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A series of studies evaluating the impact, dynamics, and distribution of the established biological control agent Hydrellia pakistanae (Diptera: Ephydridae) on Hydrilla verticillata (Hydrocharitaceae) was conducted. The studies included establishment of fly damage thresholds with artificial infestations of flies in tanks, intensive monthly field monitoring of fly densities and damage at one site, and seasonal monitoring of fly densities in six regions in Florida. The results of tank studies indicated that fly damage approached an asymptote in the top 20 cm of the hydrilla canopy when infested with 4000 larvae/m 2 . Above this level only a slight increase in hydrilla damage occurred as fly densities increased. Fly damage was concentrated in the top 20 cm (84.5%) and in the meristems (14.1%) of the hydrilla canopy. Hydrilla biomass was reduced with higher fly densities in the top 20 cm stratum when the plants were grown at the low fertilizer treatment. Field collections made in south Florida indicated that the hydrilla nitrogen content (fresh mass) was a major limiting factor that influenced fly densities. In the more northern regions of our studies, central and northern Florida, cold winter weather limited the seasonal distribution of the host plant and associated flies. In response to increased seasonal temperatures, fly densities increased each spring and summer. Consequently, field densities of flies and the associated damage to hydrilla populations never reached more than 15 adults/m 2 and 15% of the whorls damaged, respectively, about one-fifth the level estimated from cage studies that severely impact plant biomass. |
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