Biological control of Spartina alterniflora in Willapa Bay, Washington using the planthopper Prokelisia marginata: agent specificity and early results

Spartina alterniflora (smooth cordgrass) is introduced and invasive in Pacific Coast estuaries of North America. Its invasion transforms unvegetated intertidal mudflats to grass-covered marshes, eliminating habitat for birds, fish, and native and cultivated shellfish that depend on the open mudflats. The delphacid planthopper Prokelisia marginata was recently introduced into Willapa Bay, Washington for biological control of this grass. Prior to its introduction, we demonstrated the narrow host range of P. marginata with no-choice tests in the greenhouse using 23 potential species of nontarget plants, including species of native and otherwise valuable grasses and cranberry, Vaccinium marcrocarpon. P. marginata was capable of completing its life cycle only on S. alterniflora, S. anglica (also a noxious weed in Washington State), and S. foliosa (California cordgrass, native southward from San Francisco). Based on these results, we found no evidence of risk to nontarget plants in Washington State from P. marginata. The first release of P. marginata was made in Willapa Bay in August 2000, and the planthopper survived the winter at all three sites. Following additional releases in early summer of 2001 of 65,000 individuals at each site, population densities increased an average of 4.34±1.71-fold in one generation. The populations had spread 200 m from the release area by October, 2001. Macropterous (long-winged) individuals were more common (69%) within 5 m of the release center, while brachypters (reduced-winged) were more frequent (71%) at distances greater than 5 m from the release area. In field cages, P. marginata reduced S. alterniflora biomass by 50% and plant height by 15% in comparison to planthopper-free controls. These results represented short-term impacts at a localized scale. The ultimate success of this biocontrol program over wider spatial scales will only become known over a longer time period.