Insect development, hormonal control

The negative implications for pest control by precocenes themselves are clear, but it remains to be seen whether the expansion of their spectrum of activity is limited merely by the chemical structural features of precocenes or, more problematically, by the hormonal mechanisms which control insect development. In either case the JH antagonist approach to the control of larval insect pests presents a major challenge to chemical and physiological research. 

At the outset, it appeared that proteinaceous products dispatched by a special class of neurosecretory neurons, such as those of the pars intercerebral is of insects, function exclusively in a neurohormonal capacity. Following the classical discovery by Kopec (1922) of a brain hormone controlling insect metamorphosis much information has been gained on a number of essential neurohormonal functions carried out by neuropeptides in the control of postembryonic development as well as reproductive, metabolic, muscular, and additional activities. 

Our laboratory is concerned with targeting potential insecticides that disrupt normal development and metamorphosis in insects. Juvenile hormones (JHs), acting in concert with the steroid hormone ecdysone, are believed to control the timing of the larval-larval molts, larval-pupal and pupal-adult transformations of the insects. It has been demonstrated that the events leading to pupation are initiated by reduction of the JH titer in the hemolymph. In addition to a cessation of biosynthesis, this reduction in JH titer is controlled by degradative metabolism (16,17). Hydrolysis of the epoxide and ester functionalities present in active JH are two routes of degradation and subsequent inactivation of JH (18). The primary route of JH metabolism in the hemolymph of last stadium lepidopterous larvae is ester hydrolysis, and it is catalyzed by the enzyme juvenile hormone esterase (JHE). JHE has been shown to... 

Many details of the anatomy and biochemistry of insect development from egg to adult are known. Growth and development are strictly regulated by hormones, and hormone concentration, relative and absolute, is vital for this to proceed correctly. The hormones control cuticle synthesis, molting, sexual maturation, color differentiation, reproduction, etc. 

Two groups of hormones regulate development and reproduction in insects, namely the ecdysteroids and the JHs. In crustaceans too, ecdysteroids are involved in the hormonal control of growth, and a chemical compound, which is similar to the JHs of insects, is present and thought to play a role in crustacean reproduction and development. The insect JH is a species-specific acyclic sesquiterpenoid epoxide, which is synthesized in a pair of retrocerebral epithelial organs called the corpora allata (CA see Fig. 1). In decapod crustaceans, MF is the unepoxidated form of the insect JH III and it is synthesized and secreted from the MOs (see Section 3.2.4). 

Methoprene (marketed as Precor), an insect juvenile hormone mimic used in flea control products for pets, works by preventing the development of flea eggs and larvae. The effective form of methoprene, shown here, is optically active. Locate the stereogenic center and determine its configuration (R,S). 

Fig. I. Hormonal control of insect development. Three hormones are involved the brain hormone, acting on the prothoracic glands, the juvenile hormone, secreted by the corpora allata and ecdysone, secreted by the prothoracic glands. The bottom row shows the development from the caterpillar through the pupa to the moth. The larval ecdyses are controlled by ecdysone and juvenile hormone, but the pupal and imaginal ecdyses are induced only by ecdysone. (from P. Karlson, Lehrbuch der Biochemie, with kind permission of G. Thicme Verlag, Stuttgart.)...

---