Sesbania punicea(Cav.) Benth. is a perennial legume that has become a weed of major importance in South Africa. Until recently,S. puniceawas colonized by only one insect herbivore of any consequence, an introduced apionid weevil species,Trichapion lativentre(Beguin-Billecocq). The vegetative growth and reproductive capacity ofS. puniceawas shown to be significantly lower on plants at sites colonized byT. lativentrethan on plants at sites without the weevils. However, these initial comparisons did not account for the extraneous effects of intersite differences, and additional analyses have been undertaken to determine the actual effects of the weevils on the weed. Regression models showed that between 77 and 83% of the variance in vegetative growth (leaflets, rachides, and stems) ofS. puniceawas explained by the presence of weevils, by their history of colonization, and to a lesser extent, by the density of the weevils, expressed as cumulative weevil-days. Kruskal-Wallis tests showed that the weevils also almost completely nullified the reproductive capacity of the plants by destroying nearly all of the flower buds. The results confirm that the suppression of plant growth was attributable, almost entirely, to weevil herbivory and that, in moderate to dense infestations of the weed,T. lativentreis equally effective as a biological control agent, regardless of site characteristics such as the spacing, height, or size of the trees.