Adult and larval aphidophagous lady beetles (Coleoptera: Coccinellidae) feed on alfalfa weevil larvae, Hypera postica (Gyllenhal), and aphids in alfalfa fields. We studied the development of first and fourth instars of two lady beetle species ( Coccinella septempunctata L. and Harmonia axyridis Pallas) when provided live or dead weevil larvae versus pea aphids, Acyrthosiphon pisum (Harris). No individuals of either species survived to the second stadium on a diet of weevils (dead or alive), whereas almost 100% of first instars of both species did so on a diet of aphids. Nearly all individuals of H. axyridis that were provided weevils (dead or alive) during the fourth stadium survived and subsequently completed pupation. However, these individuals weighed significantly less as newly molted adults than did conspecifics that had fed on aphids as fourth instars. Only 5% of fourth-instar C. septempunctata survived to adulthood when provided live weevils (versus 100% provided aphids), but 70% did so when provided dead weevils. Both first and fourth instars of both species attacked both dead and live weevil larvae much less readily than aphids, and were often deterred from persisting in the attack by the defensive wriggling of live weevil larvae (in contrast to their persistence and success in attacking aphids). Nevertheless, even first instars of the predators succeeded in some instances in overcoming weevil larvae, and proceeded to feed upon the prey for up to the maximum of 10 min of observation. Our results suggest that although weevil larvae are not very suitable as substitute prey for aphids for the larvae of C. septempunctata and H. axyridis, consumption of weevil larvae especially by older larvae of the predators (particularly H. axyridis) may enable them to complete development in alfalfa fields even when aphid populations fall to low levels.