We report the use of RNAi in the pea aphid, Acyrthosiphon pisum, to the study the function of a novel secreted protein, C002, expressed in the principal salivary gland. This highly expressed transcript in the gland has a unique sequence having no sequence similarity to other sequences in Blast searches, leaving it as a unknown function. In order to study the function of this transcript, we performed RNA interference (RNAi) to investigate the hypomorphic phenotype. Pea aphids were injected with C002 siRNA. GFP siRNA served as a negative control. The level of C002 transcript decreased in a time-dependent manner, leading to nearly complete depletion of transcript at 96 hours after injection. Over 90% of the aphids injected with C002 siRNA die within 96 hours of injection. There was a dramatic effect on the reproduction of aphids. The average number of nymphs produced per day in C002 siRNA injected aphids was 1.2 +_ 0.3 compared to 4.5 +_ 0.4 in the control. The data suggest that C002 plays a role in the feeding of the pea aphid.