PhD

A series of experiments were carried out for ascertaining the basic
problems related to the current recommendations for managing
bunchytop disease of banana through the application of phorate.
The absorption, translocation and metabolism of phorate applied in the
soil was influenced more by the condition and age of the plant than by the
dose of the insecticide.
Since the application of 2.50 g ai/plant did not result in corresponding
increase in the residue content or the bioefficacy in the early phases of
crop growth, when compared to the 1.25 dose, the latter can be used
without significant loss in efficacy.
A definite dose-effect relationship existed between the phorate content of
plant and the mortality of P.nigronervosa confined at feeding sites.
The median lethal doses of the insecticide content of the plant tissue were
higher during declining phase of absorption as compared to those
obtained during the active absorption phase. The result indicated the
lesser toxicity of some components in the total residue during the later
phase of the crop.
Application of phorate granules in leaf axils was less effective than the
treatment done in the soil and hence the current recommendation to use
less quantity of insecticide when applied in the leaf axils has to be altered.
A simple technique for the separation, identification and quantification of
phorate and its metabolites was developed.
Phorate and phorate sulfoxide contents of the total residue showed
inverse relationship with each other while the other metabolites did not
exhibit a clear relationship among them.
Phorate and phorate sulfoxide exhibited more positive direct influences
on the morality of the vector than the other metabilites.
The absorption and toxicity of the insecticide content in plants did not
vary significantly up to 174 DAP, when applied @ 2.50 g ai/plant at
different intervals after planting. Absorption was very low when the
insecticide was applied at 180 and 210 DAP.
For ensuring residues within tolerance limits (0.10ppm) in raw fruits, the
insecticide treatment has to be limited to 150 DAP and for ripe fruits the
limit can be extended up to 180 DAP.
The absorption and persistence of phorate and metabolites was
significantly higher in plants grown in summer season than in those
grown in rainy season.
The absorption of insecticide was high in sandy soils and it was lowest in
black cotton soils during the active absorption phase.
The insecticide persisted at effective levels for 75 days in sandy, 90 days
in lateritic upland and 105 days in black cotton soil.
Sulfoxidation of the thioether moiety was the dominent metabolic
pathway in sandy soil while desulfuration pathway was predominent in
the other three soils.
Application of phorate at planting, @ 2.50 g ai/plant, did not adversely
affect the soil microflora as observed at the time of harvest of the crop.