Chemical defence in chrysomelid eggs and neonate larvae

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Abstract
Eggs and neonate larvae of chrysomelid beetles (sub‐tribes Chrysomelina and Phyllodectina) were investigated for the presence of defensive substances. The two isoxazolinone glucosides (compounds 1 and 2), characteristic of the adult defence secretion, were detected in the eggs of all studied species. Compound 2, containing a nitropropionate, is always present in concentrations (above 10 ‐2 M), which are highly deterrent to the ant Myrmica rubra. This compound is not at all or only slightly toxic to ants at 10 ‐2 M. Compound 1, devoid of nitropropionate, is a minor constituent, and is neither deterrent nor toxic to ants. The five Chrysomela species studied and Phratora vitellinae also sequester salicin in their eggs in amounts highly deterrent and toxic to ants. A single Chrysomela egg often contains enough salicin to kill an ant. While the isoxazolinones are discarded with the egg shells, salicin is used by neonate larvae as a precursor for the production of salicylaldehyde in the thoracic defence glands, already functional at hatching. No salicin could be detected in the eggs of those species whose larvae produce cyclopentanoid monoterpenes, even if they feed on Salicaceae. No larva of any species seems to be able to produce detectable amounts of monoterpenes at birth. A very early defence, possible only in those species using salicin as the precursor for their defensive secretion, could be highly advantageous in protecting the clustered larvae during the long process of hatching and in avoiding cannibalism between siblings. Only trace amounts of oleic acid were found in the eggs of Gastrophysa viridula , in contrast to previous reports on its presence in large quantities in the American G. cyanea.

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