Free and bound amino acids of spruce budworm larvae feeding on balsam fir and red and white spruce

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Levels of free and to a lesser extent bound amino acids in larvae of the spruce budworm ( Choristoneura fumiferana (Clem.)) varied significantly with food source (host species) and date of sampling (instars V and VI). Amino acids most prominent and occurring free were glutamine, glutamic acid, alanine, and proline, whereas those bound and released by acid hydrolysis were mainly aspartic and glutamic acids. Arginine contributed most to the protein N of larvae and in the host species was the main storage form of nitrogen. Percentages of twelve free amino acids including ornithine, glutamic and γ-aminobutyric acid, lysine, valine, leucine, and isoleucine reflected significantly food sources, sampling dates, and their interactions. When feeding stopped (instar VI), levels of most bound amino acids in red spruce and fir larvae increased/unit weight as levels of free amino acids dropped. Although percentages of most bound amino acids were unaffected by diet, significant differences were found with glutamic acid, histidine, and threonine. Compared to white and red spruce, balsam fir, the preferred host in the sampled area, provided larvae with the highest levels of free and bound amino acids and greatest size.