Damage to cordgrass by scale insects in a constructed salt marsh: Effects of nitrogen additions

Because tall cordgrass (Spartina foliosa) is needed for nesting by the endangered light-footed clapper rail, managers of constructed salt marshes in southern California are proposing large-scale nitrogen fertilization to improve cordgrass growth. How this might affect an existing infestation of scale insects (Haliaspis spartina) and the degree of damage these insects cause to their cordgrass hosts was unknown. We explored the effects of timing and duration of fertilization on Haliaspis damage to cordgrass, as well as the timing of Haliaspis dispersal, in a constructed marsh at Sweetwater Marsh National Wildlife Refuge in San Diego Bay, California. Fertilization did not result in increased Haliaspis abundance. After a large dispersal pulse in late May, Haliaspis establishment in the long-term fertilized plots was greater than in the controls; however, this trend reversed in August, when many more stems in the control plots were infested with large numbers of Haliaspis. Since adult Haliaspis cannot leave a feeding site, losses of individuals in the fertilized plots were apparently due to mortality, perhaps resulting from mechanical or chemical changes in the fertilized plants or increased predation. Late in the growing season, plots fertilized with 10 applications of urea over 20 wk had the lowest mean Haliaspis abundance. Plots fertilized only in March, April, June, or August did not differ from controls in mean Haliaspis abundance. Haliaspis was never abundant in the fertilized or control plots in the adjacent natural marsh. This study suggests that fertilization of constructed salt marshes in San Diego Bay may proceed without concern that further Haliaspis outbreaks will be facilitated.