The active constituents of the plant Ageratum houstonianum causing early larval instars of some insects to moult into precocious prothetelic adults are 7-methoxy-2,2-dimethylchromene and 6,7-dimethoxy-2,2-dimethylchromene1—precocenes 1 and 2 (P1, P2). P2 causes atrophy of the corpora allata (CA) in at least two species2–5 and juvenile hormone (JH) production is suppressed, simulating removal of the glands. Active CA are required for the effect of P2 in Oncopeltus5. The CA are the site of monooxygenase (MO) activity associated with JH biosynthesis6 and in appropriate conditions, those isolated from certain adult insects can effect the terminal epoxidation of endogenous E,E-methylfarnesoate to give C16JH (refs 7,8). The destructive effect of P2 on the CA, together with the finding9 that the 3,4-dihydrodiol is a significant metabolite in some insects, suggested that P2 has a specific cytotoxic action within the glands, possibly mediated by local formation of the 3,4-epoxide10. Reactive epoxides of polycyclic aromatic hydrocarbons11, bromobenzene12, or aflatoxins13, and oxidative metabolites of other xenobiotics14 have been implicated in their cytotoxic, mutagenic and/or carcinogenic effects in mammals, in some cases with support from manipulations of the critical balance between activating (oxidising) and deactivating enzymes in vivo12. We now present evidence that the action of precocenes on immature Oncopeltus and Locusta requires an oxidative bioactivation within the CA. The generation of reactive metabolites from innocuous precursors in vital tissue, if it can be linked to appropriate species and organ selectivity, offers an attractive approach to insect control.
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Brooks, G., Pratt, G. & Jennings, R. The action of precocenes in milkweed bugs (Oncopeltus fasciatus) and locusts (Locusta migratoria). Nature 281, 570–572 (1979). https://doi.org/10.1038/281570a0
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