Agriculture Reference
In-Depth Information
chlorpyrifos, quinalphos, tetraethyl pyrophosphate
(TEPP) and methyl parathion, are broad-spectrum
agents that kill insects by inhibiting their nerve impulses
Organophosphates exhibit global accessibility through
surface runoff and leaching due to their polarity and
water-solublity (Hurtig, 1972). Carbamates, such as car-
baryl, carbofuron and methyl carbamate, can be either
broad- or narrow-spectrum agents; they also kill by
deactivating the pest's nervous system and have low
persistence but they are highly toxic to honey bees and
therefore interfere with pollination. Although carbaryl
was introduced in 1959, an extensive search for carba-
mate compounds was made afterwards. Goto (1970)
reported that carbamates made up 16% of the total
consumption of insecticides.
The growing environmental and ecological hazards
caused by the continual use of these organochlorines,
organophosphates and carbamate pesticides propelled
the introduction of a new generation of pesticides dis-
playing much lower persistence levels. To embark on
this route, synthetic pyrethroids were introduced to
the market during the 1970s and soon emerged as a
shield. In the early 1960s, Elliott and his colleagues at
Rothamstead Experimental Station in the UK, initiated
an extensive study of the mechanisms of action and the
relationship between the structures and activities of the
natural pyrethroids and various synthetic analogues.
This work was published in major reviews on pyre-
throids by Elliott
et al.
(1973, 1978) and Soderlund
et al.
(2002). Casida and Quigstad (1998) summarized the
goals of pyrethroid research as mentioned below:
•Pyrethroids should be light-stable and non-
biodegradable.
•There should be target site specificity (e.g. bio-
resmethrin) or metabolic degradation (lower toxicity
for
trans
- than for
cis
-cyclopropane- carboxylate).
• Every part of the molecule should be modified in
relation to the retention of the activity.
• High insecticidal potency should be maintained with
minimum toxicity to fish (e.g. the non-ester silaflufen).
• Compunds should be improved to act as effective
fumigants and soil insecticides (e.g. tefluthrin).
• Potency should be optimized to allow corresponding
reduction in environmental contamination and hazards.
Synthetic pyrethroids are powerful, broad-spectrum
agents, and now form a diverse class of more than 1000
insecticides used to control insect pests in agriculture
and stored products (Roy & Ghosh, 1942). Pyrethroids
are semi-synthetic derivatives of the chrysanthemumic
acids derived from
Chrysanthemum
spp., developed as
insecticides that are more effective than natural pyre-
thrins and less toxic to mammals and birds in comparison
to organochlorines and organophosphates. Deltamethrin,
or (+)-
alpha
-cyano-3-phenoxybenzyl alcohol, is one of
the most widely acceptable pyrethroids. Because of its
high activity against a broad spectrum of insect pests,
deltamethrin is widely used in agriculture and forestry
(Villarini
et al.,
1998). However, its continous applica-
tion on experimental plants results in accumulation to
10 times its initial concentration in a single season,
which is higher than its degradation rate (Mueller-
Beilschmidt, 1990). Moreover, the excessive and continual
use of synthetic pyrethroids has resulted in numerous
problems such as resistance in aphids and bollworms.
Leguminous plants are widely susceptible to numerous
seed- and soil-borne fungi and hence their treatment
with different pesticides at the seed and seedling stages
is a prerequisite. Like other abiotic stresses, insecticides
may cause oxidative stress in plant cells, affecting the
various metabolic activities and growth components in
the plants (Toscano
et al.,
1982; Jones
et al.,
1986).
Toxicity in pyrethroids depends much on the three-
dimensional configuration of the molecule, i.e. its
stereochemistry. Pyrethroids like permethrin have pairs
of isomers, referred to as
cis
and
trans
isomers; each
isomer has its own toxicity but usually the
cis
isomer is
relatively more toxic than the
trans
isomer (Figure 7.1).
Some pyrethroids have as many as eight different
isomers (Mueller-Beilschmidt, 1990). Given a mixture
of two isomers, toxicity depends on the ratio of the
amounts of the two isomers in the formulation. Most
commercial formulations have a fixed isomeric ratio.
Cl
cis
-permethrin
Cl
O
O
O
trans
-permethrin
O
Cl
O
Cl
O
Figure 7.1
Structural formulae of
cis
- and
trans
-permethrin.
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