92
N. Sethunathan st al.
recalcitrance to degradation processes and subsequent biomagnification through
the food chain. But chlorinated hydrocarbon insecticides are of great importance
to the public health and agriculture of most tropical countries in view of their
indigenous availability, low cost, and broad-spectrum activity of long duration.
In 1978, about 5000 tons of various pesticides were produced in India with
imports of less than 2000 tons; hexachlorocyclohexane (HCH), a chlorinated
hydrocarbon, alone accounted for more than 67% of all pesticides produced
(Anonymous, 1979b). Price considerations and nonavailability of less-persistent
substitutes would certainly delay the replacement of chlorinated hydrocarbon
pesticides in the tropics. However, there are instances of banning or restricting
the use of certain persistent insecticides, for example endrin, in tropical agriculture. This is consistent with the strategy followed in developed countries. But
the loss of organic pesticides, through processes involving volatilization, photodecomposition, and more importantly microbial degradation, is expected to be
more rapid under tropical and subtropical conditions than in temperate environments. Moreover, the persistence of pesticides is largely influenced by the conditions and cultural practices used for specific crops. Thus, 'Y-HCH (lindane),
known for its long-term persistence under aerobic and temperate conditions,
undergoes fairly rapid decomposition in tropical soils upon flooding, the most
widely used cultural practice in rice cultivation (Raghu and MacRae, 1966).
These results emphasize the importance of realistically assessing the persistence of pesticides and their impact on the ecosystem under actual tropical
conditions before restricting the use of economically important pesticides based
on literature of no relevance to tropical conditions. Studies on the dynamics of
pesticides in tropical ecosystems are relatively few, recent, and restricted almost
entirely to microbial degradation in rice soils, as reflected in this review of
available literature.
4.2. RESIDUES IN SOIL
Although persistent chlorinated hydrocarbon insecticides are used regularly
and on a large scale in most tropical countries, information on their residues in
the tropical environment is fragmentary and far from conclusive. Moreover,
meaningful interpretation of the available data on pesticide residues in tropical
ecosystems (Bindra and Kalra, 1973; Agnihothrudu and Mithyantha, 1978) has
been difficult because of the unreliable and less-sophisticated techniques employed in most studies. In a few instances, however, residues have been determined with the help of sophisticated techniques after known applications of
pesticides under field conditions.
In a long-term field experiment, 'Y-HCH reached nonsignificant levels of
less than 0.02 ppm in soil and water within 20-30 days of its application, even
at high doses, to submerged rice fields in the Philippines (Anonymous, 1966)
N. Sethunathan st al.
recalcitrance to degradation processes and subsequent biomagnification through
the food chain. But chlorinated hydrocarbon insecticides are of great importance
to the public health and agriculture of most tropical countries in view of their
indigenous availability, low cost, and broad-spectrum activity of long duration.
In 1978, about 5000 tons of various pesticides were produced in India with
imports of less than 2000 tons; hexachlorocyclohexane (HCH), a chlorinated
hydrocarbon, alone accounted for more than 67% of all pesticides produced
(Anonymous, 1979b). Price considerations and nonavailability of less-persistent
substitutes would certainly delay the replacement of chlorinated hydrocarbon
pesticides in the tropics. However, there are instances of banning or restricting
the use of certain persistent insecticides, for example endrin, in tropical agriculture. This is consistent with the strategy followed in developed countries. But
the loss of organic pesticides, through processes involving volatilization, photodecomposition, and more importantly microbial degradation, is expected to be
more rapid under tropical and subtropical conditions than in temperate environments. Moreover, the persistence of pesticides is largely influenced by the conditions and cultural practices used for specific crops. Thus, 'Y-HCH (lindane),
known for its long-term persistence under aerobic and temperate conditions,
undergoes fairly rapid decomposition in tropical soils upon flooding, the most
widely used cultural practice in rice cultivation (Raghu and MacRae, 1966).
These results emphasize the importance of realistically assessing the persistence of pesticides and their impact on the ecosystem under actual tropical
conditions before restricting the use of economically important pesticides based
on literature of no relevance to tropical conditions. Studies on the dynamics of
pesticides in tropical ecosystems are relatively few, recent, and restricted almost
entirely to microbial degradation in rice soils, as reflected in this review of
available literature.
4.2. RESIDUES IN SOIL
Although persistent chlorinated hydrocarbon insecticides are used regularly
and on a large scale in most tropical countries, information on their residues in
the tropical environment is fragmentary and far from conclusive. Moreover,
meaningful interpretation of the available data on pesticide residues in tropical
ecosystems (Bindra and Kalra, 1973; Agnihothrudu and Mithyantha, 1978) has
been difficult because of the unreliable and less-sophisticated techniques employed in most studies. In a few instances, however, residues have been determined with the help of sophisticated techniques after known applications of
pesticides under field conditions.
In a long-term field experiment, 'Y-HCH reached nonsignificant levels of
less than 0.02 ppm in soil and water within 20-30 days of its application, even
at high doses, to submerged rice fields in the Philippines (Anonymous, 1966)