Biology Reference
In-Depth Information
in the process trigger a reduction of the weed population below an economic or eco-
logical threshold (see Norris 1999). In a rangeland, pasture, or natural situation, a
reduction in the density of an invasive weed will tend to open niches for other desirable
native or introduced plant species to reestablish, thereby restoring the ecosystem. In
some cases, such as managed forest systems, this natural restoration may need to be
supervised to maintain desirable silvicultural and forestry objectives. In certain situa-
tions, native species need to be introduced proactively, aiding reestablishment of desir-
able over less desirable species. It is also important to be vigilant to prevent colonization
of open niches by other invasive species. Barton (2004), while examining the safety
record of pathogens used in classical biological control of weeds, has reviewed this
topic and provided an in-depth analysis of the overall success of this approach.
10.2.1 Acaciasaligna-Uromycladiumtepperianum
One of the most successful classical biocontrol programs documented is the control
of
Acacia saligna
(Labill.) H.L. Wendl;
Fabaceae
in South Africa by the rust fun-
gus
Uromycladium tepperianum
(Sacc.) McAlpine, which is indigenous to
Australia. As one of the most serious invasive weeds in South Africa,
A. saligna
(Port Jackson willow) forms dense stands that replace indigenous vegetation and
interfere with agricultural practices and management of natural areas (Morris 1997).
As the weed is difficult and costly to control mechanically and chemically, it was
an excellent candidate for biological control. In 1987, a gall-forming rust fungus,
U. tepperianum
, was imported into South Africa after extensive host-range testing
undertaken in Australia confirmed its specificity (Morris 1987). Morris (1987)
studied the teliospore germination, early stages of host infection, host specializa-
tion, and the reactions of some African
Acacia
and
Albizia
species to inoculation
with
U. tepperianum
in Australia. Cross-inoculation of teliospores isolated from
A. implexa
Benth.,
A. saligna
, and
P. lophantha
Willd. spp.
lophantha
with the
same three species suggested that distinct genotypes of the rust occur on these spe-
cies. As the reactions were distinguishable at the host species level, they should,
according to Anikster (1984), be termed
formae speciales
. Morris (1987) showed
that normal galls only developed on the species from which the teliospores were
collected even though several known
U. tepperianum
host species were included in
the study. Although the results indicated that these rust genotypes may be specific
to a single host species, Morris (1987) recommended that a larger range of recorded
host species be tested prior to formally designating the
formae speciales
.
Since the initial releases in the late 1980s,
U. tepperianum
, which causes exten-
sive gall formation and subsequently spreads, has established throughout the range
of
A. saligna
(Morris 1997). Fifteen years of monitoring (1991-2005) showed that
tree density declined by between 87% and 98% with a reduction in canopy mass
compared with data recorded prior to the
U. tepperianum
release (Wood and Morris
2007). The introduction of the seed-feeding weevil,
Melanterius compactus
Lea
(Coleoptera: Cuculionidae), should accelerate a continuous decline in stand density
