Evaluating the magnitude and mechanisms of impact of insect herbivores on noxious weeds is an important research objective in weed biological control. We utilized clear Plexiglas “rhizotrons” or “root boxes” to investigate how Mecinus janthinus, an introduced weevil, affects root growth of its target weed, Dalmatian toadflax. Reductions in root biomass might be expected to influence nutrient and water acquisition and in turn affect overwintering survival of toadflax plants.