Microbial Degradation of Pesticides in Tropical Soils
101
4.3.2.3. Other Organophosphates
Methyl parathion and fenitrothion, related to parathion with a common
P~ linkage and a nitro substituent, were converted to their respective amino
analogs in a flooded soil (Adhya et al., 1981a). Particularly interesting was the
nitro group reduction of both insecticides even after repeated additions vis-a-vis
the shift from nitro group reduction to hydrolysis reported for parathion. The
relative importance of biological hydrolysis of methyl parathion and fenitrothion
in soil and water systems is yet to be established, although in at least one instance
these two insecticides were hydrolyzed with ease by a Flavobacterium sp. (Adhya
et al., 1981 b), the bacterium that hydrolyzed diazinon and parathion (Sethunathan
and Yoshida, 1973b).
4.3.3. Carbamate Insecticides
The use of carbamate pesticides in tropical agriculture is very recent. For
instance, in recent years the tremendous buildup of brown planthoppers (Nilaparvata lugens) in rice has led to the extensive and intensive use of the Nmethylcarbamates carbofuran and carbaryl, by far the most effective insecticides
for controlling this pest. Despite increasing use of these insecticides, our knowledge of their metabolic fate in tropical environments is rather limited.
Similar to several organophosphorus pesticides, carbofuran (Fullmer, 1977)
and carbaryl (Aly and EI-Dib, 1972) are susceptible to chemical hydrolysis under
alkaline conditions. But in neutral soils and in acid soils capable of attaining
near-neutral pH after incubation under flooding-a condition that hastens their
degradation-degradation of these insecticides is both chemical and biological.
Thus, heat treatment of the flooded soils prior to their incubation with insecticides
increased the persistence of both carbofuran (Venkateswarlu et ai., 1977) and
carbaryl (Venkateswarlu et al., 1980), although appreciable loss of these insecticides occurred also from heat-sterilized soils, possibly by chemical means.
Recent studies indicate that flooded soils harbor microorganisms (bacteria and
actinomycetes) capable of degrading carbofuran and carbaryl (Venkateswarlu,
1979). Of special mention is the metabolism of both carbofuran and carbaryl in
a yeast-extract-supplemented mineral medium by a Pseudomonas sp., isolated
from a flooded soil amended with carbofuran (Venkateswarlu et ai., 1980). Yet
the microbial involvement in carbofuran degradation has not been very convincing because no significant proliferation of carbofuran-degrading microorganisms occurred upon repeated additions of carbofuran to flooded soils (Siddaramappa et ai., 1978; Venkateswarlu and Sethunathan, 1978; Siddaramappa
and Seiber, 1979). In fact, chemical degradation may be more important in the
loss of carbofuran in situations where diurnal fluctuations in the pH of the flood
101
4.3.2.3. Other Organophosphates
Methyl parathion and fenitrothion, related to parathion with a common
P~ linkage and a nitro substituent, were converted to their respective amino
analogs in a flooded soil (Adhya et al., 1981a). Particularly interesting was the
nitro group reduction of both insecticides even after repeated additions vis-a-vis
the shift from nitro group reduction to hydrolysis reported for parathion. The
relative importance of biological hydrolysis of methyl parathion and fenitrothion
in soil and water systems is yet to be established, although in at least one instance
these two insecticides were hydrolyzed with ease by a Flavobacterium sp. (Adhya
et al., 1981 b), the bacterium that hydrolyzed diazinon and parathion (Sethunathan
and Yoshida, 1973b).
4.3.3. Carbamate Insecticides
The use of carbamate pesticides in tropical agriculture is very recent. For
instance, in recent years the tremendous buildup of brown planthoppers (Nilaparvata lugens) in rice has led to the extensive and intensive use of the Nmethylcarbamates carbofuran and carbaryl, by far the most effective insecticides
for controlling this pest. Despite increasing use of these insecticides, our knowledge of their metabolic fate in tropical environments is rather limited.
Similar to several organophosphorus pesticides, carbofuran (Fullmer, 1977)
and carbaryl (Aly and EI-Dib, 1972) are susceptible to chemical hydrolysis under
alkaline conditions. But in neutral soils and in acid soils capable of attaining
near-neutral pH after incubation under flooding-a condition that hastens their
degradation-degradation of these insecticides is both chemical and biological.
Thus, heat treatment of the flooded soils prior to their incubation with insecticides
increased the persistence of both carbofuran (Venkateswarlu et ai., 1977) and
carbaryl (Venkateswarlu et al., 1980), although appreciable loss of these insecticides occurred also from heat-sterilized soils, possibly by chemical means.
Recent studies indicate that flooded soils harbor microorganisms (bacteria and
actinomycetes) capable of degrading carbofuran and carbaryl (Venkateswarlu,
1979). Of special mention is the metabolism of both carbofuran and carbaryl in
a yeast-extract-supplemented mineral medium by a Pseudomonas sp., isolated
from a flooded soil amended with carbofuran (Venkateswarlu et ai., 1980). Yet
the microbial involvement in carbofuran degradation has not been very convincing because no significant proliferation of carbofuran-degrading microorganisms occurred upon repeated additions of carbofuran to flooded soils (Siddaramappa et ai., 1978; Venkateswarlu and Sethunathan, 1978; Siddaramappa
and Seiber, 1979). In fact, chemical degradation may be more important in the
loss of carbofuran in situations where diurnal fluctuations in the pH of the flood