Monoecious hydrilla tuber dynamics following various management regimes on four North Carolina reservoirs

摘要

Hydrilla [Hydra la verticillata (L.f.) Royle] is a federally listed noxious weed that has quickly spread through much of the United States. Long-term hydrilla control is complicated by persistent subterranean turions (tubers) that have been shown to remain viable for at least 6 yr. Tuber bank elimination is essential for long-term management or eradication efforts. Research was conducted on four North Carolina reservoirs to evaluate monoecious hydrilla tuber dynamics and to determine the effects of specific management techniques on monoecious hydrilla tuber densities over time. Lake Gaston, Lake Tillery, Shearon Harris Lake, and the Tar River Reservoir were sampled for up to 7 yr. Management practices and their effects on tuber density were assessed on each lake. Fluridone treatment sites were assessed on Lakes Tillery and Gaston, whereas a combination of fluridone application, drought-induced summer drawdown, and late-stage triploid grass carp stocking was assessed on the Tar River Reservoir. Sites on Lake Gaston and Shearon Harris Reservoir with no active management were also monitored. Dewatering (2007 only) and fluridone application from 2007 through 2012 plus a low-density grass carp stocking in 2013 resulted in a 100% tuber density decrease in the Tar River Reservoir. Two tubers recovered in fall 2012 were assumed to be 6 yr or older, and were still viable. On the unmanaged Shearon Harris Reservoir, average whole-lake tuber densities ranged from 838 to 2,050 tubers sq m(-1) from 2008 to 2013. Lake Gaston sites subjected to fluridone treatment every other year demonstrated a tubers m(-2) bank reduction of 28% after 2 yr and 63% after 4 yr. Conversely, Lake Gaston sites that were treated consecutively exhibited a 75% tuber density reduction in 2 yr and a 93% reduction after 3 yr. Based on these data it would take five alternate-year treatment cycles to match the tuber reduction reached in three consecutive-year treatments. Tuber densities as low as 11 m(-2) were adequate for a significant recovery in biomass and a 1,136% increase in new tuber production in just one season. Results suggest that by managing the tuber bank there is the potential to conserve management resources by switching to less intensive and costly strategies when densities are deemed to be at a nonnuisance level.