Hydrellia pakistanae, a biological control agent of hydrilla, was first introduced into Lake Seminole in 1990. Impact by this species remained low until 1998 when large numbers of individuals and associated impact, as well as increases in native plant diversity, were observed in several locations. In 1999, large-scale reductions in hydrilla were observed throughout many areas of the lake. These changes were correlated with changes in insect numbers where large increases in fly populations were correlated with decreases in tube numbers and increases in species richness. While insect numbers were reduced in 2000, significantly decreases in biomass and increases in native plant diversity were observed when numbers of immatures and/or feeding damage were high. While the evidence presented herein indicates that the flies played a major role in the hydrilla decline on Lake Seminole, other factors may have contributed to the observed reductions. This includes such obvious changes as decreases in lake levels brought on by drought that may have increased light penetration in shallower areas, thus allowing for increased growth of natives. More likely, a complex of factors, including IL pakistanae feeding damage, contributed to the observed changes in hydrilla status on Lake Seminole.