Hormones

8.13.3.1 Introduction

Insect hormones regulate growth and maturation from the larva to the adult. The juvenile hormone controls the development of the immature larva through successive growth stages. If, however, it remains present during the metamorphosis of the larva to the adult, a deformed larva results or the adult that is formed soon dies. The molting hormone, ecdysone, is required for the differentiation to the adult. If the immature larva is treated with the molting hormone, it passes through its life cycle at a rapid rate and dies prematurely. Insect growth and development could be controlled if these hormones, or compounds with similar biological activity, could be applied at critical stages in the development of the larva to the adult. For example, compounds have been developed that bind to ecdysone receptors and accelerate the growth of the larvae of the tobacco hornworm, an insect that eats tobacco plants.

CH9CH3 CH3

CH3CH2C-CH(CH2)2C=CH(CH2)2CH=CCO2CH3

trans cis

Juvenile hormone

a-Ecdysone (molting hormone)

a-Ecdysone (molting hormone)

8.13.3.2 Antijuvenile Hormones from Plants

The continuing battle between insects and plants led to the synthesis by the common ageratum plant of two compounds that block the action of juvenile hormones. Compounds with this general biological effect are called antialla-trotropins. The particular compounds isolated from ageratum were named precocene I and precocene II because they caused premature metamorphosis of the larvae of a number of insects.

The adult females that develop after treatment with the precocenes are sterile, and only a few of the males are capable of successful mating with normal females. In addition, treatment of newly developed adult females with preco-cenes prevents the development of ovaries and, in the case of the cockroach, prevents the secretion of sex pheromones. Treatment of the adult potato beetle with precocenes caused it to prematurely stop feeding and go into a dormant stage. Precocene II also prevented the development of the eggs of the milkweed bug and the Mexican bean beetle.

The wide range of biological activity exhibited by the precocenes suggests that the antiallatrotropins could be very effective agents for insect control. They have a decided advantage over juvenile hormones in that their toxic properties are effective on the adult and on the egg as well as on the larva. In addition, their effect on the larval stage is to accelerate development to the

Precocene I

Precocene II

Precocene I

Precocene II

adult, thereby shortening the larval stage, which is generally the most destructive.

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