A pheromone precursor and its uptake in maleDanaus butterflies

Abstract

1. MaleDanaus chrysippus butterflies the larvae of which have been raised indoors on their natural foodplants (Asclepias) lack the pyrrolizidinone pheromone (Fig. 1, I) which is known from the hairpencils of field-caught butterflies. 2. MaleD. chrysippus have been observed actively approaching withered parts of aHeliotropium plant in Kenya. These observations could be repeated in the laboratory. On the plants, the males suck extensively. 3. A pyrrolizidine alkaloid (Fig. 1, II) has been isolated and characterized as lycopsamine from thisHeliotropium species. 4. Significant and up to “normal” amounts of I are found in the hairpencils after the indoors-raised males were allowed to: a) suck on withered and remoistenedHeliotropium, or b) feed on a methanol extract ofHeliotropium, or c) feed on a solution of alkaloid (II) isolated fromHeliotropium (see Table 1, Fig. 2). It therefore appears that substance II is a dietary precursor of I. 5. Electroantennogram recordings revealed the presence of antennal olfactory receptors for both substances I and II, as well as for the odor of the withered and remoistenedHeliotropium (Fig. 3). 6. Experiments in which radiolabelled compounds were administered toD. gilippus berenice males also suggest that the pyrrolizidine pheromone (I) is biosynthesized from an exogenous alkaloid precursor.