Homopteran insect pests can cause severe economical damage to crop plants by both direct physical means and as vectors of plant viral diseases. They are notoriously difficult insects to control by conventional methods, primarily due to their ability to evolve resistance-breaking biotypes within a relatively short time period. The production of genetically modified crop plants, expressing insecticidal genes, offers a novel method of control for a wide range of insect species. Once suitable gene products, such as plant- derived proteins, have been identified as having insecticidal effect against specific insects in vitro, their effect can be determined in vivo by expressing the relevant gene in transgenic plants. Insect feeding trials were carried out to determine the effects of incorporating a range of plant-derived proteins into artificial diets fed to planthopper, leafliopper and aphid pests and to aphids in planta. The lectins Galanthus nivalis agglutinin (GNA) and wheat germ agglutinin (WGA), and the enzyme soybean lipoxygenase (LPO) were shown to exhibit significant antimetabolic effects towards first and third instar nymphs of rice brown planthopper (Nilaparvata lugens Stal) when incorporated into artificial diet at 0 1% {w/v}, 0-1% (w/v) and 0 08% {w/v} levels respectively. The lectin GNA was also shown to exhibit a significant antimetabolic effect towards third instar nymphs of the rice green leafhopper (Nephottetix cinciteps Uhler) and the peach potato aphid {Myzus persicae Sulzer). A number of inert proteins, lectins, protein inhibitors and enzymes also tested showed relatively little or no effect towards both insects. The mechanism of action of all three effective proteins was examined using BPH as a model insect. As judged by honeydew production, the proteins all had a deterrent effect on insect feeding. However, subsequent toxic effects are also indicated. When fed sub-optimal concentrations of effective proteins in combination no synergistic or additive effects were observed, indicating that pyramiding the genes of these effective proteins would be of no advantage in protecting the crop against BPH.