IMPACT OF HIGH HERBIVORE DENSITIES ON INTRODUCED SMOOTH CORDGRASS, SPARTINA-ALTERNIFLORA, INVADING SAN-FRANCISCO BAY, CALIFORNIA

Spartina alterniflora, smooth cordgrass, invading San Francisco Bay, C alifornia (USA), is attacked by high densities of a plant hopper, Prok elisia marginata, and a mirid bug, Trigonotylus uhleri. Both herbivore s are sap-feeders. We investigated the impact of these herbivores on S . alterniflora's growth rate, vegetative spread, and seed production b y manipulating herbivore densities in the field and in a greenhouse. H erbivore densities in the field peaked in early fall, with P. marginat a averaging more than 300 individuals per mature culm of S. alterniflo ra (about 100,000 per m(2)) and T. uhleri densities exceeding 10 per c ulm (about 3,000 per m(2)). Field reductions of herbivore densities by approximately 70% with insecticidal soap did not result in greater ve getative growth rates or lateral spread of plants; plants grew vigorou sly with the highest densities of insects. In the greenhouse study, co nducted with seedlings, herbivory significantly reduced plant mass and tiller number in some but not all replicate herbivory treatments. In both field and greenhouse, there were significant differences between some clones' growth rates independent of herbivory. Inflorescence prod uction in the held was not affected by reduced-herbivory treatments. S eed set was low under conditions of both natural and reduced herbivory , averaging 0.4%. Despite densities of P. marginata and T. uhleri that are much higher than typically observed in areas where S. alterniflor a is native, herbivory by these particular insects appears to have lit tle impact and in unlikely to limit S. alterniflora's spread through S an Francisco Bay.