Insect hormones

M.SREENIVAS

VHM/12-17

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.

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.

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.

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.

MOULTING HORMONES: Responsible for normal moulting, growth and

maturation of insects. secreted by Prothorasic glands.

Types of moulting hormones are Ecdysone. Ecdysterone.

Ecdysteroids.

These are steroidal compounds responsible for moulting , growth and maturation of insects.

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.

GENERAL STRUCTURE

HO

HO

O

OH

R₁OH

R₂

H₂

C

OH

R₃

R₁ R₂ R₃

Ecdysone H H H

20-hydroxy ecdysone

OH H H

26-hydroxy ecdysone

H H OH

20,26-dihydroxy ecdysone

OH H OH

Makisterone A OH CH₃ H

20-Deoxymakisterone

H CH₃ H

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

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.

BIOSYNTHESIS 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.

In insects, juvenile hormones regulate a

variety of functions including

1. Metamorphosis,

2. Vitellogenesis,

3. Diapause and

4. Polymorphism.

FUNCTIONS

METAMORPHOSIS 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.

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.

VITELLOGENESIS 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.

Types of juvenile hormones:

Lepidoptera

1) juvenile hormones-02) juvenile hormones- i3) juvenile hormones-ii4) 4-Methyl Juvenile hormone-I

Non Lepidoptera(except Diptera)

1. juvenile hormones-iii

JHO – C₁₉ H₃₂ O₃

JH1 – C₁₈ H₃₀ O₃

JH2 – C₁₇ H₂₈ O₃

JH3 – C₁₆ H₂₆ O₃

JHB3 – C₁₆ H₂₆ O₄

STRUCTURES

JHs have limited scope in pest control.

Unstable in U.V light.Rapidly metabolized by insects.

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.

JUVENILE HORMONE ANTAGONISTS

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.

Important aspects of the typical insect endocrine system.

from Gulen & Cranston 2000

=> PTTH pro-thoracico-tropic-hormone

PTTH (storage)

=> JH juvenile hormone

=> Ecdysone (ecdysteroids)

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