nontoxic N-oxides, and store them in metabolically inactive tissues [21]. Although
the specific mechanism of alkaloid sequestration remains unknown in the case of
Asota moths, it has been suggested that they can use host alkaloids for their
protection [44]. Indeed, both Asota adults and caterpillars are highly toxic to
predators. They also show high toxicity to humans to such an extent that there
were cases of mass lepidopterism fever outbreaks during an Asota population
explosion in India [45].
In addition, herbivores have evolved behavioral adaptations and mechanisms to
avoid defences of their hosts. Passiflora lobata has its leaves protected by dense
hook-shaped trichomes, which have strong negative effects on non-adapted herbivores. However, larvae of specialized Heliconius charithonia butterfly are able to
release themselves from the hooked trichomes if entrapped. Moreover, they produce
silk mats and bite off the hooked tips of the trichomes in order to be able to move
around the leaf. Such adaptations significantly improve their feeding efficiency.
While trichomes deter generalist herbivores, specialists thus do not seem to be
strongly affected by them in this particular case [46].
Another behavioral adaptation includes herbivores able to cope with hosts producing latex. Production of latex has independently evolved in multiple plant
lineages, including around 10% of flowering plants species [47]. Latex serves as a
mechanical protection directly interfering with insect feeding. In addition, latex
serves as a vessel for various defensive compounds. In Ficus, latex contains high
concentrations of cysteine proteases, interfering with processes in the insect mid-gut
and being among the traits with the most pronounced effects on Ficus herbivores [7,
48]. In milkweeds, latex contains cardenolides, inhibiting the Na
+
/K
+
-ATPase
enzyme and showing high toxicity to most animals [49]. As such, latex is usually
an efficient form of defence, which probably supported diversification of plant
lineages possessing latex [6]. To avoid latex, specialized herbivores have evolved
behavioral adaptations including cutting leaf veins, impairing latex transportation
and outflow. On hosts with non-articulated venation, herbivores cut the main vein
only. Such a behavior can be observed in later instars of monarch butterfly caterpillars (Danaus plexippus), for example. On hosts with articulated venation, herbivores
have to cut multiple veins by creating trenches over large parts of the leaf blade
(Fig. 3) [47].
4
Nutrients, Natural Enemies, and Induced Defences
Host-plant defences can increase herbivore mortality directly (e.g., by intoxication)
or indirectly through enhanced risk of predation or parasitism [50]. Negative effects
of host defences may prolong the time herbivores need for feeding. Such a prolonged
period of feeding exposes herbivores to higher risks of being predated or parasitized.
A caterpillar has ca. 100Â higher risk of being predated or parasitized when active
and feeding [3, 51]. Therefore, the effects of host traits can be modulated by natural
enemies of herbivores – by predators and parasitoids. There is some evidence that
high predation can even facilitate host-shifts to novel hosts. For example,
4 Differential Response of Herbivores to Plant Defence
83
the specific mechanism of alkaloid sequestration remains unknown in the case of
Asota moths, it has been suggested that they can use host alkaloids for their
protection [44]. Indeed, both Asota adults and caterpillars are highly toxic to
predators. They also show high toxicity to humans to such an extent that there
were cases of mass lepidopterism fever outbreaks during an Asota population
explosion in India [45].
In addition, herbivores have evolved behavioral adaptations and mechanisms to
avoid defences of their hosts. Passiflora lobata has its leaves protected by dense
hook-shaped trichomes, which have strong negative effects on non-adapted herbivores. However, larvae of specialized Heliconius charithonia butterfly are able to
release themselves from the hooked trichomes if entrapped. Moreover, they produce
silk mats and bite off the hooked tips of the trichomes in order to be able to move
around the leaf. Such adaptations significantly improve their feeding efficiency.
While trichomes deter generalist herbivores, specialists thus do not seem to be
strongly affected by them in this particular case [46].
Another behavioral adaptation includes herbivores able to cope with hosts producing latex. Production of latex has independently evolved in multiple plant
lineages, including around 10% of flowering plants species [47]. Latex serves as a
mechanical protection directly interfering with insect feeding. In addition, latex
serves as a vessel for various defensive compounds. In Ficus, latex contains high
concentrations of cysteine proteases, interfering with processes in the insect mid-gut
and being among the traits with the most pronounced effects on Ficus herbivores [7,
48]. In milkweeds, latex contains cardenolides, inhibiting the Na
+
/K
+
-ATPase
enzyme and showing high toxicity to most animals [49]. As such, latex is usually
an efficient form of defence, which probably supported diversification of plant
lineages possessing latex [6]. To avoid latex, specialized herbivores have evolved
behavioral adaptations including cutting leaf veins, impairing latex transportation
and outflow. On hosts with non-articulated venation, herbivores cut the main vein
only. Such a behavior can be observed in later instars of monarch butterfly caterpillars (Danaus plexippus), for example. On hosts with articulated venation, herbivores
have to cut multiple veins by creating trenches over large parts of the leaf blade
(Fig. 3) [47].
4
Nutrients, Natural Enemies, and Induced Defences
Host-plant defences can increase herbivore mortality directly (e.g., by intoxication)
or indirectly through enhanced risk of predation or parasitism [50]. Negative effects
of host defences may prolong the time herbivores need for feeding. Such a prolonged
period of feeding exposes herbivores to higher risks of being predated or parasitized.
A caterpillar has ca. 100Â higher risk of being predated or parasitized when active
and feeding [3, 51]. Therefore, the effects of host traits can be modulated by natural
enemies of herbivores – by predators and parasitoids. There is some evidence that
high predation can even facilitate host-shifts to novel hosts. For example,
4 Differential Response of Herbivores to Plant Defence
83