Cc. Daehler et Dr. Strong, IMPACT OF HIGH HERBIVORE DENSITIES ON INTRODUCED SMOOTH CORDGRASS, SPARTINA-ALTERNIFLORA, INVADING SAN-FRANCISCO BAY, CALIFORNIA, Estuaries, 18(2), 1995, pp. 409-417
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.