The History and Future of Biocontrol in Riparian Areas, the 20/20 Perspective
 
Dan Bean1, Tom Dudley2
 
1Colorado Department of Agriculture, Palisade Insectary, Palisade, CO, USA; dan.bean@state.co.us
2Marine Science Institute, University of California, Santa Barbara, CA, USA
 
Biological control of invasive plants is ecological in nature, increasing stress on the target plant by reintroducing the target to natural enemies.  Biological control diminishes the ability of the target to outcompete native plants, offering resources managers assistance in target control.  By its nature, biological control takes time and should be incorporated into management plans with long term goals, making it ideally suited for a 20/20 perspective. We will discuss two plants that have invaded riparian ecosystems as examples of biological control targets and how biological control can be a valuable component of riparian resources management.  The first target was tamarisk or saltcedar, a group of related species in the genus Tamarix. Tamarisk biocontrol began just over 20 years ago when the northern tamarisk beetle, Diorhabda carinulata, was released at seven sites in western North America. Over the first twenty years beetles moved, either naturally or with human assistance, through most tamarisk infested river basins in North America.  The impact on tamarisk density has been variable, surpassing 40% mortality in some locations while resulting in little or no mortality in others.  Tamarisk biological control is most valuable as part of larger scale management strategies.  Russian knapweed, Rhaponticum repens, has invaded some of the same watersheds that tamarisk has.  Two gall-forming biological control agents are being used against Russian knapweed, a gall midge Jaapiella ivannikovi and a gall wasp Aulacidea acroptilonica.  In areas where the agents have become well established, they have had a visible impact on the structure of knapweed plants.  Monitoring is underway to determine the long-term impact of the two agents on plant density which will used to inform resources managers of the potential value in managing knapweed. Over the next twenty years we expect that biological control will provide a target-specific means to diminish the competitive ability of invasive riparian plants, benefitting management of invaded riparian ecosystems.