New methods of insect pest management include the application and development of the existing natural resistance mechanisms, which protect plants against insect feeding. Aphids (Homoptera) have the exceptionally modified mouthparts, the piercing-sucking type, which enables feeding on plant sap directly from the sieve elements of the phloem tissue. Most aphid species are oligophagous; i. e. their host plant range is restricted to selected related plant species. The crucial step in host-plant recognition by these insects is the gustatory examination of the plant sap. Aphids do not possess external contact chemoreceptors, so this step requires stylet penetration of the plant and the subsequent sap sampling. The effect of chemical properties of a plant, and in fact the acceptability of a plant to an aphid, can be investigated directly by using the electronic monitoring of stylet penetration (EPG = Electrical Penetration Graph) technique. In this system the aphid and the plant are made parts of an electric circuit and any changes in electric characteristic of the circuit express aphid activities, such as: stylet penetration of peripheral plant tissues, feeding on phloem sap, salivation into phloem elements, e.t.c. The duration and frequency of these activities can be used as indicators of plant suitability. The existence and tissular localisation of factors determining the level of plant resistance against aphids can also be derived from these data.