The bacterial endosymbiont of aphids, Buchnera aphidicola, often provides amino acids to its hosts. Plasmid amplification of leucine (leuABCD) and tryptophan (trpEG) biosynthesis genes may be a mechanism by which some Buchnera over-produce these nutrients. We used quantitative polymerase chain reaction to assess the leuABCD/trpEG copy variability within Uroleucon ambrosiae, an aphid with a wide diet breadth and range. Both leuABCD and trpEG abundances are: (i) similar for aphids across 15 populations, and (ii) low compared to Buchnera from other aphid species (particularly trpEG). Consequently, the plasmid location of trpEG combined with Buchnera's chromosomal polyploidy may functionally limit, rather than increase, tryptophan production within Uroleucon ambrosiae.