Insulin in vertebrates plays a crucial role in maintaining homeostasis of blood sugar level. Insulin-like peptide (ILP) has been identified in insects, such as Drosophila melanogaster and Aedes aegypti. Plasma sugars and polyols of the diamondback moth, Plutella xylostella were separated by a Bio-LC. Among seven peaks, trehalose was the most predominant blood sugar and maintained at approximately 3.5 mM in the larval plasma. However, the feeding activity affected the plasma trehalose level, in which starvation significantly up-regulated the trehalose level. Analysis of ILP expression upon feeding indicated that feeding stimulated the gene expression of ILP. Interestingly, an injection of a vertebrate insulin significantly suppressed the hypertrehalosemia induced by starvation. These results suggest that ILP is a endocrine signal to down-regulate the plasma trehalose level in P. xylostella.