Biodegradable Pesticides in Public Health Entomology
199
of malaria varies from one locality to another, depending upon climate, species
of parasites and species of vectors present, and customs of the people (Macdonald, 1957, and others), most malaria transmission takes place inside houses
at night. The main method of attack therefore has been application of residual
insecticides to the inside walls of domiciles.
For various reasons, the attack phase with insecticides lasted longer than
expected in many areas and, as is well known, the greatest deterrent to success
in the program has been development of resistance by many of the major vector
species to a succession of insecticides used as intradomiciliary sprays. DDT
resistance has developed in populations of 15 anopheline vector species and
dieldrin resistance in populations of 37 species (Brown et al., 1976). In India,
four of the nine vector anophelines are resistant to DDT-An. culicifacies, An.
stephensi, An. fluviatilis, and An. philippinensis, with the first two also resistant
to BHC and dieldrin (Brown et al., 1976; Sharma, 1972) and with An. culicifacies
now starting to show resistance to malathion (Rajagopal, 1977). In southern
Turkey, An. sacharovi is not only DDT- and dieldrin-resistant but has gone the
route through malathion and propoxur resistance. In El Salvador and Nicaragua,
the same is true of An. albimanus. An. albimanus is one of a number of examples
of the importance of agricultural insecticides in bringing selection pressure to
bear on important vectors. The malathion and propoxur resistance of An. albimanus in El Salvador has been mainly induced by the use of malathion, methyl
parathion, and carbaryl on cotton fields (Georghiou, 1972). As resistance appears, substitutions become expensive, malathion costing 5 times and propoxur
20 times more than DDT when substituted for it in the late 1960s and early
1970s (Brown et al., 1976). In tests in India against An. culicifacies, Bhatnagar
et al. (1974) found that three rounds of fenitrothion would be required to cover
one malaria transmission season, compared to two rounds for DDT, and the
substitution of fenitrothion for DDT would result in a 1O.66-fold cost increase.
A number of other factors have contributed to the currently complicated
situation: the appearance of parasite resistance to chloroquine and related 4aminoquinolines in many areas of southeast Asia and South America (Peters,
1974); exophily of vectors, in which the vector bites outdoors and thus does not
contact the wall deposits, for example, An. balabacensis in southeast Asia (Scanlon and Sandhinand, 1965); the excito-repellent effect of DDT deposits, causing
some species to exit from houses soon after biting; absorption of insecticide
applied to mud walls, thereby reducing its availability to vectors; DDT resistance
in bedbugs and other household pests, thus making spraymen less welcome
(Rafatjah, 1971); nomadism of the people in some geographic areas; operational
problems caused by reduced finanacial support from international sources; inflation; and shortages of insecticides (Brown et al., 1976). Despite the systematic
screening of more than 1400 compounds by WHO in its search for suitable DDT
substitutes (Wright, 1971), only a few promising compounds have surfaced.
199
of malaria varies from one locality to another, depending upon climate, species
of parasites and species of vectors present, and customs of the people (Macdonald, 1957, and others), most malaria transmission takes place inside houses
at night. The main method of attack therefore has been application of residual
insecticides to the inside walls of domiciles.
For various reasons, the attack phase with insecticides lasted longer than
expected in many areas and, as is well known, the greatest deterrent to success
in the program has been development of resistance by many of the major vector
species to a succession of insecticides used as intradomiciliary sprays. DDT
resistance has developed in populations of 15 anopheline vector species and
dieldrin resistance in populations of 37 species (Brown et al., 1976). In India,
four of the nine vector anophelines are resistant to DDT-An. culicifacies, An.
stephensi, An. fluviatilis, and An. philippinensis, with the first two also resistant
to BHC and dieldrin (Brown et al., 1976; Sharma, 1972) and with An. culicifacies
now starting to show resistance to malathion (Rajagopal, 1977). In southern
Turkey, An. sacharovi is not only DDT- and dieldrin-resistant but has gone the
route through malathion and propoxur resistance. In El Salvador and Nicaragua,
the same is true of An. albimanus. An. albimanus is one of a number of examples
of the importance of agricultural insecticides in bringing selection pressure to
bear on important vectors. The malathion and propoxur resistance of An. albimanus in El Salvador has been mainly induced by the use of malathion, methyl
parathion, and carbaryl on cotton fields (Georghiou, 1972). As resistance appears, substitutions become expensive, malathion costing 5 times and propoxur
20 times more than DDT when substituted for it in the late 1960s and early
1970s (Brown et al., 1976). In tests in India against An. culicifacies, Bhatnagar
et al. (1974) found that three rounds of fenitrothion would be required to cover
one malaria transmission season, compared to two rounds for DDT, and the
substitution of fenitrothion for DDT would result in a 1O.66-fold cost increase.
A number of other factors have contributed to the currently complicated
situation: the appearance of parasite resistance to chloroquine and related 4aminoquinolines in many areas of southeast Asia and South America (Peters,
1974); exophily of vectors, in which the vector bites outdoors and thus does not
contact the wall deposits, for example, An. balabacensis in southeast Asia (Scanlon and Sandhinand, 1965); the excito-repellent effect of DDT deposits, causing
some species to exit from houses soon after biting; absorption of insecticide
applied to mud walls, thereby reducing its availability to vectors; DDT resistance
in bedbugs and other household pests, thus making spraymen less welcome
(Rafatjah, 1971); nomadism of the people in some geographic areas; operational
problems caused by reduced finanacial support from international sources; inflation; and shortages of insecticides (Brown et al., 1976). Despite the systematic
screening of more than 1400 compounds by WHO in its search for suitable DDT
substitutes (Wright, 1971), only a few promising compounds have surfaced.