agents (Hill et al. 2020, Chap. 19). Introduction pathways have important implications for biotic interactions in the receiving environment, for example host-parasite
relationships. Many other factors also play a role, including residence time of
different partners in host-parasite relationships, whether the parasite has had any
co-evolutionary history with the host, competition among hosts or parasites, complexity in food webs such as cascading effects in multi-trophic systems, and hosts
shifts. While some alien insect hosts are parasitised by alien or native parasites, the
opposite is also true, and there are examples of biological control agents infiltrating
native communities. Thus, the relationships between parasite and host for insect
invaders are complex, and include direct and indirect effects that shape the dynamics
of whole communities. Here we focus on parasitoids, since the information on
pathogens, fungi and other parasites such as nematodes in South Africa has either
been reviewed elsewhere (Wingfield et al. 2001), is very scarce (Haelewaters et al.
2016), or has focused on the selection of biological control agents under controlled
experimental conditions (e.g., nematodes, Malan and Moore 2016). We provide
some examples of biotic interactions involving insect hosts and parasitoids for
biological invasions in South Africa.
Alien insects can be parasitised by native or alien parasitoids. In South Africa, the
invasive Harlequin Ladybeetle, Harmonia axyridis, a notorious predator of aphids
and other coccinellid species and native to Asia (see Janion-Scheepers and Griffiths
2020, Box 7.4, Chap. 7), was first detected in the Western Cape Province in the early
2000s and then spread rapidly across the country (Stals and Prinsloo 2007; Roy et al.
2016). Beetles sourced in the USA, but originating from Japan, were intentionally
introduced to South Africa to control an aphid pest in 1980, but failed to establish.
Invasive populations of H. axyridis in the country are thought to have originated
from a separate subsequent and accidental introduction (Roy et al. 2016). Population
genetic analyses revealed that Western Cape populations originated from an invasive
population in eastern North America, described as a bridgehead for the worldwide
invasion of this species (Lombaert et al. 2010). The wasp Dinocampus coccinellae
(Hymenoptera: Braconidae), a koinobiont endoparasitoid of coccinellid species,
with a widespread global distribution, was later reported to utilise H. axyridis as a
host (Fig. 14.3f), in addition to three other native and one alien host (Minnaar et al.
2014). The wasp was initially collected from native hosts in South Africa in the late
1940s and 1960s, suggesting that its occurrence preceded the introduction of
H. axyridis. Interestingly, the level of parasitism by this parasitoid on H. axyridis
was much lower than rates found on native hosts (Minnaar et al. 2014), and is
consistent with findings from other global regions in the species’ invasive range
(Comont et al. 2014; Ceryngier et al. 2018). Despite this, further work is needed to
identify the mechanisms underlying the release from this native enemy in
South Africa. A possible explanation is that the invasive ecotype of H. axyridis
garners higher immunity or resistance to parasitoids than native species, as several
studies highlight the diversity of chemical defences (harmonine and antimicrobial
peptides; Röhrich et al. 2012; Vilcinskas et al. 2013a) and prevalence of obligate
parasitic microsporidia in this species (Vilcinskas et al. 2013b). Nonetheless, the
parasitoid may adapt further via changes in host location mechanisms or parasite
402
J. J. Le Roux et al.
relationships. Many other factors also play a role, including residence time of
different partners in host-parasite relationships, whether the parasite has had any
co-evolutionary history with the host, competition among hosts or parasites, complexity in food webs such as cascading effects in multi-trophic systems, and hosts
shifts. While some alien insect hosts are parasitised by alien or native parasites, the
opposite is also true, and there are examples of biological control agents infiltrating
native communities. Thus, the relationships between parasite and host for insect
invaders are complex, and include direct and indirect effects that shape the dynamics
of whole communities. Here we focus on parasitoids, since the information on
pathogens, fungi and other parasites such as nematodes in South Africa has either
been reviewed elsewhere (Wingfield et al. 2001), is very scarce (Haelewaters et al.
2016), or has focused on the selection of biological control agents under controlled
experimental conditions (e.g., nematodes, Malan and Moore 2016). We provide
some examples of biotic interactions involving insect hosts and parasitoids for
biological invasions in South Africa.
Alien insects can be parasitised by native or alien parasitoids. In South Africa, the
invasive Harlequin Ladybeetle, Harmonia axyridis, a notorious predator of aphids
and other coccinellid species and native to Asia (see Janion-Scheepers and Griffiths
2020, Box 7.4, Chap. 7), was first detected in the Western Cape Province in the early
2000s and then spread rapidly across the country (Stals and Prinsloo 2007; Roy et al.
2016). Beetles sourced in the USA, but originating from Japan, were intentionally
introduced to South Africa to control an aphid pest in 1980, but failed to establish.
Invasive populations of H. axyridis in the country are thought to have originated
from a separate subsequent and accidental introduction (Roy et al. 2016). Population
genetic analyses revealed that Western Cape populations originated from an invasive
population in eastern North America, described as a bridgehead for the worldwide
invasion of this species (Lombaert et al. 2010). The wasp Dinocampus coccinellae
(Hymenoptera: Braconidae), a koinobiont endoparasitoid of coccinellid species,
with a widespread global distribution, was later reported to utilise H. axyridis as a
host (Fig. 14.3f), in addition to three other native and one alien host (Minnaar et al.
2014). The wasp was initially collected from native hosts in South Africa in the late
1940s and 1960s, suggesting that its occurrence preceded the introduction of
H. axyridis. Interestingly, the level of parasitism by this parasitoid on H. axyridis
was much lower than rates found on native hosts (Minnaar et al. 2014), and is
consistent with findings from other global regions in the species’ invasive range
(Comont et al. 2014; Ceryngier et al. 2018). Despite this, further work is needed to
identify the mechanisms underlying the release from this native enemy in
South Africa. A possible explanation is that the invasive ecotype of H. axyridis
garners higher immunity or resistance to parasitoids than native species, as several
studies highlight the diversity of chemical defences (harmonine and antimicrobial
peptides; Röhrich et al. 2012; Vilcinskas et al. 2013a) and prevalence of obligate
parasitic microsporidia in this species (Vilcinskas et al. 2013b). Nonetheless, the
parasitoid may adapt further via changes in host location mechanisms or parasite
402
J. J. Le Roux et al.