Manduca diuresin (MD), a 30 amino acid peptide, was isolated and identified from the type IIb median neurosecretory cells in the brain of adult Manduca sexta (Blackburn et al., Biochem. Biophys. Res. Commun., 181, 927-932, 1991). The synthetic hormone stimulates water loss in the adult moth both in vitro using isolated Malphigian tubule preparations and in vivo using decapitated adult moths as bioassay animals (Blackburn and Ma, Arch. Insect Biochem. Physiol., 27, 3-10, 1994). In the present study, MD was shown to have adverse physiological effects on neonates of Manduca sexta when fed synthetic hormone-treated leaf discs. With a sustained diet of MD-treated discs, neonates exhibited symptoms of reduced food consumption and marginal weight increases. In addition to slowed growth and reduced developmental rates, a high percentage of these neonates failed to molt into second instar larvae and death usually followed shortly thereafter. This is a first report that a synthetic insect neuropeptide has shown detrimental effect on insect larvae by ingestion. The possibility of utilizing this peptide in plant protection is discussed.