D. Nano-spheres
Unlike nano-capsules where there exists a central cavity, in polymeric nano-spheres, the pesticide is dispersed throughout the nano-sphere along with the
polymer as a solid sphere.
E. Nano-gel
The active pesticide is dispersed in a gel, which consists
of nanoscale building blocks. This type of formulation
gives the active ingredient protection from premature
evaporation.
F. Nano-fibres
Active pesticide is loaded onto rod like nano-fibres.
This method has a benefit that nano-fibres have a higher
loading efficiency, meaning that the pesticides can be
more densely packed (Balaure et al. 2017).
4.2.2 Non-carrier-Based Nano-pesticides
A. Solid nanoparticle pesticides
Solid nanoparticles themselves are applied on crops
either in solution or in dry form to act as pesticides.
They act on the pests directly and achieve control via
disruption of normal physiological functions.
B. Metals and metal oxide-based nano-pesticides
Metals and metal oxide nanoparticles, when used as
nano-pesticides, can harm the pest in several ways;
photocatalytic damage and release of superoxide radicals, membrane lysis leading to leaking of cellular
contents and uptake of metal ions, lead to disruption of
normal cytological processes and eventual death of the
pest. Silver, copper, copper oxide and titanium oxide
have been investigated for this purpose.
C. Non-metal-based nano-pesticides
Some nanoparticles are naturally toxic to pests such as
silica and alumina, which damage the wax protective
coating on cuticle of insects (Hayles et al. 2017).
4.3 Mechanism of Action of Pesticides
Pesticides may be classified based on several criteria: class
of pests they act on, bio-degradability, solubility, chemical
nature, etc. Here, we will consider and classify them
according to their mode of action on pests. Broadly, on the
basis of mode of action, pesticides may be classified as ionic
pumps, neurotransmitter, neural disruptors, hormonal disruptors, juvenile hormone mimics and others.
A. Ionic pump
Ionic pumps such as sodium and potassium are necessary to maintain the ionic homeostasis of the neurons,
which enables them to polarize and depolarize and
thereby transmit impulses. Altering the natural state of
these pumps changes their permeability to the ions and
thus causes the neurons and by extension, the nerve
fibres to continuously fire. This produces convulsions,
controlled twitching, loss of coordination and eventual
death in the pest. Organochloride chemical class of
pesticides is included in this mode of action. The main
problem associated with this class is that they are
indiscriminate and hence act on mammals as well. Also,
these are stable, hence making them persist in the
environment (Das 2013).
B. Neurotransmitter
This class of pesticides act on the junction between two
neurons, where the signal is bridged by a neurotransmitter. They include chemical classes such as
organophosphates and carbamates, which act on the
enzyme, cholinesterase (ChE), which removes the
neurotransmitter Acetylcholine (ACh) from the neural
or neuromuscular junction (Das 2013). As a result,
accumulation of acetylcholine causes uncontrolled
contractions, twitching and eventual death of the pest.
C. Muscle disruptors
This includes the chemical classes of pesticides, such as
diamedes, which bind to and open calcium channels in
the muscle and cause uncontrolled spasms followed by
death, much like neurotransmitter disruptors.
D. Hormonal disruptors
Among pests, especially of insects, life cycles are
heavily regulated by hormones. Therefore, disruption in
this hormonal cycle can inhibit pest action. One benefit
of exploiting the hormonal system of insects is that
unlike the neural system, they are more unique to the
pests or at least insects and therefore have less unintentional physiological effects on mammals and by
extension humans.
E. Juvenile Hormone Mimics
For the immature larvae to metamorphosize into an
adult, the concentration of juvenile hormone needs to be
decreased. The decrease of this juvenile hormone,
prompting transition into adulthood is governed by
several physiological, nutritional and environmental
conditions. Juvenile hormone mimics, when used as a
pesticide, suppress metamorphosis and hence disrupt
the life cycle of the pest (Das 2013).
F. Others
• Chitin inhibitors
Chitin is a long-chain polysaccharide, which is the
main constituent in the exo-skeleton of insects.
Chitin synthesis inhibitors act by inhibiting an
enzyme called chitin synthases, thereby inducing
chitin deficiency in the pest. Insects exposed to this
160
N. Sarkar et al.
Unlike nano-capsules where there exists a central cavity, in polymeric nano-spheres, the pesticide is dispersed throughout the nano-sphere along with the
polymer as a solid sphere.
E. Nano-gel
The active pesticide is dispersed in a gel, which consists
of nanoscale building blocks. This type of formulation
gives the active ingredient protection from premature
evaporation.
F. Nano-fibres
Active pesticide is loaded onto rod like nano-fibres.
This method has a benefit that nano-fibres have a higher
loading efficiency, meaning that the pesticides can be
more densely packed (Balaure et al. 2017).
4.2.2 Non-carrier-Based Nano-pesticides
A. Solid nanoparticle pesticides
Solid nanoparticles themselves are applied on crops
either in solution or in dry form to act as pesticides.
They act on the pests directly and achieve control via
disruption of normal physiological functions.
B. Metals and metal oxide-based nano-pesticides
Metals and metal oxide nanoparticles, when used as
nano-pesticides, can harm the pest in several ways;
photocatalytic damage and release of superoxide radicals, membrane lysis leading to leaking of cellular
contents and uptake of metal ions, lead to disruption of
normal cytological processes and eventual death of the
pest. Silver, copper, copper oxide and titanium oxide
have been investigated for this purpose.
C. Non-metal-based nano-pesticides
Some nanoparticles are naturally toxic to pests such as
silica and alumina, which damage the wax protective
coating on cuticle of insects (Hayles et al. 2017).
4.3 Mechanism of Action of Pesticides
Pesticides may be classified based on several criteria: class
of pests they act on, bio-degradability, solubility, chemical
nature, etc. Here, we will consider and classify them
according to their mode of action on pests. Broadly, on the
basis of mode of action, pesticides may be classified as ionic
pumps, neurotransmitter, neural disruptors, hormonal disruptors, juvenile hormone mimics and others.
A. Ionic pump
Ionic pumps such as sodium and potassium are necessary to maintain the ionic homeostasis of the neurons,
which enables them to polarize and depolarize and
thereby transmit impulses. Altering the natural state of
these pumps changes their permeability to the ions and
thus causes the neurons and by extension, the nerve
fibres to continuously fire. This produces convulsions,
controlled twitching, loss of coordination and eventual
death in the pest. Organochloride chemical class of
pesticides is included in this mode of action. The main
problem associated with this class is that they are
indiscriminate and hence act on mammals as well. Also,
these are stable, hence making them persist in the
environment (Das 2013).
B. Neurotransmitter
This class of pesticides act on the junction between two
neurons, where the signal is bridged by a neurotransmitter. They include chemical classes such as
organophosphates and carbamates, which act on the
enzyme, cholinesterase (ChE), which removes the
neurotransmitter Acetylcholine (ACh) from the neural
or neuromuscular junction (Das 2013). As a result,
accumulation of acetylcholine causes uncontrolled
contractions, twitching and eventual death of the pest.
C. Muscle disruptors
This includes the chemical classes of pesticides, such as
diamedes, which bind to and open calcium channels in
the muscle and cause uncontrolled spasms followed by
death, much like neurotransmitter disruptors.
D. Hormonal disruptors
Among pests, especially of insects, life cycles are
heavily regulated by hormones. Therefore, disruption in
this hormonal cycle can inhibit pest action. One benefit
of exploiting the hormonal system of insects is that
unlike the neural system, they are more unique to the
pests or at least insects and therefore have less unintentional physiological effects on mammals and by
extension humans.
E. Juvenile Hormone Mimics
For the immature larvae to metamorphosize into an
adult, the concentration of juvenile hormone needs to be
decreased. The decrease of this juvenile hormone,
prompting transition into adulthood is governed by
several physiological, nutritional and environmental
conditions. Juvenile hormone mimics, when used as a
pesticide, suppress metamorphosis and hence disrupt
the life cycle of the pest (Das 2013).
F. Others
• Chitin inhibitors
Chitin is a long-chain polysaccharide, which is the
main constituent in the exo-skeleton of insects.
Chitin synthesis inhibitors act by inhibiting an
enzyme called chitin synthases, thereby inducing
chitin deficiency in the pest. Insects exposed to this
160
N. Sarkar et al.