Evolutionary implications of herbivory onVernonia (Compositae)

A field insect feeding experiment was conducted with two species ofVernonia which produce the sesquiterpene lactone glaucolide-A, one species which does not contain this bitter compound, and two F₁ hybrid combinations. Also, naturalVernonia populations were observed over a two year period for their degree of herbivory. Contrary to the pattern observed in laboratory feeding preference tests,Vernonia flaccidifolia, the species lacking sesquiterpene lactones, was consistently fed upon less by insects than wereV. gigantea andV. glauca. Even though glaucolide-A appears to adequately protect some Vernonias against herbivores, especially mammals,Vernonia flaccidifolia has lost this compound. ApparentlyV. flaccidifolia has evolved an alternative defensive mechanism which is more effective against insects but less effective against mammalian herbivores.