Lecture 23: INSECT GROWTH REGULATORS
Insect Growth Regulators (IGRs) are compounds which interfere with the
growth, development and metamorphosis of insects. IGRs include synthetic analogues
of insect hormones such as ecdysoids and juvenoids and non-hormonal compounds
such as precocenes (Anti JH) and chitin synthesis inhibitors.
Natural hormones of insects which play a role in growth and development are
1. Brain hormone: The are also called activation hormone(AH). AH is secreted
by neuro secretory cells (NSC) which are neurons of central nervous system
(CNS). It’s role is to activate the corpora allata to produce juvenile hormone
(JH).
2. Juvenile hormone (JH): Also called neotinin. It is secreted by corpora allata
which are paired glands present behind insect brain. Their role is to keep the
larva in juvenile condition. JH I, JH II, JH III and JH IV have been identified
in different groups of insects. The concentration of JH decreases as the larva
grows and reaches pupal stage. JH I, II and IV are found in larva while JH III
is found in adult insects and are important for development of ovary in adult
females.
3. Ecdysone: Also called Moulting hormone (MH). Ecdysone is a steroid and is
secreted by Prothoracic Glands (PTG) present near prothoracic spiracles.
Moulting in insects is brought about only in the presence of ecdysone.
Ecdysone level decreases and is altogether absent in adult insects.
IGRs used in Pest management
a) Ecdysoids: These compunds are synthetic analogues of natural ecdysone. When
applied in insects, kill them by formation of defective cuticle. The development
processes are accelerated bypassing several normal events resulting in integument
lacking scales or wax layer.
b) Juvenoids (JH mimics) : They are synthetic analogues of Juvenile Hormone
(JH). They are most promising as hormonal insecticides. JH mimics were first
identified by Williams and Slama in the year 1966. They found that the paper
towel kept in a glass jar used for rearing a Pyrrhocoris bug caused the bug to die
before reaching adult stage. They named the factor from the paper as ‘paper
factor’ or ‘juvabione’. They found that the paper was manufactured from the
wood pulp of balsam fir tree (Abies balsamea) which contained the JH mimic.
Juvenoids have anti-metamorphic effect on immature stages of insect. They
retain status quo in insects (larva remains larva) and extra (super numerary)
, moultings take place producing super larva, larval-pupal and pupal-adult
intermediates which cause death of insects. Juvenoids are larvicidal and ovicidal
in action and they disrupt diapause and inhibit embryogenesis in insects.
Methoprene is a JH mimic and is useful in the control of larva of hornfly, stored
tobacco pests, green house homopterans, red ants, leaf mining flies of vegetables
and flowers
c) Anti JH or Precocenes: they act by destroying corpora allata and preventing JH
synthesis. When treated on immature stages of insect, they skip one or two larval
instars and turn into tiny precocious adults. They can neither mate, nor oviposit
and die soon. Eg. EMD, FMev, and PB (Piperonyl Butoxide)
d) Chitin Synthesis inhibitors: Benzoyl phenyl ureas have been found to have the
ability of inhibiting chitin synthesis in vivo by blocking the activity of the enzyme
chitin synthetase. Two important compounds in this category are Diflubenzuron
(Dimilin) and Penfluron. The effects they produce on insects include
Disruption of moulting
Displacement of mandibles and labrum
Adult fails to escape from pupal skin and dies
Ovicidal effect.
Chitin sysnthesis inhibitors have been registered for use in many countries and
used successfully against pests of soybean, cotton, apple, fruits, vegetables, forest
trees and mosquitoes and pests of stored grain
IGRS from Neem : Leaf and seed extracts of neem which contains azadirachtin as
the active ingredient, when applied topically causes growth inhibition, malformation,
mortality and reduced fecundity in insects.
Hormone mimics from other living organisms: Ecdysoids from plants
(Phytoecdysones) have been reported from plants like mulberry, ferns and conifers.
Juvenoids have been reported from yeast, fungi, bacteria, protozoans, higher animals
and plants.
Advantages of Using IGRs
Effective in minute quantities and so are economical
Target specific and so safe to natural enemies
Bio-degradable, non-persistent and non-polluting
Non-toxic to humans, animals and plants
Insect Growth Regulators (IGRs) are compounds which interfere with the
growth, development and metamorphosis of insects. IGRs include synthetic analogues
of insect hormones such as ecdysoids and juvenoids and non-hormonal compounds
such as precocenes (Anti JH) and chitin synthesis inhibitors.
Natural hormones of insects which play a role in growth and development are
1. Brain hormone: The are also called activation hormone(AH). AH is secreted
by neuro secretory cells (NSC) which are neurons of central nervous system
(CNS). It’s role is to activate the corpora allata to produce juvenile hormone
(JH).
2. Juvenile hormone (JH): Also called neotinin. It is secreted by corpora allata
which are paired glands present behind insect brain. Their role is to keep the
larva in juvenile condition. JH I, JH II, JH III and JH IV have been identified
in different groups of insects. The concentration of JH decreases as the larva
grows and reaches pupal stage. JH I, II and IV are found in larva while JH III
is found in adult insects and are important for development of ovary in adult
females.
3. Ecdysone: Also called Moulting hormone (MH). Ecdysone is a steroid and is
secreted by Prothoracic Glands (PTG) present near prothoracic spiracles.
Moulting in insects is brought about only in the presence of ecdysone.
Ecdysone level decreases and is altogether absent in adult insects.
IGRs used in Pest management
a) Ecdysoids: These compunds are synthetic analogues of natural ecdysone. When
applied in insects, kill them by formation of defective cuticle. The development
processes are accelerated bypassing several normal events resulting in integument
lacking scales or wax layer.
b) Juvenoids (JH mimics) : They are synthetic analogues of Juvenile Hormone
(JH). They are most promising as hormonal insecticides. JH mimics were first
identified by Williams and Slama in the year 1966. They found that the paper
towel kept in a glass jar used for rearing a Pyrrhocoris bug caused the bug to die
before reaching adult stage. They named the factor from the paper as ‘paper
factor’ or ‘juvabione’. They found that the paper was manufactured from the
wood pulp of balsam fir tree (Abies balsamea) which contained the JH mimic.
Juvenoids have anti-metamorphic effect on immature stages of insect. They
retain status quo in insects (larva remains larva) and extra (super numerary)
, moultings take place producing super larva, larval-pupal and pupal-adult
intermediates which cause death of insects. Juvenoids are larvicidal and ovicidal
in action and they disrupt diapause and inhibit embryogenesis in insects.
Methoprene is a JH mimic and is useful in the control of larva of hornfly, stored
tobacco pests, green house homopterans, red ants, leaf mining flies of vegetables
and flowers
c) Anti JH or Precocenes: they act by destroying corpora allata and preventing JH
synthesis. When treated on immature stages of insect, they skip one or two larval
instars and turn into tiny precocious adults. They can neither mate, nor oviposit
and die soon. Eg. EMD, FMev, and PB (Piperonyl Butoxide)
d) Chitin Synthesis inhibitors: Benzoyl phenyl ureas have been found to have the
ability of inhibiting chitin synthesis in vivo by blocking the activity of the enzyme
chitin synthetase. Two important compounds in this category are Diflubenzuron
(Dimilin) and Penfluron. The effects they produce on insects include
Disruption of moulting
Displacement of mandibles and labrum
Adult fails to escape from pupal skin and dies
Ovicidal effect.
Chitin sysnthesis inhibitors have been registered for use in many countries and
used successfully against pests of soybean, cotton, apple, fruits, vegetables, forest
trees and mosquitoes and pests of stored grain
IGRS from Neem : Leaf and seed extracts of neem which contains azadirachtin as
the active ingredient, when applied topically causes growth inhibition, malformation,
mortality and reduced fecundity in insects.
Hormone mimics from other living organisms: Ecdysoids from plants
(Phytoecdysones) have been reported from plants like mulberry, ferns and conifers.
Juvenoids have been reported from yeast, fungi, bacteria, protozoans, higher animals
and plants.
Advantages of Using IGRs
Effective in minute quantities and so are economical
Target specific and so safe to natural enemies
Bio-degradable, non-persistent and non-polluting
Non-toxic to humans, animals and plants
