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Plant (Asclepias) - insect (Oncopeltus) chemical relationship Duffey, Sean Stephen

Abstract

The association of the Large Milkweed Bug, Oncopeltus fasciatus Dallas, with, a potentially poisonous Asclepiad, Asclepias, has been investigated to determine the fate of sequestered cardiac glycosides in this insect and to investigate the possibility that these compounds and/or odorous and volatile alkyl secretions of this insect may be serving as (an) anti-predator device(s). Nineteen species of Asclepias from diverse parts of North America have been shown to contain cardiac glycosides. Evidence is also given that Oncopeltus plus several other species of brightly coloured Coleopterans and Hemipterans, which are associated with Asclepias as a food-host, contain cardenolides which could function as the chemical basis for a Mullerian mimicry complex. The large quantities of polar cardiac glycosides sequestered by Oncopeltus fasciatus (approximately 111 micrograms) from the seeds of Asclepias syriaca were found to be concentrated in a complex of dorso-lateral abdominal and thoracic secretory glands. Various parameters of the uptake and entry of the natural cardiac glycosides of Asclepias syriaca and unnatural isotopic cardiac glycosides into the dorso-lateral glands were examined. The high levels of polar glycosides in Oncopeltus is also related to other aspects of the insect's physiology and the cardenolide composition of the food-host. The literature cites that lipid cardenolides are more emetic to birds than are the polar glycosides: therefore, the high levels of polar glycosides in this Hemipteran feeding on the above plant could make it non-emetic. Oncopeltus fasciatus was shown to be aposematic to chickens, turtles, lizards and starlings because of the volatile secretions of the ventral metathoracic glands. Frogs and toads did not consider this insect to be aposematic. The cardiac glycosides that had been sequestered from the seeds of this northern Asclepiad by Oncopeltus were not shown to be effective in causing rejection by the above predators in laboratory conditions. The predation studies on Oncopeltus suggest that the responses of various predators to a complex of glycoside containing mimics are not equivalent. This study also shows that along with predator responses being a critical feature in a palatability spectrum, the insect's physiology and its behavioural association with the plant are poignant aspects of the insect's potential to be unpalatable.

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