herbivores on Brassicaceae. Brassicaceae possess a strong chemical defence, known
as the glucosinolate-myrosinase system, generally maintaining an efficient protection of Brassicaceae hosts to generalist herbivores [31, 32]. When the host tissue is
damaged, formerly compartmentalized myrosinase enzyme gets into contact with
glucosinolates. These are then hydrolyzed into isothiocyanates, which are toxic to
herbivores [32]. Specialized herbivores, such as Pieris butterflies or Plutella xylostella diamondback moths, have evolved mechanisms to detoxify this defence [32,
33]. Larvae of Pieris rapae express a nitrile-specifier protein in their midgut,
promoting the formation of nitrile breakdown products instead of toxic isothiocyanates [32]. Plutella xylostella diamondback moths employ glucosinolate
sulfatase to desulfate glucosinolates, producing metabolites that no longer act as
substrates for isothiocyanate production by myrosinases [33]. This illustrates that
detoxification mechanisms have evolved several times independently in insect
specialists even within a single insect-plant system. Although such detoxification
mechanisms facilitate a similar response to host plant defences, their background is
often different and crucial for understanding how tolerance to plant defences in
particular cases is maintained.
Furthermore, some specialized herbivores were able to adapt to host defences to
such an extent that they can use them for their own benefit. In these cases, specialized
herbivores often respond positively to host plant chemical defences. This is also the
case for several specialized herbivores adapted to salicylates in Salicaceae hosts.
Salicylates are phenolic glycosides typical for willows and poplars. Although various derivatives of salicyl alcohol can be found in many plant lineages (e.g., [34]), in
Salicaceae they reach the highest diversity with many compounds being unique and
novel for this family [35]. Salicylates have been reported to serve an anti-herbivorous function primarily and their documented impact on generalist herbivores
involves deterrent effects, retarded larval growth, and increased mortality [36, 37].
However, certain specialist herbivores show preference for willow hosts with high
salicylate content, using salicylates as feeding cues [38]. This is probably because
they are able to sequester salicylates and even use them to their own benefit. Such an
ability was best documented in Phratora and Chrysomela leaf beetles. Larvae of
these beetles use salicylates as a precursor for salicylaldehyde, a metabolite deterring
invertebrate predators including ants and lady beetles [39, 40]. They secrete
salicylaldehyde from specialized abdominal glands (Fig. 1). In addition, larval
growth in several specialized leaf beetle and sawfly species has been shown to be
promoted on hosts with high salicylate content, providing them with an additional
advantage [41].
Another example of specialized herbivores showing a strong preference for
highly toxic hosts are Asota tiger moths. Asota tiger moths are broadly distributed
in Africa, south Asia, and tropical parts of the Australian region. Both adults and
larvae are brightly colored and usually feed on alkaloidal hosts (Fig. 2). In lowland
tropical forests in Papua New Guinea, Asota moths are largely specialized on Ficus.
The abundance of the larvae shows a strong positive correlation with the content of
phenantroidolizidine alkaloids in the host leaf tissue [7]. Other tiger moth species
have been reported to be able to sequester host alkaloids, convert them into their
4 Differential Response of Herbivores to Plant Defence
81
as the glucosinolate-myrosinase system, generally maintaining an efficient protection of Brassicaceae hosts to generalist herbivores [31, 32]. When the host tissue is
damaged, formerly compartmentalized myrosinase enzyme gets into contact with
glucosinolates. These are then hydrolyzed into isothiocyanates, which are toxic to
herbivores [32]. Specialized herbivores, such as Pieris butterflies or Plutella xylostella diamondback moths, have evolved mechanisms to detoxify this defence [32,
33]. Larvae of Pieris rapae express a nitrile-specifier protein in their midgut,
promoting the formation of nitrile breakdown products instead of toxic isothiocyanates [32]. Plutella xylostella diamondback moths employ glucosinolate
sulfatase to desulfate glucosinolates, producing metabolites that no longer act as
substrates for isothiocyanate production by myrosinases [33]. This illustrates that
detoxification mechanisms have evolved several times independently in insect
specialists even within a single insect-plant system. Although such detoxification
mechanisms facilitate a similar response to host plant defences, their background is
often different and crucial for understanding how tolerance to plant defences in
particular cases is maintained.
Furthermore, some specialized herbivores were able to adapt to host defences to
such an extent that they can use them for their own benefit. In these cases, specialized
herbivores often respond positively to host plant chemical defences. This is also the
case for several specialized herbivores adapted to salicylates in Salicaceae hosts.
Salicylates are phenolic glycosides typical for willows and poplars. Although various derivatives of salicyl alcohol can be found in many plant lineages (e.g., [34]), in
Salicaceae they reach the highest diversity with many compounds being unique and
novel for this family [35]. Salicylates have been reported to serve an anti-herbivorous function primarily and their documented impact on generalist herbivores
involves deterrent effects, retarded larval growth, and increased mortality [36, 37].
However, certain specialist herbivores show preference for willow hosts with high
salicylate content, using salicylates as feeding cues [38]. This is probably because
they are able to sequester salicylates and even use them to their own benefit. Such an
ability was best documented in Phratora and Chrysomela leaf beetles. Larvae of
these beetles use salicylates as a precursor for salicylaldehyde, a metabolite deterring
invertebrate predators including ants and lady beetles [39, 40]. They secrete
salicylaldehyde from specialized abdominal glands (Fig. 1). In addition, larval
growth in several specialized leaf beetle and sawfly species has been shown to be
promoted on hosts with high salicylate content, providing them with an additional
advantage [41].
Another example of specialized herbivores showing a strong preference for
highly toxic hosts are Asota tiger moths. Asota tiger moths are broadly distributed
in Africa, south Asia, and tropical parts of the Australian region. Both adults and
larvae are brightly colored and usually feed on alkaloidal hosts (Fig. 2). In lowland
tropical forests in Papua New Guinea, Asota moths are largely specialized on Ficus.
The abundance of the larvae shows a strong positive correlation with the content of
phenantroidolizidine alkaloids in the host leaf tissue [7]. Other tiger moth species
have been reported to be able to sequester host alkaloids, convert them into their
4 Differential Response of Herbivores to Plant Defence
81