How FACs-producing generalist and specialist herbivores regulate their FACs-hydrolyzing enzyme L-ACY-1 to balance FACs' beneficial vs. detrimental effects remains unknown. To address this question, we compared L-ACY-1 expression in Helicoverpa armigera and Helicoverpa assulta, a pair of closely related sibling species differing mainly in their host range, by the same sets of hostplants, protein to digestible carbohydrate (P:C) ratios, or allelochemical. L-ACY-1 expression remained low/unchanged in H. armigera, but was induced by hot pepper fruits and repressed by cotton bolls in H. assulta. The representative allelochemicals of the tested hostplants significantly (capsaicin) or insignificantly (gossypol and nicotine) induced L-ACY-1 expression in H. armigera, but insignificantly inhibited (capsaicin and gossypol) or induced (nicotine) it in H. assulta. L-ACY-1 expression remained low/unaltered on balanced (P50:C50 and P53:C47) or protein-biased diets and induced on carbohydrate-biased diets in H. armigera, but was at the highest level on balanced diets and reduced on either protein- or carbohydrate-biased diets in H. assulta. Furthermore, L-ACY-1 expression was significantly higher in H. assulta than in H. armigera for most of feeding treatments. Such expressional divergences suggest that FACs are utilized mainly for removal of excessive nitrogen in generalists but for nitrogen assimilation in specialists.