RELEASE AND ESTABLISHMENT OF HYDRELLIA-BALCIUNASI (DIPTERA, EPHYDRIDAE) FOR THE BIOLOGICAL-CONTROL OF THE SUBMERSED AQUATIC PLANT HYDRILLA-VERTICILLATA (HYDROCHARITACEAE) IN THE UNITED-STATES

The Australian leaf-mining fly, Hydrellia balciunasi, was first releas ed as a biological control agent for the management of the submersed a quatic plant, Hydrilla verticillata in 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. balciunasi levels have remained relative ly low at these sites, averaging, at best, less than 1000 immatures/kg wet plant weight. All other collections were substantially lower. Rea sons for poor establishment are unknown but probably relate to a compl ex of factors, including competition with another introduced leaf-mini ng species, parasitism by native wasps, poor host quality, genetic dif ferences between Australian and United States hydrilla resulting in mi smatched physiological strains of agent and host plant, and possibly i nbreeding depression in laboratory colonies used for releases. (C) 199 7 Academic Press.