The Southeast Asian citrus-feeding butterfly Princeps (Papilio) demoleus (L.) was recently introduced into the Americas, causing an imminent threat to citrus production and ornamental flora. The human nutrient amino acid methionine has been shown by us to disrupt aminoacid-modulated ion transport systems in caterpillars and other insect larvae that possess an alkaline midgut. Heraclides (Papilio) cresphontes was bioassayed as a United States Department of Agriculture permitted surrogate of the Florida quarantined P. demoleus to test the potential efficacy of methionine. Larvae were allowed to feed ad libitum on wild lime plants with leaves treated with methionine or proline. Methionine caused 100% mortality in first through fourth instars in a time- and dose-dependent manner, as determined by probit analysis whereas proline was not toxic. Wild lime host plants did not exhibit phytotoxicity with methionine treatments during a 14 d test period. It is concluded that methionine is an effective larvicide against H. cresphontes, and therefore may be a candidate environmentally safe biorational pesticide for use against invasive P. demoleus in the Americas.