2. CHEMICALS BASED ON ENDOCRINE and
nervous system
In
insects, virtually all life processes are regulated
by neural and endocrine systems.
Basically
Three types of hormones
a) Brain hormones
b) Moulting hormones
c) Juvenile hormones
These hormones are involves in the life processes of
insects and regulating insect development , so
studying of these can be utilized for insect pest
control.
3. BRAIN HORMONES:
Also known as neurohormones.
Produced by the central nervous system to
regulate various events in the body known as
brain hormones.
The first neuropeptide Proctolin was isolated
from Periplaneta americana in 1975.
4.
All the fully sequenced brain hormones are oligopeptides
or small protein molecules.
The important brain hormones are:
a)Diuretic hormone.
b)Anti diuretic hormone.
c) Allatotrophins.
d)Allatostatins.
e)PTTH.
f)Pheromone biosynthesis stimulating
hormone.
5.
The brain hormone or prothoracicotropic hormone
(PTTH) secreted by neurosecretory cells of the
brain.
Hormones exert a critical control over all developmental,
reproductive and metabolic activities hence disruption or
removal of these lead to mortality of the insects.
These neuropeptides which are commercially produced
used for the pest control.
But the major problem in commercial exploitation is
costly synthesis , inability to penetrate cuticle and
photoinstability.
6. MOULTING HORMONES:
Responsible
for normal moulting, growth and
maturation of insects.
secreted by Prothorasic glands.
Types
These
of moulting hormones are
Ecdysone.
Ecdysterone.
Ecdysteroids.
are steroidal compounds responsible for
moulting , growth and maturation of insects.
7.
Depending on the stage of insect , ecdysone acts as
hormone or as precursor for the more active
compounds like
20-hydroxy ecdysone,
20,26-dihydroxy ecdysone or
26 hydroxy ecdysone.
10.
The MHs are hydrophilic because of a number of hydroxy
groups present on the molecule.
the body of insect , MHs are produced from cholesterol
from a number of phytosterols with an intermediate
desmosterol.
Usage in pest control:
Blocking the biosynthesis of MHs disrupt the moulting
process.
eg. Triparanol , 22,25-diazacholesterol.
Some of the Azasterols , Modified Azasteroids.
eg .Tebufenozide
11. JUVENILE HORMONES
The role of juvenile hormones ( JH) in the growth of
insects was recognized by C.M. Williams
JHs are group of acyclic sesquiterpenoids that
regulate many aspects of insect physiology.
In insects JHs ensures growth of larva, while
preventing metamorphosis.
These are also important for reproduction of
eggs in female insects.
12.
JHs are secreted by a pair of endocrine glands behind
the brain called as corpora allata
JHs are terpenoidal compounds synthesised from acetyl
COA via farnesyl pyrophosphate.
13. FUNCTIONS
In insects, juvenile hormones regulate a variety
of functions including
1. Metamorphosis,
2. Vitellogenesis,
3. Diapause
and
4. Polymorphism.
14.
JH found in haemolymph control the stages of
development of insects.
They maintain a juvenile state in insects.
Its level gradually decreases during development of
insect allowing it to proceed to successive instars with
each molt.
V.B. Wriggles Worth studies.
Decreased level of JH results in – diminutive adult at
next molt.
Increased level of JH produce supernumerary juvenile
instars.
In presence of high titer of JH, a larva moults into alarva,
while at low titer of JH it moults into pupa.
15.
16.
There is a complex interaction between JH and
ecdysone hormone.
In insects, as long as there is enough JH , the ecdysone
promotes larva to larva moults.
Low levels of JH ,ecdysone promotes pupation.
Complete absence of JH results in adult formation.
17.
JH and Vitellogenin levels in general show inverse
pattern.
Vitellogenin levels are high at adult stage.
JHs also involved in queen and worker caste
differentiation during larval stage.
18. Types of juvenile hormones:
Lepidoptera
Non Lepidoptera
(except Diptera)
1) juvenile hormones-0
1.
2) juvenile hormones- i
3) juvenile hormones-ii
4) 4-Methyl Juvenile hormone-I
juvenile hormones-iii
20. JHs have limited scope in pest control.
Unstable in U.V light.
Rapidly metabolized by insects.
21. JUVENILE HORMONE ANTAGONISTS
Exogenous application of JHAS is effective only when
endogenous levels of JHs in insects is low.
So, chemicals blocking the biosynthesis of natural JHS
are used, named as anti-JH agents.
example:
Procene 1 and 2 - Plant, Ageratum houstonianum .
Piperonyl butoxide .
Imidazole.
Acetylenic compounds.
22. USE AS INSECTICIDE
Exogenous application of JH result in formation of
Supernumerary nymphal/larval instars.
Larval- pupal intermediates.
Pupal- adult intermediates.
These abnormal forms are unable to reproduce, feed or
fly.
Application of JHAs to adult females or eggs results in
ovicidal or chemosterilising effects.
23. => PTTH pro-thoracico-tropic-hormone
PTTH (storage)
=> JH juvenile hormone
=> Ecdysone (ecdysteroids)
from Gulen & Cranston 2000
Important aspects of the typical insect endocrine system.
