Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 385

When the match between a weed, its imported natural enemies, and the
environment is correct, the results can be quite spectacular. In Australia,
where various species of the New World cactus Opuntia escaped from cultiva-
tion and heavily infested rangeland,importation and release of insects for bio-
logical control were initiated in the 1920s (Grossman,1989; Julien & Griffiths,
1998). Among the agents that proved effective were a moth ( Cactoblastis cac-
torum ) from Argentina,whose larvae mine and destroy pads of the cactus; scale
insects ( Dactylopius austrinus , D. ceylonicus , D. opuntiae , and D. tomentosus ) from
Argentina,Brazil,India,Sri Lanka,and the USA,which attack cactus pads and
fruits; a bug ( Chelinidea tabulata ) from the USA, which attacks new shoots and
fruits; and a beetle ( Archlagocheirus funeatus ) from Mexico, whose larvae feed in
woody stem tissue and cause collapse of adult plants.
Within a decade of their release in Australia, a sufficient number of these
agents were established to maintain effective biological control of Opuntia
cacti, and millions of hectares once considered useless for grazing were ren-
dered productive (Grossman, 1989). For certain control agents used against
Opuntia spp., seasonal reinoculation is still necessary in some locations. In
New South Wales, for example, low winter temperatures reduce populations
of Dactylopius austrinus to a level at which no control of Opuntia aurantiaca is
achieved. To overcome this, augmentative releases of D. austrinus are carried
out in the spring by government employees (Tisdell,Auld & Menz, 1984).
Successful biological control of a weed through the introduction of multi-
ple herbivore species attacking different life stages has also been accom-
plished in agricultural regions of South Africa infested by the South American
tree Sesbania punicea (Moran, 1995). The bud-feeding weevil Trichapion lativen-
tre was imported, released, and established in the 1970s, and found capable of
reducing pod production of the target weed by

98%.The impact of T.lativen-
tre was enhanced by introduction of a second weevil species, Rhyssomatus mar-
ginatus ,whose larvae destroy the seeds of S.punicea. The combined action of the
two agents lowered S. punicea fecundity by 99%, but little effect was evident

on the density of mature plants.Athird weevil, Neodiplogrammus quadrivittatus ,
which feeds in the branches and stems of mature host plants, was then intro-
duced in 1986 from Brazil.Although it was ineffective against juvenile stages,
it proved to be quite deadly against adult trees. Combination of the three
control agents has been effective in reducing both seed production (and hence
seedling recruitment) and standing biomass of the weed in the Southwest
Cape Province (Moran, 1995).
The importance of T.lativentre in the biological control of S.punicea has been
emphasized by results of an inadvertent, large-scale experiment. Hoffman &
Moran (1995) found that organophosphate insecticide drift onto thickets of S.