Comparison of the Physiological and Realized Host-Ranges of a Biological Control Agent from Australia for the Control of the Aquatic Weed,Hydrilla verticillata

Evaluating the safety of a potential weed control agent usually involves laboratory host-range tests. However, while these laboratory tests may define the “physiological” range of hosts upon which an organism can feed and develop, many researchers are unconvinced that this is necessarily predictive of the “realized” range of hosts upon which that organism will develop in the field. We present the results of our laboratory and field studies in Australia of a stem-boring weevil,Bagous hydrillae(Coleoptera: Curculionidae), a biological control agent for the submersed weed, hydrilla,Hydrilla verticillata.During these studies, the same array of hosts was evaluated in the laboratory and the field. Laboratory studies indicated that in addition to hydrilla (the primary host), the physiological range included 16 feeding hosts and 11 oviposition hosts. However, during our field surveys, which involved extracting herbivores from 1532 quantitative collections of 48 different aquatic plant species, we found the weevil only on 7 plant species besides hydrilla, and on only one of these (Vallisneria spiralis) was it ever common. Based solely on laboratory tests,B. hydrillaeʹs safety as a biocontrol agent was uncertain. The narrow host-range indicated by extensive field data from Australia was a key factor in obtaining permission for the release of this weevil in Florida.