As sessile organisms, plants are unable to escape from attack by herbivorous insects. To cope with this pressure, plants have evolved several defense strategies, including the production of secondary metabolites, specialized chemicals with ecological functions. Most studies have focused on the role of secondary metabolites in plant defense against chewing insects. Little is known about what compounds are present in phloem sap and how they affect phloem feeding insects. Therefore, I investigated the effects of phenolic compounds on phloem feeders, using Chaitophorus aphids in bioassays with wildtype and transgenic poplar overexpressing the transcription factor MYB 134, which results in elevated levels of tannins and reduced levels of phenolic glycosides. Aphids produced significantly more offspring on MYB 134 plants but showed a significant preference for lower tannin leaf tissue. Analysis of poplar phloem exudates and aphid extracts provides direct evidence that the phenolic glycosides salicin, salicortin and tremulacin are present in poplar phloem and are ingested by aphids. These results are discussed in relation to what is driving the differences in aphid fecundity and choice between plant types. / Graduate / 0309 / 0307 / 0329 / alphareghanwong@gmail.com