Research Reports: Population Increases of Planthoppers on Fertilized Salt-Marsh Cord Grass may be Prevented by Grasshopper Feeding

Abstract

We fertilized four separate areas of salt-marsh cord grass, Spartina alterniflora, in northwest Florida to examine the effect of increased foliar nitrogen on herbivorous insects. Two of the experimental plots were located where Spartina grew as a shoreline fringe; two others were pure isolated offshore islets of Spartina. Application of fertilizer in winter and early spring resulted in significant increases in foliar nitrogen through the following August at all sites. The two most common and damaging herbivores throughout the growing season were the planthopper Prokelisia marginata (Homoptera: Delphacidae) and the grasshopper Orchelimum fidicinium (Orthoptera: Tettigoniidae). Populations of planthoppers on islets responded positively to Spartina fertilization. The increase in Prokelisia densities disappeared in August shortly after nitrogen levels of fertilized islets dropped to approximate those of controls. In the fall, planthopper populations on islets increased dramatically, despite low nitrogen levels. On mainland patches, Prokelisia densities did not always increase on fertilized patches, and there were no large population outbreaks of Prokelisia in the fall. Densities of grasshoppers increased on mainland fertilized patches, whereas on offshore islets grasshoppers were rarely observed. These observations are consistent with the idea that grasshoppers can depress Prokelisia populations. Cage experiments confirmed that grasshoppers can reduce the numbers of Prokelisia. Simulation of grasshopper feeding by clipping of the tops of Spartina plants also reduced Prokelisia densities. On offshore islets, because grasshoppers were very rare, they did not have a sufficient effect to depress Prokelisia densities.