Abstract: In this study the ability of gut extracts from the larva and adult of cotton bollworm, Helicoverpa armigera and armyworm, Mythimna separata to inhibit juvenile hormone (JH) synthesis was tested in vitro by radiochemical assay. The allatostatin (AS)like material that can inhibit JH synthesis in corpora allata (CA) occurred in the larvae and adult guts of these two species of insects. The inhibition of JH synthesis by 1 gut equivalent extract from larvae M.separata (43%) was greater than that from H. armigera (26%). Using 5 midgut equivalent of female or 10 midgut equivalent of male, the inhibition of JH synthesis achieved about 47% for the cotton bollworm and 35% for the armyworm. This indicates that the content of ASlike material in the gut from female is higher than that from male in both H. rmigera and M. separata. The inhibition of JH synthesis of CA was significantly higher by midgut extract than by hindgut extract in both insects. The inhibition of CA activity by midgut extract was completely re

Key words: Helicoverpa armigera, Mythimna separata, allatostatin, gut