AlthoughArgentinorhynchus bruchi(Hustache) from northern Argentina and Paraguay is extremely rare, it has the potential to cause heavy damage to waterlettuce (Pistia stratiotesL.). In the laboratory, adults ate 1 cm2of leaf surface/day. Field-collected females laid an avg 1575 eggs each on the leaf surface after they were brought into the laboratory. The eggs hatched in 7.6 days. First instars fed within the spongy leaf tissue, and 2nd and 3rd instars fed in the crown. Larvae developed through the 3rd instar in 12 days. The 4th instars could not be reared; they always exited from the plant and drowned in the water of the container. Adults fed and oviposited only on waterlettuce (except for slight feeding onSpirodela) among 26 plant species tested. Larvae fed slightly on 6 species of plants, but all died within 7 days except those on waterlettuce. In the laboratory, 6, 8, or 10 larvae/plant killed waterlettuce within a month.