Interest in the relationship between above and below ground herbivores has increased in the last decade. These herbivores compete for resources indirectly by changing the chemistry of their host plant. We evaluated the effect of two nitrogen levels (50 and 150 mg/L) on the interaction between darkwinged fungus gnats (DWFG), Bradysia impatiens, and silverleaf whitefly (SLWF), Bemisia tabaci biotype B, on poinsettias and their combined effect on the amino acid composition of poinsettia. Our results show that SLWF was positively impacted by the presence of DWFG as SLWF F1 oviposition was higher when DWFG were present. Conversely, DWFG were negatively impacted by SLWF presence. There were fewer DWFG pupae found in the soil when SLWF were present. This relationship was mediated by nutrition as we found stronger effects when the plants were grown at the high nitrogen level. Higher amino acid levels were found with the higher nutrition treatment. The presence of both herbivores increased the total amino acid concentration of poinsettia and individual amino acids such as lysine and threonine and decreased alanine concentration in the petioles at the high nitrogen level. The plants attacked by DWFG alone had negative impact on total amino acids of both petioles and roots. Overall, SLWF benefited from the competition while DWFG were negatively impacted. This study provided evidence that SLWF and DWFG change the plant quality by modifying the levels of total and individual amino acids.