to facilitate their own protection against herbivores. These defences rely largely on
volatile infochemicals [57, 58]. While many volatiles are produced by plants even
when not attacked by herbivores [3], the true complexity of these interactions is
revealed after a herbivore attack and an induction of the host defences. The previous
examples in this chapter largely focused on constitutive defences, which are more or
less steadily present in the plant tissue and their level is not directly governed by
external stimuli, such as herbivory. Induced defences are deployed after a herbivore
attack and represent an alternative form of plant defence with possibly differential
effects on various insects.
Induced defences are based on several complementary mechanisms and often show
a high degree of specificity, which makes them an efficient protection against a variety
of herbivores. When induced, plants can upregulate defences (secondary metabolites,
trichomes, leaf thickness, etc.) that target the herbivore [59, 60]. These defences
directly affect herbivore preference and performance. Herbivores should be able to
cope with the induction of direct defences in a largely similar way to constitutive
defences, although it may require them to habituate to increased defence levels (e.g.,
they need to increase the efficiency of their detoxification mechanisms [61]).
However, plants can also employ elaborate indirect defences which help them to
attract natural enemies of herbivores through the production of herbivore induced
plant volatiles (HIPVs) [57, 58, 62]. HIPVs, such as shikimic acid derivatives,
terpenoids, or alcohols, are generally well detectable even in complex environments,
unlike the scents emitted directly by herbivores themselves, and help predators and
parasitoids to navigate efficiently toward their prey [63]. Importantly, the induced
responses in HIPV production seem to differ between herbivores, showing a large
degree of specificity [64]. In a greenhouse experiment, Danner et al. [65] demonstrated that the responses in indirect induced defences differed among herbivores
from different feeding guilds (see also below). Leaf-chewing herbivores induced a
strong response in HIPVs, while sap-sucking herbivores were able to suppress their
production. Induced responses may also differ between specialist and generalist
herbivores, but these differences seem to be much more subtle and vary among
systems [65, 66].
So far, the relative importance of different forms of induced defences in plant
defence and their effects on specialist and generalist herbivores remain largely
unknown [62]. For example, the attraction of predators is likely to benefit plants
through the immediate removal of herbivores [59]. On the other hand, parasitism
does not lead to an immediate termination of herbivory and in some cases it can even
prolong the feeding period of parasitized larvae [59]. Several common direct
defences, such as some phenolic secondary metabolites, have only limited effects
on immediate insect mortality on their own, though they retard larval growth [60].
Their main defensive value can possibly result from an interplay with indirect
defences as they can prolong larval growth and increase the exposure of herbivores
to predators or parasitoids attracted by HIPVs. The effect of defensive traits is thus
highly dependent on the third trophic level context.
HIPVs have been long known to attract insect parasitoids or predators [67, 68].
Recent results suggest that these volatiles can also be perceived by birds [58]. For
4 Differential Response of Herbivores to Plant Defence
85
volatile infochemicals [57, 58]. While many volatiles are produced by plants even
when not attacked by herbivores [3], the true complexity of these interactions is
revealed after a herbivore attack and an induction of the host defences. The previous
examples in this chapter largely focused on constitutive defences, which are more or
less steadily present in the plant tissue and their level is not directly governed by
external stimuli, such as herbivory. Induced defences are deployed after a herbivore
attack and represent an alternative form of plant defence with possibly differential
effects on various insects.
Induced defences are based on several complementary mechanisms and often show
a high degree of specificity, which makes them an efficient protection against a variety
of herbivores. When induced, plants can upregulate defences (secondary metabolites,
trichomes, leaf thickness, etc.) that target the herbivore [59, 60]. These defences
directly affect herbivore preference and performance. Herbivores should be able to
cope with the induction of direct defences in a largely similar way to constitutive
defences, although it may require them to habituate to increased defence levels (e.g.,
they need to increase the efficiency of their detoxification mechanisms [61]).
However, plants can also employ elaborate indirect defences which help them to
attract natural enemies of herbivores through the production of herbivore induced
plant volatiles (HIPVs) [57, 58, 62]. HIPVs, such as shikimic acid derivatives,
terpenoids, or alcohols, are generally well detectable even in complex environments,
unlike the scents emitted directly by herbivores themselves, and help predators and
parasitoids to navigate efficiently toward their prey [63]. Importantly, the induced
responses in HIPV production seem to differ between herbivores, showing a large
degree of specificity [64]. In a greenhouse experiment, Danner et al. [65] demonstrated that the responses in indirect induced defences differed among herbivores
from different feeding guilds (see also below). Leaf-chewing herbivores induced a
strong response in HIPVs, while sap-sucking herbivores were able to suppress their
production. Induced responses may also differ between specialist and generalist
herbivores, but these differences seem to be much more subtle and vary among
systems [65, 66].
So far, the relative importance of different forms of induced defences in plant
defence and their effects on specialist and generalist herbivores remain largely
unknown [62]. For example, the attraction of predators is likely to benefit plants
through the immediate removal of herbivores [59]. On the other hand, parasitism
does not lead to an immediate termination of herbivory and in some cases it can even
prolong the feeding period of parasitized larvae [59]. Several common direct
defences, such as some phenolic secondary metabolites, have only limited effects
on immediate insect mortality on their own, though they retard larval growth [60].
Their main defensive value can possibly result from an interplay with indirect
defences as they can prolong larval growth and increase the exposure of herbivores
to predators or parasitoids attracted by HIPVs. The effect of defensive traits is thus
highly dependent on the third trophic level context.
HIPVs have been long known to attract insect parasitoids or predators [67, 68].
Recent results suggest that these volatiles can also be perceived by birds [58]. For
4 Differential Response of Herbivores to Plant Defence
85