Insect endocrine system

Although there are many examples of plant secondary compounds which interfere with the physiology of insects [6, 7], there is debate as to the nature of the selective forces responsible for the evolution of these compounds [9, 10]. The argument has been made that although it appears that these secondary metabolites serve to protect the plant against insect herbivory today, it is likely that they were initially selected as a response to vertebrate herbivory [11]. However, phytochemicals which specifically interfere with the insect endocrine system possibly represent compounds which have evolved as protection against insect attack and therefore represent plant adaptations to insect herbivory. 

Insects are so successful because of their mobility, high reproductive potential, ability to exploit plants as a food resource, and to occupy so many ecological niches. Plants are essentially sessile and can be seen to produce flowers, nector, pollen, and a variety of chemical attractants to induce insect cooperation in cross-pollination. However, in order to reduce the efficiency of insect predation upon them, plants also produce a host of structural, mechanical, and chemical defensive artifices. The most visible chemical defenses are poisons, but certain chemicals, not intrinsically toxic, are targeted to disrupt specific control systems in insects that regulate discrete aspects of insect physiology, biochemistry, and behavior. Hormones and pheromones are unique regulators of insect growth, development, reproduction, diapause, and behavior. Plant secondary chemicals focused on the disruption of insect endocrine and pheromone mediated processes can be visualized as important components of plant defensive mechanisms. 

Pesticide is used to control pests of different kinds, such as target insects, vegetation, and fungi. Pesticides are known poisons used specifically for the control of crop pests and rodents. Some are very poisonous, or toxic, and may seriously injure or even kill humans. Others are relatively nontoxic. Pesticides can irritate the skin, eyes, nose, or mouth. The health effects of pesticides depend on the type of pesticide. The organophosphate and carbamate pesticides affect the nervous system. Others cause irritation to the skin, eyes, and mucous membranes. Several pesticides are carcinogens and some others cause disturbances to the hormone or endocrine system in the body. 

An interesting feature which the neuroendocrine axis of vertebrates and insects has in common is the interaction of stimulatory and inhibitory neurohormonal directives to the respective glands of the endocrine system. Their existence has been demonstrated by a combination of structural and experimental studies. 

The endocrine system of insects is a potential target for so-called third generation pesticides (27). Compounds which specifically interfere with the insect hormone system would have several advantages over existing pesticides ... 

It is apparent from these examples that plants produce compounds which are able to interfere with the endocrine system of insects. It is tempting to speculate that they may be responsible, or at least partially responsible, for the protection of the plant against insect attack. Unfortunately, most... 

Since juvenoids affect the insect specific endocrine system, they are expected to become insect-specific insecticides [2]. Some naturally occurring compounds have juvenoid activity, e.g., Schmialek [3] isolated farnesol (1) and farnesal (2) with juvenoid activity from the feces of mealworm, Tenehrio molitor, (Fig. 25.2.1). 

Larval development is so finely controlled, that the general belief has been that a minor disruption may result in a major effect for the insect. Disruption of the endocrine system for insect control was first proposed by Williams (19,20), Considerable attention has been paid to JH analogs (JHAs) for insecticidal use in the field. The JHAs have been exploited for control of a number of pest insects including stored product pests, fleas and mosquito larvae (21,22). 

Abstract Pheromones are utilized by many insects in a complex chemical communication system. This review will look at the biosynthesis of sex and aggregation pheromones in the model insects, moths, flies, cockroaches, and beetles. The biosynthetic pathways involve altered pathways of normal metabolism of fatty acids and isoprenoids. Endocrine regulation of the biosynthetic pathways will also be reviewed for the model insects. A neuropeptide named pheromone biosynthesis activating neuropeptide regulates sex pheromone biosynthesis in moths. Juvenile hormone regulates pheromone production in the beetles and cockroaches, while 20-hydroxyecdysone regulates pheromone production in the flies. 

It has been widely reported that the decline of bird populations is associated with an increase in the use of organochlorine pesticides, such as DDTs, for insect control in urban and agricultural environments (Blus et al., 1979 Cade et al., 1971 Peakall, 1974). It is also known that some chemicals, in particular organochlorines, interfere with the functioning of reproductive, endocrine, immune, and nervous systems (Yamashita et al., 1993 Jimenez, 1997). 

Barth R. H., Jr (1965) Insect mating behavior endocrine control of a chemical communication system. Science 149, 882-883. 

The similarities between ecdysone synthesis in the insect prothoracic gland and the ovary are obvious and in each case synthesis is initiated in response to a hormone originating in the brain. Both bear a striking resemblance to the mammalian system where steroid hormone synthesis in the various endocrine tissues is initiated in response to the release of appropriate hormones from the anterior pituitary. 

The endocrine organs of insects are of two types neurosecretory cells within the nervous system and specialized endocrine glands, such as the corpora allata, the corpora cardiaca, and the pro thoracic glands. Corpora cardiaca are a pair of organs closely associated with the main vessel (aorta). Corpora allata and the pro thoracic glands are more diffuse glandular bodies. 

Most of the endocrine processes in insects are controlled by neuropeptides. The neuroendocrine system in insects is comprised of neuropeptide-synthesizing neurons located in the cerebral ganglia (brain),... 

The probability that nematode endocrine control involves some compounds similar in structure to vertebrate or insect steroid hormones has prompted several investigations of the effects upon nematodes of exogenous application of various concentrations of several of these compounds (53-62). When observed, effects have consisted primarily of inhibition or stimulation of growth, reproduction or molting. Although the high concentrations of added steroid or the nature of the assey system in some of these experiments might lead to speculation about the in vivo extrapolation of such results, the bo<(y of work is compatible with the hypothesis that steroids are hormonally active within nematodes. 

Insect growth regulators (IGR). IGR s are chemicals that are able to act on the endocrine and hormone systems of insects. Highly specific in their action, they have received much attention as potential insecticides for wood protection. Many of the newer IGR s mimic the juvenile hormone and effectively inform the insect to remain in the immature state. Adult insects treated with such products are also unable to moult... 

---