We performed laboratory tests to examine effects of different densities ofPulvinariella mesembryanthemi(Vallot) on population growth, development, survivorship, and reproduction. We also measured survivorship and growth of its ice plant host,Carpobrotus edulis×aqualateris(Aizoaceae), supporting various scale densities. Scale body size and fecundity were inversely related to density, indicating intraspecific competition; both scale and host-plant survivorship were strongly density-dependent. Scale mortality at high densities was attributed to limited settling sites, honeydew fouling, physical crowding, and death of host plants and plant parts supporting scales. On dying plant tissues, scales reproduced prematurely and produced fewer offspring. Plant growth was negatively correlated with scale densities, and at high scale densities plants stopped production of new leaves and shoots, reducing the resource base for subsequent generations of scale.