GM trees hold promises of increased quality and yield and reduced use of herbicides and pesticides but could also have ecological consequences.  We investigated whether modification of a non-defensive trait unintentionally influenced plant traits important for plant-herbivore interactions.  We found that over-expression of sucrose phosphate synthase (SPS), which is known to increase mesophyll sucrose content and biomass production in GM aspens, also unintentionally induced changes in the concentration of plant phenolics and nitrogen.  One of the GM lines, SPS33A, had higher concentrations of salicin, tremuloidin, condensed tannins and nitrogen and lower concentrations of coumaric acid and four flavonoids compared with the isogenic wild type.  Line SPS33A was also utilized less by the leaf-beetle Phratora vitellinae than the isogenic wild type.  Ecological consequences such as this are not specific to GM trees or GM plants but can occur as a result of the introduction of all introduced new varieties of crops or trees.  Nevertheless, the results underline the need to consider these unexpected effects when evaluating both the potential benefits and the potential risks with GM plants, and highlight the need to establish and implement comprehensive product-by-product evaluation protocols for GM plants.