Abstract: An outdoor experiment was conducted for the inhibitory effect of Rhopalosiphum nymphaeae various in population density and growth stage on growth of water hyacinth,Eichhornia crassipes (Mart.) Solms. Results show that growth of water hyacinth was significantly inhibited by adult aphids,which was demonstrated in reduction of fresh plant weight,number of leaves and sprouts,and increase in the number of wizened leaves. R. nymphaeae feed on water hyacinth,thus affecting their growth. However the effect is positively related to their density,i.e. if the density is higher than 20 aphids per plant,the effect is significant and will become more and more significant as time goes by. But it should also be noted that R. nymphaeae have a relatively broad range of hosts and may endanger other aquatic plants in the neighborhood. Thus,the applicability of R. nymphaeae as natural enemy to control water hyacinth needs to be further studied.

Key words: Eichhornia crassipes, Rhopalosiphum nymphaeae, growth, inhibitory effect