Biology and preliminary host range of Hydrotimetes natans Kolbe (Coleoptera: Curculionidae), a natural enemy candidate for biological control of Cabomba caroliniana Gray (Cabombaceae) in Australia

Cabomba caroliniana, otherwise known as cabomba, is a submerged, rooted macrophyte with heavily dissected leaves that produces flowers that extend above the water surface. It has been disseminated around the world by aquarium traders and enthusiasts, and has become a noxious weed in numerous countries, including Australia, where it often produces dense monospecific stands. It is considered native to southern parts of North and South America, and is a common plant in floodplain streams and lakes in north-east Argentina. Because of the lack of alternative control methods, a search for biological control agents was initiated in 2003. The most promising natural enemy found during surveys of C. caroliniana in Argentina was the aquatic weevil Hydrotimetes natans. The entire life cycle is completed on the plant, primarily underwater, except when the adults climb onto emergent flowers to mate. Larvae mine the stems, pupal cases are constructed in the leaf axils where the petiole joins the stem, and adults feed on the leaves and the stems, primarily at the growing tips. Field surveys of C. caroliniana and other submerged plant species, as well as results from preliminary laboratory host range trials suggest that the weevil is specific to cabomba. Adults were found only on other plant species when intertwined with C. caroliniana in the field and did not move onto other plant species in aquaria trials. The distribution of the larvae in the field was studied in relation to depth and plant size. No preferences for stem width or plant length were found, although a significant portion of the larval mines were located near the root crown of the plant. This information on the host specificity and biology of H. natans in its native range may guide researchers studying the laboratory fundamental host range and determining the potential effectiveness of the weevil for biological control of cabomba in its introduced range.