 88, 40, 30 and 550 times higher respectively than that reported earlier for the Pondicherry strains of the same species (Das and Rajagopalan 7 ,1979; Das et al. 9 ,1980).
The explanation might be that the strains used in the present study are from a laboratory colonies which have been maintained for about 14 years.It is therefore likely that even if the natural population were resistant, this phenomenon night have been reversed in the laboratory colony due to the removal of selection pressure.
Susceptibility status was determined for larvae of the same mosquito to folithion (fenitrothion), The values showed that Cx. tritaeniorhynchu and Ae.albopictus (field & lab.) were more susceptible than the other species tested.American strains of Ae. albopictus were also reported to be suscep-tible to malathion.Conversely to this, Ae. albopictus resistant to folithion has been reported in Japan and the USA (Wesson 22 , 1990).In the present study An.stephensi was found to be least susceptible to deltamethrin, temephos and folithion.Another interesting observation relates to the fact that LC 50 values determined in the current study are considerable lower than the discriminating dosages recommended by WHO for the insecticides tested.This strongly suggests the need for determining the discriminating dose for different vector species from different areas, so that some base line data are made available for monitoring the resistance/susceptibility status of the vector mosquitoes in the field.Development of resistance cannot be detected early, if the high discriminating dosages recommended by the WHO are used.
When adults were exposed to the diagnostic doses of malathion, folithion, alphamethrin and deltamethrin recommended by WHO (1981 24 ), a 100% mortality was observed in all species except Cx. quinquefasciatus from field to malathion (52.5%).However, adults of Ae. albopictus were highly susceptible to malathion with LT 50 value of 6.65 min (Chi sq.0.594).Earlier studies had showed that Cx. quinquefasciatus adults collected in the field from Pondicherry were highly suscepti-ble to malathion (Das et al. 9 , 1980).The present study has shown that Cx. quinquefasciatus has a potentiality to develop resistance to this O.P. compound.Malathion resistance in the adults of Ae. albopictus adults has been observed in Sri Lanka and Texas (Robert and Olson 20 ,1990; WHO 26 ,1986).
The residual effect of deltamethrin, alphamethrin, malathion and folithion at 25 mg(ai)/m 2 on different surfaces against six species of vector mosquitoes is presented in Fig. 1-4.In general both pyrethroid compounds, deltamethrin and alphamethrin were more effective than malathion and folithion.
As between the two synthetic pyrethroids tested, alphamethrin show the highest residual activity on all surfaces against all the mosquito species tested, in accordance with the results reported earlier (Amalraj et al 2 , 1987).However, the bioassay conducted on mud, cement, thatch and plywood showed that the residual efficacy of deltamethrin and alphamethrin lasted longer (8-17 wks) on the thatched surface than on the other three surfaces.Earlier reports showed that the residual efficacy of alphamethrin tested on the same surfaces at 100 mg(ai)/m 2 lasted for 20 wks against Cx.quinquefasciatus and An.stephensi, but it was more effective on the cement surface (Almaraj et al. 2 , 1987).
Malathion and folithion showed very low residual activity on all surfaces and against all species of mosquitoes tested.
Even though many alternatives are being sought with a views to overcoming the problem of pesticide use, insecticides continue playing a major role in vector and disease control.It is concluded, from the present study, that, of the insecticides tested, synthetic pyrethroids, particularly, alphamethrin, can be effectively used for controlling mosquito vectors.
In spite of above mentioned and although until recently pyrethroid resistance has not been a serious problem, there have been various report of pyrethrin resistance arising as early as 1956 (Keller 15 , 1956).More recently, WHO 27 (1991) has reported that pyrethroids resistance was detected in 15 mosquito species from 32 countries and in 9 other arthropods of public health or veterinary importance in 27 countries.