The Weevils Argentinorhynchus breyeri, A. bruchi and A. squamosus (Coleoptera: Curculionidae), Candidates for the Biological Control of Waterlettuce (Pistia stratiotes)

H.A. CORDO; A.J. SOSA

Simposio; X International Symposium on Biological Control of Weeds; 1999

Eleven species of apparently monophagous weevils, genera Neohydronomus,Pistiacola and Argentinorhynchus, occur in South America plus 7 species of oligophagousinsects of other groups. Biological control of waterlettuce was successful in Australia andSouth Africa using the weevil Neohydronomus affinis Hustache from South America. Inthe 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 theNeotropical Region from south-central Mexico to Argentina. Three species occur inArgentina, A. bruchi Hustache, A. breyeri Brethes and A. squamosus Hustache. All adultsfeed at night cutting round holes in the leaves. Larvae of the three species have differentfood niches: A. breyeri feed on leaf primordia of the center of Pistia rosettes, A. bruchitunnels 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 allspecies of Argentinorhynchus. The full grown larva abandons the plant and buries itself inthe bottom of the water body. The further dessication of the pond seems important for thenormal development of the pupa in the subterraneous cell. This implies an adaptation forsporadic population of waterlettuce. In the laboratory, A. breyeri showed potential to killplants if enough larvae develop in one plant.