Release and Establishment ofHydrellia balciunasi(Diptera: Ephydridae) for the Biological Control of the Submersed Aquatic PlantHydrilla verticillata(Hydrocharitaceae) in the United States

The Australian leaf-mining fly,Hydrellia balciunasi,was first released as a biological control agent for the management of the submersed aquatic plant,Hydrilla verticillatain August 1989 in southern Florida. Since then, 284,684 individuals have been released at seven Florida and four Texas sites. Despite this effort, populations have persisted at just two Texas sites: Sheldon Reservoir near Houston and Lake Raven in Huntsville State Park.H. balciunasilevels have remained relatively low at these sites, averaging, at best, less than 1000 immatures/kg wet plant weight. All other collections were substantially lower. Reasons for poor establishment are unknown but probably relate to a complex of factors, including competition with another introduced leaf-mining species, parasitism by native wasps, poor host quality, genetic differences between Australian and United States hydrilla resulting in mismatched physiological strains of agent and host plant, and possibly inbreeding depression in laboratory colonies used for releases.