The routes of introduction of alien species are discussed in more detail elsewhere
in this volume (Faulkner et al. 2020, Chap. 12), but in contrast to many vertebrate or
plant introductions, the majority of invertebrates introductions appear to have been
accidental, as contaminants or stowaways (Faulkner et al. 2016), although 92 species
are known to have been deliberately introduced as biological control agents. However, the exact pathways of introduction for less than 50% of invertebrates are
properly known (Faulkner et al. 2015). This is partly due to their small size, which
has resulted in many species having been imported undetected along with commercial goods, but is also because the identification of some introduced taxa remains
problematic. This is especially concerning for phytophagous species, which may
cause considerable damage to crops, resulting in economic losses and threats to food
security (Giliomee 2011).
Both dates and routes of introduction are linked to geographical patterns of
importation and trade, and how these have changed over time. For example, seed
insects are usually introduced along with seeds and herbivorous invertebrates with
plants, both of which may be introduced either as crops, or as food. In early colonial
days, these products were mostly imported from Europe, whereas in more recent
times other major trade routes have opened up, notably those to Asia, which has
now become the source of some 25% of all terrestrial invasions (Picker and
Griffiths 2017).
Changes in technology may also have impacts, for example, air freight of fresh
produce may now allow the importation of short-lived, delicate species that would
not have survived earlier, longer-duration passages by sea. Another example of
changing vector patterns is an increase in the use of wooden crates in general
trade, which has resulted in an increase in the number of woodborer beetles
introduced into the USA (Herms and McCullough 2014).
7.4 Biological Control Agents
Twenty percent of the terrestrial invertebrates listed here (Table 7.1) were deliberately introduced biological control agents. These are highly selective natural enemies
(herbivores, predators, parasites or pathogens) used to control populations of invasive species, usually plants, but also some insect pests. Over the past century, the use
of such biological control agents has become widespread and now takes place in
about 130 countries, with over 550 biological control agents released globally
(Zachariades et al. 2017). The use of biological control agents has many advantages
over traditional manual or chemical control techniques, primarily economic ones, in
that they are relatively cheap to apply and then usually self-sustaining, so that their
benefits continue to accrue indefinitely. There are some costs involved, however, in
7 Alien Terrestrial Invertebrates in South Africa
191
in this volume (Faulkner et al. 2020, Chap. 12), but in contrast to many vertebrate or
plant introductions, the majority of invertebrates introductions appear to have been
accidental, as contaminants or stowaways (Faulkner et al. 2016), although 92 species
are known to have been deliberately introduced as biological control agents. However, the exact pathways of introduction for less than 50% of invertebrates are
properly known (Faulkner et al. 2015). This is partly due to their small size, which
has resulted in many species having been imported undetected along with commercial goods, but is also because the identification of some introduced taxa remains
problematic. This is especially concerning for phytophagous species, which may
cause considerable damage to crops, resulting in economic losses and threats to food
security (Giliomee 2011).
Both dates and routes of introduction are linked to geographical patterns of
importation and trade, and how these have changed over time. For example, seed
insects are usually introduced along with seeds and herbivorous invertebrates with
plants, both of which may be introduced either as crops, or as food. In early colonial
days, these products were mostly imported from Europe, whereas in more recent
times other major trade routes have opened up, notably those to Asia, which has
now become the source of some 25% of all terrestrial invasions (Picker and
Griffiths 2017).
Changes in technology may also have impacts, for example, air freight of fresh
produce may now allow the importation of short-lived, delicate species that would
not have survived earlier, longer-duration passages by sea. Another example of
changing vector patterns is an increase in the use of wooden crates in general
trade, which has resulted in an increase in the number of woodborer beetles
introduced into the USA (Herms and McCullough 2014).
7.4 Biological Control Agents
Twenty percent of the terrestrial invertebrates listed here (Table 7.1) were deliberately introduced biological control agents. These are highly selective natural enemies
(herbivores, predators, parasites or pathogens) used to control populations of invasive species, usually plants, but also some insect pests. Over the past century, the use
of such biological control agents has become widespread and now takes place in
about 130 countries, with over 550 biological control agents released globally
(Zachariades et al. 2017). The use of biological control agents has many advantages
over traditional manual or chemical control techniques, primarily economic ones, in
that they are relatively cheap to apply and then usually self-sustaining, so that their
benefits continue to accrue indefinitely. There are some costs involved, however, in
7 Alien Terrestrial Invertebrates in South Africa
191