Agriculture Reference
In-Depth Information
treatment. The upper leaves were dead, and lower parts
of the trees became twisted, chlorotic (yellowing) and
brown leading to the death of the trees within 60 days
after application.
fatty acid content of this plant (Rosa & Cheng, 1973). It
was suggested that the higher lipid content in oil chan-
nels of oilseeds facilitates the uptake and distribution of
lipophilic chemicals like DDT. Keith (1970) also sup-
ported this theory and suggested that oils and fatty acids
in the cytoplasm probably help in solubilization and dis-
solution of liphophilic chemicals, like DDT, within the
cell. When these lipophilic chemicals bind to the lipid
fraction of the cell membrane, membrane permeability
and root transport are impaired. This, in turn, may
affect the uptake and translocation of essential plant
nutrients from soil to plants and ultimately result in
inhibition of plant growth.
Pesticide monitoring programmes in California bet-
ween 1982 and 1985 found permethrin (a pyrethroid)
residue on cabbage, lettuce and tomatoes (Mott &
Snyder, 1988). US Food and Drug Administration (FDA)
monitoring programmes between 1985 and 1990 con-
sistently found residues of various pesticides like
permethrin, fenvalerate and cypermethrin in around 45
different vegetables, fruits and meat products. On
average up to 6 ppm permethrin was found on leafy
vegetables such as spinach, lettuce, kale, collards and
turnip greens (FDA, 1990b). The maximum permethrin
residue level set by the FDA on leafy vegetables is
20  ppm. To date no legal limits (tolerance levels) for
pyrethroid residues on many of these products have
been established.
According to FDA reports the fluvalinate residues in
honey are 22 times higher than the legal limits (FDA,
1990c). It has also been reported that the pyrethroid
residues (e.g. from cyfluthrin, deltamethrin, cyperme-
thrin, fenvalerate and permethrin) remain effective for
more than 10 months in grains even after milling and
baking with minimal losses (Dicke et al., 1988).
7.2.2 effects of pesticide residues
in food and water
Continual and excessive use of pesticides leaves pesti-
cide residues or their remains as a mixture of substances
in food, soil, water and air. Pesticide residues include the
conversion products, metabolites, reaction products and
impurities. Pesticides exhibit biomagnification, where
the residues of pesticides, especially insecticides, tend to
increase up to lethal levels from lower to higher trophic
levels in the food chain. Pesticides also undergo bioac-
cumulation as a characteristic feature; for example,
unicellular organisms like phytoplankton accumulate
pesticide in their cells at concentrations many times
higher than that in the surrounding water while
growing in pesticide-contaminated rivers. Zooplankton
in the rivers feed on these phytoplankton and thus the
residues of pesticides pass through the food chain and
ultimately reach the top of the chain in higher concen-
trations. Fishes in polluted water accumulate DDT at
concentrations more than 10,000 times that of their sur-
roundings (Kenaga, 1971). Crop plants like potatoes
and carrot grown on soil treated with organochlorine
insecticides like aldrin at 0.5 kg/acre are reported to
accumulate higher levels of pesticide residues than
other crops grown in similar conditions.
Bioaccumulation is exhibited by both animals and
plants. Chlorinated hydrocarbons have the greatest ten-
dency for biological concentration. The indiscriminate
use of pesticides can be considered a major source of
pollutants with effects on the biochemistry and phys-
iology of the plants (Kenaga, 1972). The residues of
organochlorine and organophosphorus insecticides were
analysed by Ahmad et al. (1999). About 50% of the 149
samples of different vegetables investigated were found
to be contaminated by various pesticide residues, and
16% of samples contained residues above the maximum
residual limit (MRL).
Plants containing high fatty acid contents rapidly take
up various organochlorine and organophosphate pesti-
cides. It has been reported by Nash et  al. (1974) that
pesticide residues in the seeds are directly related to
their oil contents. Increased uptake of pesticides like
DDT by tobacco plants was correlated with the high
7.3 pyrethroids: mode of action
DDT and pyrethroids are by far the most important
insecticides.On the basis of mode of action of insecti-
cides they are classified into two types. Type 1 includes
DDT and all pyrethroids lacking a cyano-group, whereas
type 2 compounds include pyrethroids that contain
an  α-cyano-3-phenoxybenzyl alcohol. There is ample
evidence to suggest that both DDT and the pyrethroids
have a common mode of action in acting on voltage-
gated sodium channels. Pyrethroids prolong the period
Search WWH ::




Custom Search