Insects are unable to biosynthesize the steroid nucleus and generally require an exogenous source of sterols. Two salient areas of insect steroid metabolism are the dealkylation and conversion of dietary C28 and C29 plant sterols to cholesterol and other C27 sterols, and the biosynthesis and metabolism of the steroidal insect molting hormones. Certain azasteroids and nonsteroidal amines block this conversion of 24-alkyl sterols to cholesterol and/or disrupt molting and development in insects. These inhibitors have served in charting metabolic pathways for steroids in insects and are serving as models in developing selective pesticidal chemicals and chemotherapeutic agents for use against insects and other invertebrate pests and parasites. The mode of action of some of these inhibitors on molting and development has been investigated in vivo and in vitro. Certain of these inhibitors represent a new class of insect hormonal compounds with a novel mode of action-the disruption of molting hormone metabolism. Research on sterol metabolism in insects provides important information on the comparative biochemistry and physiological functions of steroids in living systems.

Ingestion of certain synthetic ecdysone analogs inhibited larval growth and development in several species of insects, whereas 20-hydroxyecdysone was inactive or considerably less active. Natural 20-hydroxyecdysone and ponasterone A, and a synthetic ecdysone analog inhibited ovarian maturation and egg production in the adult housefly. These effects appeared to be related to hormonal activity.

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The two major molting hormones of insects, alpha ecdysone and 20-hydroxyecdysone, were isolated in crystalline form from dry pinnae of the bracken fern, Pteridium aquilinum (L.) Kuhn. Three unidentified substances with molting hormone activity were also detected. Bracken is the first plant found to contain both of the major insect ecdysones, and it is the first known plant source of alpha ecdysone.