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Explora: Environment
and Resource Data should determine biocontrol success

have increased the complexity of managing A. saligna. Another seed-reducing agent, the flower-bud galling
Impson et al. have similarly argued that the gall midge wasp Trichilogaster acaciaelongifoliae Froggatt (Figure 4),
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Dasineura dielsi (Cecidomyiidae) has caused a decline in was found to reduce the reproductive potential of
tree density of Acacia cyclops before and after fire events. A. longifolia (Andr.) Wild within the first few years of
However, they have not indicated the area covered by the introduction at several sites within South Africa, although
plant, and consequently, the extent to which this agent will mostly sampling concentrated within the Western and
cause a decline in the area occupied by A. cyclops is unclear. Eastern Cape. 80,82,83 However, since the introduction of this
agent on A. longifolia, there have been no direct measures
A B of this invasive plant’s abundance and distribution
after 25 years of biocontrol. Using scale-area curves,
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Veldtman et al. showed that A. longifolia indeed has
115
variable tree density across its geographic range but that
T. acaciaelongifoliae is present everywhere throughout this
range. They suggest that A. longifolia is a niche-limited
invasive weed species in South Africa and that propagule
C reduction will not enforce a range limit but rather as
a consequence of a combination of a lack of favorable
vegetation type and climatic suitability for the invasive
plant itself. Using the climatic modeling by Rouget et al.
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38
to project the potential distribution of A. longifolia thus
overestimates the potential range by not taking soil types
into account. Therefore, by comparing the realized range
to the overestimated potential distribution, the proposed
Figure 2. Galling biocontrol agents can remain visible on the target weed limiting action of T. acaciaelongifoliae is erroneously
but do not necessarily target future flower buds, which would require new concluded.
infections to prevent seed production. (A) Uromycladium morrisii galls
on Acacia saligna; (B) close-up of tree branch; and (C) branch tip showing Impson et al. have also argued that there is a strong
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the position of new flower buds relative to existing galls. theoretical basis to suggest that biocontrol agents will
reduce the rate of spread of their hosts. This was based
A on the work of Le Maitre et al. where they assessed
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A C

B

D
B

Figure 3. A reduction of the density of the target weed could be from self-
thinning as certain individuals outcompete congeners. (A) Hypothetical
illustration of self-thinning in Acacia salgina populations without Figure 4. Galling biocontrol agents are normally species-specific with
disturbance events in between fire intervals resulting in a mature little risk of non-target host plant effects. (A) Acacia longifolia galled by
population with trees of similar size and age. The size of the circle indicates the biological control agent Trichilogaster acaciaelongifoliae in a high-
the relative tree size in a monospecific stand. (B) Hypothetical illustration density stand in the Western Cape Province, South Africa. (B) This
of self-thinning in Acacia salgina populations with disturbance events agent attacks the flower buds of the host plant before inflorescences
in between fire intervals resulting in a mature population with trees of fully develop; consequently, gall development indicates the successful
different sizes and ages. The size of the circle indicates relative tree size in prevention of seed pod formation. (C) Dissected gall showing pupae of
a monospecific stand. T. acaciaelongifoliae. (D) Adult female wasp.

Volume 2 Issue 1 (2025) 11 doi: 10.36922/eer.5876