Dual chemical barriers protect a plant against different larval stages of an insect

The host plants of the native American butterfly, Pieris napi oleracea, inc lude most wild mustards. However, garlic mustard, Alliaria petiolata, a hig hly invasive weed that was introduced from Europe, appears to be protected from this insect. Although adults will oviposit on the plant, most larvae o f P. n. oleracea do not survive on garlic mustard. We used feeding bioassay s with different larval stages of the insect to monitor the isolation and i dentification of two bioactive constituents that could explain the natural resistance of this plant. A novel cyanopropenyl glycoside (1), alliarinosid e, strongly inhibits feeding by first instars, while a flavone glycoside (2 ), isovitexin-6"-D-beta -glucopyranoside, deters later instars from feeding . Interestingly, the first instars are insensitive to 2, and the late insta rs are little affected by 1. Furthermore, differential effects of dietary e xperience on insect responses suggest that 1 acts through a mechanism of po st-ingestive inhibition, whereas 2 involves gustatory deterrence of feeding .