Oviposition-stimulatory activity of phenanthroindolizidine alkaloids of host-plant origin to a danaid butterfly, Ideopsis similis

Oviposition response of Ideopsis similis (L.) (Lepidoptera: Danaidae) was e xamined for 12 phenanthroindolizidine alkaloids present in its host plant, Tylophora tanakae (Maxim.) (Asclepiadaceae). At least five alkaloids, i.e. (+)-isotylocrebrine (3,4,6,7-tetramethoxyphenanthroindolizidine; l), (+)-3- demethyliso- tylocrebrine (3), (+)-isotylocrebrine N-oxide (5), (+)-6-demet hyltylocrebrine (8) and (-)-7-demethyltylophorine (10), were found to indiv idually stimulate oviposition by females. Of these, compounds 1, 3 and 10 w ere regarded as key components most responsible for host recognition or pre ference. However, female egg-laying was much higher in response to a mixtur e of the five alkaloids. In two-choice bioassays, more eggs were deposited on samples comprising the five alkaloids than on samples consisting of a si ngle alkaloid. This suggests strongly that host selection by the butterfly is mediated by the synergistic action of several phenanthroindolizidine alk aloids present in the host plant.