The weevils Argentinorhynchus breyeri, A. bruchi and A. squamosus (Coleoptera: Curculionidae), candidates for the biological control of waterlettuce (Pistia stratiotes).

Eleven species of apparently monophagous weevils, genera Neohydronomus, Pistiacola and Argentinorhynchus, occur in South America plus 7 species of oligophagous insects of other groups. Biological control of waterlettuce was successful in Australia and South Africa using the weevil Neohydronomus affinis Hustache from South America. In the US, N. affinis was released in Florida during April 1987 but after a promising beginning, it failed to provide control. Other insects with potential for BC are the aquatic weevils Argentinorhynchus spp. with six species (presumed strictly monophagous) in the Neotropical Region from south-central Mexico to Argentina. Three species occur in Argentina, A. bruchi Hustache, A. breyeri Brethes and A. squamosus Hustache. All adults feed at night cutting round holes in the leaves. Larvae of the three species have different food niches: A. breyeri feed on leaf primordia of the center of Pistia rosettes, A. bruchi tunnels the upper part of the rhizome and A. squamosus feeds in the lower rhizome. Pupation is the most critical stage of the cycle and until now, it was a mystery for all species of Argentinorhynchus. The full grown larva abandons the plant and buries itself in the bottom of the water body. The further dessication of the pond seems important for the normal development of the pupa in the subterraneous cell. This implies an adaptation for sporadic population of waterlettuce. In the laboratory, A. breyeri showed potential to kill plants if enough larvae develop in one plant.