that have been used, or what they do, or how they damage the target plant
species. We focus instead on evaluations (informed opinions, evidence and quantifications) of what has been achieved in South Africa by using biological control to
suppress populations and impacts of invasive plant species. Satisfactory long-term
evaluations of outcomes are difficult and expensive, but many have been done,
providing ample evidence that biological control is often highly successful. We use
case studies from South Africa to support this assertion and to make the point that
successes may be largely forgotten in a relatively short time. Biological control of
invasive plants in South Africa is demonstrably cost effective and has become
generally accepted as a preferred management strategy. However, it is not a panacea,
and we discuss several issues which complicate our understanding of its effectiveness and which raise research and implementation challenges. Further studies are
needed on the economic and social benefits of biological control of invasive plants to
inform and involve the wider South African community.
19.1 Introduction
Biological control of invasive alien plants relies on suitably host-specific natural
enemies (agents), mainly plant-feeding insects and mites, and fungal pathogens. The
agent species are sourced from their respective countries of origin and released into
the invaded range to suppress the aggressiveness of the targeted plant species. The
rapid and spectacularly successful outcome of biological control against cactus
species, particularly in Australia in the 1930s (Dodd 1940; Mann 1970), set an
early precedent suggesting, deceptively, that biological control and the evaluation of
its success is easy. This also led, ironically, to the misguided perception that other
biological control projects targeting invasive plant species that had less-dramatic
outcomes were relative failures. Over the years, unequivocal successes have made
up a diminishing proportion of the escalating number of projects that have been
initiated, and an increasing number of projects have resulted in significant levels of
control, but which have been less obvious and harder to gauge (Raghu and Walton
2007).
Biological control, as a management tool, and as a supplement to physical
destruction or removal of the problem plants, and to herbicidal applications, has
been used in South Africa since 1913 and has accumulated an impressive number of
documented successes (Moran et al. 2005, 2013; Klein 2011; Zachariades et al.
2017). Initially (from 1913 to the 1930s) invasive plants in agriculture, particularly
invasive cacti, were the targets of biological control introductions (Zimmermann
et al. 2009). Between the 1950s and 1980s, biological control projects were initiated
on several species of plants that were invasive in the Cape Floristic Region, a
‘biodiversity hotspot’, including Hypericum perforatum (Clusiaceae; St. John’s
Wort; Gordon et al. 1986; Gordon and Kluge 1991), Hakea sericea (Proteaceae;
Silky Hakea; Esler et al. 2010; Gordon and Fourie 2011), several Australian Acacia
species (Fabaceae; wattles) (Impson et al. 2011), and Leptospermum laevigatum
(Myrtaceae; Australian Myrtle; Gordon 2011). This era also saw projects initiated
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M. P. Hill et al.
species. We focus instead on evaluations (informed opinions, evidence and quantifications) of what has been achieved in South Africa by using biological control to
suppress populations and impacts of invasive plant species. Satisfactory long-term
evaluations of outcomes are difficult and expensive, but many have been done,
providing ample evidence that biological control is often highly successful. We use
case studies from South Africa to support this assertion and to make the point that
successes may be largely forgotten in a relatively short time. Biological control of
invasive plants in South Africa is demonstrably cost effective and has become
generally accepted as a preferred management strategy. However, it is not a panacea,
and we discuss several issues which complicate our understanding of its effectiveness and which raise research and implementation challenges. Further studies are
needed on the economic and social benefits of biological control of invasive plants to
inform and involve the wider South African community.
19.1 Introduction
Biological control of invasive alien plants relies on suitably host-specific natural
enemies (agents), mainly plant-feeding insects and mites, and fungal pathogens. The
agent species are sourced from their respective countries of origin and released into
the invaded range to suppress the aggressiveness of the targeted plant species. The
rapid and spectacularly successful outcome of biological control against cactus
species, particularly in Australia in the 1930s (Dodd 1940; Mann 1970), set an
early precedent suggesting, deceptively, that biological control and the evaluation of
its success is easy. This also led, ironically, to the misguided perception that other
biological control projects targeting invasive plant species that had less-dramatic
outcomes were relative failures. Over the years, unequivocal successes have made
up a diminishing proportion of the escalating number of projects that have been
initiated, and an increasing number of projects have resulted in significant levels of
control, but which have been less obvious and harder to gauge (Raghu and Walton
2007).
Biological control, as a management tool, and as a supplement to physical
destruction or removal of the problem plants, and to herbicidal applications, has
been used in South Africa since 1913 and has accumulated an impressive number of
documented successes (Moran et al. 2005, 2013; Klein 2011; Zachariades et al.
2017). Initially (from 1913 to the 1930s) invasive plants in agriculture, particularly
invasive cacti, were the targets of biological control introductions (Zimmermann
et al. 2009). Between the 1950s and 1980s, biological control projects were initiated
on several species of plants that were invasive in the Cape Floristic Region, a
‘biodiversity hotspot’, including Hypericum perforatum (Clusiaceae; St. John’s
Wort; Gordon et al. 1986; Gordon and Kluge 1991), Hakea sericea (Proteaceae;
Silky Hakea; Esler et al. 2010; Gordon and Fourie 2011), several Australian Acacia
species (Fabaceae; wattles) (Impson et al. 2011), and Leptospermum laevigatum
(Myrtaceae; Australian Myrtle; Gordon 2011). This era also saw projects initiated
554
M. P. Hill et al.