Insect hormones

insect hormones

1. Insect hormones M.SREENIVAS VHM/12-17

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 of moulting hormones are Ecdysone. Ecdysterone. Ecdysteroids. These 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.

8. GENERAL STRUCTURE HO HO O OH R₁ OH R₂ H₂ C OH R₃

9. 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

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. 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.

13. In insects, juvenile hormones regulate a variety of functions including 1. Metamorphosis, 2. Vitellogenesis , 3. Diapause and 4. Polymorphism. FUNCTIONS

14. 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.

15. 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.

16. 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.

17. Types of juvenile hormones: Lepidoptera 1) juvenile hormones-0 2) juvenile hormones- i 3) juvenile hormones-ii 4) 4-Methyl Juvenile hormone-I Non Lepidoptera (except Diptera) juvenile hormones-iii

18. JHO – C₁₉ H₃₂ O₃ JH1 – C₁₈ H₃₀ O₃ JH2 – C₁₇ H₂₈ O₃ JH3 – C₁₆ H₂₆ O₃ JHB3 – C₁₆ H₂₆ O₄ STRUCTURES

19. JHs have limited scope in pest control. Unstable in U.V light. Rapidly metabolized by insects.

20. 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

21. 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.

22. 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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