Magnolia virginiana Neolignan compounds as chemical barriers to swallowtail butterfly host use.

The role of toxins and deterrents in preventingtroilus group species (Lepidoptera, Papilionidae) from feeding on magnoliaceous hosts was investigated using bioassay-directed isolation ofMagnolia virginiana allelochemicals. A fraction consisting of three neolignan compounds significantly reduced survival of first instarP. palamedes, atroilus group member. Two of these compounds, magnolol and a biphenyl ether, were tested individually and were both toxic toP. palamedes. The larval survival ofP. troilus, anothertroilus group species, was also significantly reduced by magnolol but not by the biphenyl ether. In contrast,P. glaucus, a polyphagousglaucus group species that feeds on magnoliaceous hosts, was not affected by either compound. The effect of these compounds against a. polyphagous nonpapilionid was examined using the fall webworm (Hyphantria cunea, Lepidoptera: Arctiidae). The biphenyl ether but not magnolol significantly lowered webworm first-instar survival, demonstrating that polyphagy does not preadapt lepidopterans to feeding on this neolignan. These results demonstrate that although phagostimulants play a role in the specialization of thetroilus group on the Lauraceae, the presence of toxins and/or deterrents in nonhosts is also important in determining food plant patterns in these species.