In the course of long term coevolution with their host plants, phloemsucking insects have evolved a special stylet that facilitates feeding on phloem sap. Using aphids as an example, we investigated the feeding mechanisms used by phloemsucking insects. Aphids secrete both viscous and watery saliva to reduce the resistance of their host plants during the feeding process. The watery saliva contains a complex mixture of enzymes that not only make it easier for the aphid to penetrate the phloem but which also appear to trigger the plant’s chemical defense mechanisms. Generally, aphids activate the plant’s defenses via the salicylic acid signaling pathway. However, previous research demonstrates that the jasmonic acid and ethylene signaling pathways are also involved in plant defenses against aphids. Aphids have evolved a variety of adaptations to counter plant defenses. For example, aphid feeding activity renders plant forisome protein inactive, thereby allowing aphids to continue feeding on their host plants. Our observations suggest that components of the saliva of phloemsucking insects are likely to play an important role in the interaction between herbivorous insects and their hostplants.