Plants have evolved sophisticated defense mechanisms, most of which are concentrated in the seeds since these are the vehicles for propagation and survival of the species. Seed tissues may accumulate, constitutively or after induction, a wide array of defensive compounds that confer resistance against phytophagous predators. In this work, we investigated the effect of crude extract of T. esculenta seeds on A. gemmatalis development, a lepidoptera insect that is a serious pest of soybeans. Artificial diets containing crude extract at concentration of 2% (w/w) were obtained. Larvae of fourth instar submitted to this diet, were analyzed in terms of weight, number of survival and nutritional analysis (n = 60). The proteolytic activity of midgut and feces was determined using BApNA as substrate. The crude extract at 2% didn't have effect on the survival but, reduced the weight of the larvae in 50% and reduced the nutritional indices ECI and ECD, in terms of 17.4 and 34.8% respectively. A drop in ECI indicates that more food is being metabolized for energy and less is being converted to body substance (i.e., growth). ECD also decreases as the proportion of digested food metabolized for energy increases. However, MC and AD increase about 10.5 and 21% respectively in relation to the control, suggesting the antifeedant activity of the extract. When the larvae were exposed to crude extract (2%, w/w) resulted in significantly increased levels of trypsin-like activity about 200 and 70% in the midgut and feces respectively, compared to controls. A change in the enzymes recycling mechanism may provide an alternative explanation for the observed increases in the tryptic activity of faecal extracts. These enzyme activities can also leads to poor nutrient use and retarded development larval. More complex assays should be accomplished to best understand the mechanism action of this crude extract.