There has been little documentation of the success of introduced agents for classical weed biological control. Field evaluation of an insect's establishment, spread and early host impact within its new environment must be performed before agent success can either be doucmented or predicted. Population attributes of the ednophagous biological control agent, Mecinus janthinus Germar (Coleoptera: Curculionidae), and interactions with its target weed, Dalmation toadflax, (Linaria dalmatica (L.) Mill.) (Scrophulariaceae), were explored across variable levels of resource availability and insect abundance. Patterns of population growth and impact of this biocontrol agent were very consistent throughout this study. Within four years of release, populations of M. janthinus achieved outbreak population levels and virtually eliminated the seed producing shoots from toadflax stands. There is a tight but flexible relationship between oviposition site selection and offspring performance in this endophagous herbivore, maximizing offspring survival even under moderate to high M. janthinus densities. These attributes allow M.janthinus to be an effective biocontrol agent under changing levels of resource availability. / ix, 134 leaves : ill. ; 28 cm.