A synoptic review of Tamarix biocontrol in North America: tracking success in the midst of controversy

Woody shrubs in the genus Tamarix L. (Tamaricaceae) were introduced into western North America in the nineteenth century and have invaded riparian areas, acting as drivers of ecosystem change by altering fire cycles, soil chemistry, hydrology and native plant composition. The scope and severity of the invasions provided impetus for a classical weed biological control program using Diorhabda spp. (Coleoptera: Chrysomelidae). Since the first releases in 2001 Diorhabda spp. have moved into many of the areas dominated by Tamarix resulting in defoliations, canopy dieback, and in some locations substantial Tamarix mortality. Success of the program has been overshadowed by concern that Tamarix is used by a federally-listed bird sub-species, the southwestern willow flycatcher. The controversy has led to lawsuits, cancelled biological control research and release permits and to a negative perception of Tamarix biocontrol by some. Long term success is likely, but only with continued monitoring and riparian restoration will the program reach its full potential.