The effect of oxadiazolyl 3(2H)-pyridazinone (ODP), a new insect growth regulator, on growth of larvae of the armyworm, Pseudaletia separata Walker (Lepidoptera: Noctuidae) was evaluated in comparison to the insecticide, toosendanin, a tetranortriterpenoid extracted from the bark of Melia toosendan that has multiple effects on insects. The digestive physiological properties of these compounds on insects were investigated by feeding them maize leaves dipped in these compounds. The results showed that ODP inhibited the growth of P. separata significantly, causing a slowed development and a prolonged larval period, smaller body size and sluggish behavior, delayed pupation and a reduced eclosion rate of pupae and adults. Moreover, ODP strongly inhibited the activities of weak alkaline trypsine-like enzyme, chymotrypsin-like enzyme and alpha amylase in the midguts of fifth instar P. separata larvae, in vivo, and inhibited the activity of alpha amylase, in vitro. These data suggest that ODP has severe consequences on the larval carbohydrate assimilation and/or nutrient intake and thereby causes inhibition of larval growth. The regulatory action of ODP on larval growth development was similar to that of toosendanin; both could be used to decrease the growth of insect populations.