Experiments in Costa Rica showed that cassava plants grown in weedy ground suffered more severe plant stunting than plants grown in less weedy ground. The damage to cassava was due to root-feeding by white grubs. Phyllophaga spp, (Coleoptera: Scarabaeidae). A significant correlation (P < 0.05) was found between the amount of groundcover and plant damage due to grub feeding at two plant population densities. The correlations were highly significant (P < 0.01) in pure stands of wither of two varieties of cassava, but were not significant in mixed stands. Possible biological mechanisms responsible for the results are discussed.