The influence of host plant phenotype on the impact caused to Mimosa pigra L. (Mimosaceae) plants by the feeding activity of larvae of Carmenta mimosa Eichlin and Passoa (Lepidoptera, Sesiidae) and the relationship between plant phenotype (through plant quality) and larval development were investigated under controlled conditions. Plants grown under conditions of reduced light availability were most prone to the detrimental effects of the feeding activities of C. mimosa larvae. The relative growth rate (RGR) of most plant phenotypes could be reduced when infested with C. mimosa larvae compared to uninfested plants. Small plants infested with C. mimosa and grown under reduced light availability exhibited significant reductions in RGR sooner than large plants grown in full sun. The physical quality of stems was correlated with the severity of impact, i.e., whether stem breakage occurred. Little larval-induced mortality of plants was observed, suggesting that vascular tissues remained sufficiently intact to allow translocation. Systemic stem death arose through infection by secondary pathogens. Differences in biomass of infested versus uninfested plants of all phenotypes were associated with the loss of stems and to a lesser extent leaves. The phenotype of plants had a significant effect on the development of C. mimosa larvae. Larvae in stems of plants with good access to reserves of soil moisture reached more advanced stages of development sooner than did those in plants which were often water stressed. This response was mediated through the combined influence of availability and nutritional quality of food. The implications of these findings to the impact of this agent in the field and the factors which may significantly influence its population dynamics are discussed.