Status of Hydrellia spp. (Diptera: Ephydridae) Release Sites in Texas as of December 1998.

Abstract

Abstract : Beginning in 1991 and continuing through the present, two species of introduced Hydrellia (i.e., H. balciunasi and H. pakistanae) were released and their populations monitored at several sites in eastern and southeastern Texas. Currently, populations of the introduced Hydrellia spp. appear to be established throughout eastern Texas. Hydrellia pakistanae is more widespread and found in higher numbers at most sites relative to H. balciunasi. Levels of H. pakistanae, although low at many locations, seem to be expanding in range with several new populations being discovered in areas where no releases have previously been made. These include several ponds at the Lewisville Aquatic Ecosystem Research Facility and on the Rio Grande near Brownsville, TX. Hydrellia balciunasi also appears to be expanding in range, although at a slower pace than H. pakistanae. New populations of H. balciunasi were observed at lake Cypress Springs near Mt. Vernon, TX, during 1997. At this location, releases of H. pakistanae were made during 1997, but only H. balciunasi was recovered during sampling conducted during 1998. While populations of either species of Hydrellia spp. appear low at most locations, at some sites leaf-mining fly populations may have reached high enough levels to impact of hydrilla. For example, at Coleto Creek Reservoir, the hydrilla was substantially reduced at two of the release sites during 1998 relative to what was observed during past sampling endeavors. Evidently, populations of H. pakistanae were high enough to impact the hydrilla at these sites since no other reason explaining the observed declines in the hydrilla is evident. Another location where Hydrellia spp. may be stressing the plants based on observed damage is Lake Raven at Huntsville State Park.