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Explora: Environment
and Resource Data should determine biocontrol success

the potential impact of biocontrol on the population the efficacy of clearing operations on invasive Australian
dynamics and rate of spread of Hakea sericea. However, Acacia spp. and who concluded “Even reducing plants to
Le Maitre et al. used the opinion of “experts” to discern <1 plant per hectare (our aim) would leave a few scattered
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possible dispersal distances of this plant in the presence plants capable of seeding on the landscape, which could lead
of its biocontrol agent. The theory is, therefore, largely to problematic regeneration relatively quickly.”
influenced by expert opinion rather than quantitative Despite these limitations, the release of gall-forming
data and, therefore, subject to possible bias. Regardless, and seed-feeding agents was promoted as the best strategy
it is questionable to use the modeled dynamics (which is to control the invasion of Australian Acacia spp. in South
an approximation of reality) of one species and growth Africa while not impacting their economic use. 28,32,119
form and agent combination to make a case for another Arguably, this “win-win” approach has led to a suboptimal
biocontrol test case. The question should be asked why basic management decision – do not attempt to control
information, such as the rate of spread of A. cyclops or any vegetative growth but rather target the reproductive output
other Acacia spp., was never collected. Any appropriately of the tree. However, has there ever been a case where
collected and analyzed data will provide far more support >99.9% of the reproductive output of invasive plants has
than modeled dynamics based on parameters derived from been reduced by a released agent? A more realistic figure
expert opinion. would be a 50% reduction of seeds. 14,15 However, Australian
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3. How much is enough? Acacia spp. are not seed-limited and 1 seed per m is
sufficient to maintain cover after disturbance (Table 4).
Irrespective of whether seed production, seed bank size, Strydom et al. 13,14 documented seed production per m
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or rate of spread has changed, the pertinent question that with established agents much greater than the few seeds
should be asked is how much reproductive output must required (140 – 350 seeds per m ). This critique specifically
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be reduced to have an appreciable impact on the area of addresses the use of seed-reducing agents in the control of
occupancy, rate of spread, and management costs. To Australian Acacia spp. and we are not aware of any released
reduce a plant population’s area of occupancy, seed input biological control agents used for attacking the vegetative
needs to be reduced below the carrying capacity of mature parts of these invasive plants. We can thus not say whether
populations. In this situation, density-dependent seedling the use of the latter would be more effective. Our argument
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mortality will not be able to compensate for the differences instead is that seed reduction of Australian Acacia spp. is
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in seedling densities. Therefore, the population needs to unlikely to be effective given the biological response of the
be seed-limited for seed-reducing agents to be effective. 118 released agents and the fact that these plants are not seed-
Considering the carrying capacity and seed bank size limited. Data on whether agents targeting reproductive or
of mature invasive Australian Acacia spp. populations in vegetative parts would be more effective for other plant
the presence of their released biocontrol agents in South species would not be relevant here because we are not
Africa, their associated seed-reducing biocontrol agents making general statements on the effectiveness of different
will have to further reduce seed input with more than types of biocontrol agents.
99.9% to reduce the area occupied by these plants (Table 4). The South African literature on the effectiveness of
This clearly demonstrates that invasive Australian Acacia seed-reducing biocontrol agents of Australian Acacia spp.
spp. are not seed-limited and that the seed-reducing is characterized by review articles, especially in special
biocontrol program on these plants is a wasted effort. This issues, with few primary research papers. 5-12 The literature
is further supported by Cheney et al. 33,p.281 who investigated cited in these reviews are the few original works when the

Table 4. Average tree density (m ), average seed rain (m ), and average seed bank size (m ) of invasive Australian Acacia
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populations under biological control, with a near complete to closed canopy cover (90 – 100%) in the Western Cape of South Africa
Species Tree density (m ) Seed rain (m ) % Damage required Seed bank (m ) % Damage required
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Acacia longifolia 1±0.2 745±406 >99.9 1017±303 >99.9
Acacia mearnsii 1.4±0.2 NA 8564±5131 >99.9
Acacia pycnantha 1.7±0.7 314±110 >99.0 17261±9800 >99.9
Acacia saligna 3.3±0.8 1942±311 >99.9 14153±4075 >99.9
Note: Data are adapted from Strydom et al., 13,14 including the percentage reduction in the seed rain and seed bank size required to reduce seedling
recruitment below adult-carrying capacity.

Volume 2 Issue 1 (2025) 12 doi: 10.36922/eer.5876