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Explora: Environment
and Resource Data should determine biocontrol success

acacias (wattles – Acacia spp.), is a lauded example of the biocontrol program on invasive Australian Acacia spp.,
success, which has been a major focus in invasive alien we argue the opposite based on a lack of quantitative data
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plant management in South Africa in terms of effort and in the literature to support this qualitative notion. For a
funding. 12,24,25 biocontrol program to be successful, the released biological
Management of invasive Australian Acacia spp. can control agent should reduce the area of occupancy and
potentially include any one or combination of biological, rate of spread of its host plant to such an extent that it
chemical, and mechanical controls. 25,26 Although all has benefits for biodiversity, ecosystem functioning, or
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strategies are potentially viable depending on the Acacia management (reduced costs).
species being managed, a preconceived bias exists toward To properly test the asserted assumption that seed
using biocontrol as a management intervention. In this biocontrol agents have meaningfully reduced the
instance, the biocontrol program on invasive Australian reproductive potential and subsequent area of occupancy
Acacia spp. in South Africa has become a classic example of Australian Acacia spp. in South Africa, 8,10,21,28 pre-
of perceived success. 7-12 release data available on seed set, germination rates, and
The biocontrol program on invasive Acacia spp. in recruitment to the reproductive population of trees over its
South Africa has widely been reported as successful over full geographic extent of the invasion are needed. Failing
the past 34 years. 5-12,27 This program has included 17 seed- this, collecting data in the core range of these species should
reducing biological control agents released on 10 invasive be attempted. After agents are released and enough time
Australian Acacia spp. (Table 1). The reported success has has elapsed to facilitate establishment and autonomous
been based on the qualitative assessment of the extensive spread, the same data should be collected again. However,
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control of these agents on their target species. “Extensive” due to a paucity of research with the aim of collecting this
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implies that damage levels are very high or the agents type of data, such complete information has still not
cause reduced plant survival, arrested plant growth, or been collected in South Africa, despite new agents being
minimal or even no production of seed. This was later released as recently as 2016 (Table 1).
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further qualified as “substantial” that means agents reduce A basic assessment of whether released biocontrol
the amount of effort and management to control the agents have been successful in reducing the area of
target weeds. It has also been argued that the agents have occupancy or rate of spread of their host plants is to
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impacted their host populations by reducing the area of consider their distribution in their invaded range over
occupancy, rate of spread, and management costs. 16,28-30 time. In general, there is a lack of adequate distributional
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Based on this perceived success described in the peer- and population data for all species on all relevant spatial
reviewed literature, it has been argued that seed-reducing scales to prove that the released biocontrol agents have
biocontrol agents are the most effective and practical caused a decline in the area of occupancy and rate of
means of managing these invasive plants. Furthermore, spread of their host plants. This is further exacerbated
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and based on the success of these programs, it has been by available data having been collected and expressed in
suggested that biocontrol should form an integral different ways over time. 30,33-35 However, with regard to
part of any invasive plant management program. For available distributional data for the invasive Australian
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example, Moran et al. 32,p.438] state the following: “There is Acacia spp. in South Africa, there is no evidence that they
overwhelming evidence from the studies of biological control have disappeared from any areas occupied before the
of invasive trees in South Africa that any reduction in seeding release of their associated biocontrol agents or that their
levels aids management. Agents that reduce seed production area of occupation has been reduced. 30,34-37 On the contrary,
should always be in the front line of the attack (De Loach the distribution range of these plants in South Africa has
1981). As a general principle in weed biological control, we increased since the release of their biocontrol agents. 30,35
advocate that agents that reduce seed production should take This is also the case even after the biocontrol agents have
priority during the exploration phases and be amont the first been reported to have established across their entire host’s
agents released.” distribution range 30,35 and have been present at all localities
long enough to have a demonstrable impact. 13-16
The reported success of the biocontrol program on
invasive Australian Acacia spp. in South Africa has also Consequently, based on distributional data, the
stimulated motivation for the use of these agents in suggested impact of the released seed-reducing biocontrol
other parts of the world where these plants have become agents on their invasive Acacia hosts has not translated
invasive. Subsequently, these agents have been released into a measurable reduction in the area of occupancy of
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in Portugal, Israel, and New Zealand. Although an these plants. Furthermore, there is also no evidence from
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abundance of literature exists arguing for the success of distributional data that a change in the rate of spread

Volume 2 Issue 1 (2025) 3 doi: 10.36922/eer.5876