Abstract BACKGROUND: The activity of the major digestive cysteine proteinase detected in the intestinal tract of larvae of the bean weevil, Acanthoscelides obtectus (Say), was efficiently inhibited by the well‐characterized cysteine proteinase synthetic inhibitor E‐64 and also by a recombinant form of chagasin (r‐chagasin), a tight‐binding cysteine proteinase inhibitor protein from Trypanosoma cruzi . RESULTS: Incorporation of r‐chagasin into an artificial diet system at 0.1 g kg −1 retarded growth rate, decreased larval survival and led to complete mortality of A. obtectus at the end of the trial. The observed differences in growth rates occurred particularly in the first and second development stages. Artificial seeds containing high levels of r‐chagasin (0.5–30 g kg −1 ) completely inhibited larval penetration. CONCLUSION: Together, the results reported in this paper support the hypothesis that the inhibitory activity of r‐chagasin towards the major insect gut cysteine proteinase in vitro and in vivo is an accurate prediction of its insecticidal effects. The selectivity of this inhibitor against insect digestive proteinases supports the key role in parasite virulence by affecting the endogenous proteinase activity in its natural host. Copyright © 2008 Society of Chemical Industry