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Aspects of the chemical ecology of lygaeid bugs (Oncopeltus fasciatus and Lygaeus kalmii kalmii) feeding of milkweeds (Asclepias species) in central California Isman, Murray Bruce

Abstract

A plant-insect allomonal system was investigated, involving seed bugs (Lygaeidae) on milkweeds (Asclepias spp.). The ability of the insects to sequester secondary compounds from host plants was studied in detail in central California. A colorimetric assay was used to quantify the amount of cardenolides (cardiac glycosides) in the lygaeid bugs Oncopeltus fasciatus and Lygaeus kalmii kalmii and nine species of milkweed host plants. The cardenolide content of individual adult insects, determined in microgram equivalents of digitoxin, varied from zero to over 300 μg per insect. Sources of variation of cardenolide content in the insects included interspecific and intraspecific differences in the cardenolide content of the host plant species, and also differences in the content of plant organs on which insects were feeding. When reared in the laboratory on a diet of milkweed seeds, the uptake of cardenolides by the bugs was proportional to the cardenolide content of the seeds. However, field collected lygaeids contained fewer cardenolides than were available in the plants on which they were feeding. This suggests that several ecological parameters, such as different reproductive phenologies and morphologies of the different host species, may interfere with the bugs' acquisition of cardenolides. O. fasciatus and L. k. kalmii differ in their feeding requirements, host plant utilization, and ability to sequester cardenolides from their hosts. However, the larval growth and development of O. fasciatus and L. k. kalmii in the laboratory on seeds of different milkweed species did not vary with plant species or cardenolide content of the seeds. Therefore, seeds of all plant species were equally suitable as food sources for the bugs. The colonization pattern of O. fasciatus on species of Asclepias in north central California suggest that this species does not maximize its opportunities to sequester large quantities of cardenolides from potential hosts. Over 50 per cent of field collected lygaeids in this study contained less than 50 μg of cardenolide. The ecological significance of cardenolide sequestration by lygaeids as a defensive strategy may depend on several modes of pharmacological activity in potential vertebrate predators, not simply emetic potential. Alternately, the presence of volatile secretions from the scent glands and histamine-like compounds in the dorso-lateral space fluid of O. fasciatus and L. k. kalmii could be more important than sequestered cardenolides as anti-predator strategies. The data suggest that the effectiveness of cardenolide sequestration as a chemical defence strategy may vary over the geographical range of these insect species, and may vary in the course of a season at a particular location. Therefore, cardenolide sequestration does not appear to be a crucial aspect in the feeding and population ecology of these lygaeid species at least in central California.

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