Herbivorous insects may incorporate plant natural products into exocrine and nonexocrine defensive secretions. By selectively adding proven plant repellent compounds to their own deterrent secretions, insects can increase the effectiveness of their chemical deterrents. These plant-derived compounds are generally unrelated to the constituents in the defensive exudates of their herbivores. In all likelihood, these plant additives may augment the repellency of the deterrents by reacting with olfactory chemoreceptors different from those targeted by the insect-derived repellents.
The large milkweed bug, Oncopeltus fasciatus, in common with many species of true bugs, uses the secretion of the metathoracic scent gland as an effective defensive exudate. Nymphs of this species generate defensive secretions with midorsal glandular fluid. The repellent secretions also contain cardenolides derived from the milkweed host plants of this species. These toxic and emetic steroids undoubtedly augment the deterrent effectiveness of the de novo synthesized compounds in the glandular exudates.
Similarly, R. guttata sequesters in the metathoracic defensive glands plant allelochemicals that can considerably augment the deterrent effectiveness of the secretion. Unlike O. fasciatus, R. guttata is a generalist that feeds on and sequesters a potpourri of plant natural products. As a consequence, the compositions of the glandular exudates can be variable, sometimes resulting in secretions that are considerably more repellent than those derived from insects that had fed on a limited number of host plant species.
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