Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 399

Figure 8.2 Effects ofcompetition from subterranean clover and infection by
Puccinia chondrillina on biomass production of Chondrilla juncea in a glasshouse pot
experiment.(Adapted from Groves & Williams,1975.)
clipped to a height of 5 cm, or left unaltered. Cages were used to regulate S.
jacobaea exposure to two herbivores: the moth Tyria jacobaeae , and the flea
beetle Longitarsus jacobaeae. In addition to caging, herbivores were removed
from “protected” treatments using the insecticide rotenone. The moth and
beetle fed on S. jacobaea at different times of year, so it was possible to vary
attack by the two herbivore species independently, by opening and closing
cages at different times.
Without competition from desired grasses, L. jacobaeae was incapable of
eliminating S. jacobaea from the pasture . Plant competition alone eliminated
actively growing S. jacobaea individuals in four years, but the combination of
competition and herbivory by L. jacobaeae eliminated all S. jacobaea individuals
except those in the buried seed bank within two years. Tyria jacobaeae reduced
S. jacobaea seed production, but had no detectable effect on the weed's biomass
production during four years of measurements.
Because both the total removal of competitors and their partial suppres-
sion by clipping helped maintain S. jacobaea populations, even when L. jaco-
baeae was present, this experiment strongly suggests the importance of
integrating grazing practices with weed biocontrol agents. If grazing by