Box 2 (continued)
Recent studies have suggested that indices measuring herbivore specialization should comprise affinities among resources as well as their co-occurrence with consumers (e.g., [130]). Such indices allow to define specialists as
herbivores using significantly clustered sets of resources, feeding on related or
otherwise similar hosts. For example, specialization can be measured as
phylogenetic or chemical relatedness of the used resources. This allows
distinctions to be made between lineage specialists, herbivores tracking a
lineage of hosts, and trait specialists, herbivores tracking certain host defences
[130]. On the other hand, generalists can be defined as herbivores using overdispersed resources. Intermediate species are classed as indiscriminate
consumers.
There are some notable exceptions, and some herbivores feeding on multiple
hosts, such as tiger moth Grammia geneura, are able to feed on plants with highly
toxic defences. G. geneura is sometimes considered as a generalist as it feeds on
several unrelated hosts high in pyrrolizidine alkaloids [21]. But even such herbivores
are rarely indiscriminate consumers. In this particular case, G. geneura feeds on a
pool of locally available alkaloidal hosts, making it a pyrrolizidine alkaloid specialist
(Box 2). This may be rather common under sequential radiation and phylogenetic
tracking scenarios [22], when herbivores colonize already existing plant lineages. In
such a situation, host shifts can track similarities in host defences rather than strictly
follow host phylogeny. For example, host shifts in Melitaeini nymphalid butterflies
feeding on 16 plant families have been shown to follow the presence of iridoid
glycosides [23].
Highly polyphagous insect species feeding on large number of hosts from various
lineages disregarding their traits are relatively rare even among herbivores considered to be generalists [24]. In many terrestrial ecosystems, such highly polyphagous
herbivores represent only a small portion of the herbivore community [25]. When
forced to feed on toxic diet, consisting of a narrow set of toxic hosts, these herbivores
generally perform poorly. This is because instead of employing elaborate detoxification mechanisms, some of these herbivores rather mix different diets to achieve
optimal quality and dilute toxins [26, 27]. Their diet thus often includes a diverse set
of plants belonging to different functional groups under natural conditions [28].
3
Tolerance and Adaptations of Specialized Insect
Herbivores
Insect specialists have repeatedly evolved adaptations to overcome toxic or deterrent
effects of host plant defences. For example, specialized sawflies on birch are able to
detoxify flavonoid aglycones by glycosylation [29, 30]. One of the most iconic
examples of secondary metabolite detoxification by insects involves specialized
80
M. Volf
Recent studies have suggested that indices measuring herbivore specialization should comprise affinities among resources as well as their co-occurrence with consumers (e.g., [130]). Such indices allow to define specialists as
herbivores using significantly clustered sets of resources, feeding on related or
otherwise similar hosts. For example, specialization can be measured as
phylogenetic or chemical relatedness of the used resources. This allows
distinctions to be made between lineage specialists, herbivores tracking a
lineage of hosts, and trait specialists, herbivores tracking certain host defences
[130]. On the other hand, generalists can be defined as herbivores using overdispersed resources. Intermediate species are classed as indiscriminate
consumers.
There are some notable exceptions, and some herbivores feeding on multiple
hosts, such as tiger moth Grammia geneura, are able to feed on plants with highly
toxic defences. G. geneura is sometimes considered as a generalist as it feeds on
several unrelated hosts high in pyrrolizidine alkaloids [21]. But even such herbivores
are rarely indiscriminate consumers. In this particular case, G. geneura feeds on a
pool of locally available alkaloidal hosts, making it a pyrrolizidine alkaloid specialist
(Box 2). This may be rather common under sequential radiation and phylogenetic
tracking scenarios [22], when herbivores colonize already existing plant lineages. In
such a situation, host shifts can track similarities in host defences rather than strictly
follow host phylogeny. For example, host shifts in Melitaeini nymphalid butterflies
feeding on 16 plant families have been shown to follow the presence of iridoid
glycosides [23].
Highly polyphagous insect species feeding on large number of hosts from various
lineages disregarding their traits are relatively rare even among herbivores considered to be generalists [24]. In many terrestrial ecosystems, such highly polyphagous
herbivores represent only a small portion of the herbivore community [25]. When
forced to feed on toxic diet, consisting of a narrow set of toxic hosts, these herbivores
generally perform poorly. This is because instead of employing elaborate detoxification mechanisms, some of these herbivores rather mix different diets to achieve
optimal quality and dilute toxins [26, 27]. Their diet thus often includes a diverse set
of plants belonging to different functional groups under natural conditions [28].
3
Tolerance and Adaptations of Specialized Insect
Herbivores
Insect specialists have repeatedly evolved adaptations to overcome toxic or deterrent
effects of host plant defences. For example, specialized sawflies on birch are able to
detoxify flavonoid aglycones by glycosylation [29, 30]. One of the most iconic
examples of secondary metabolite detoxification by insects involves specialized
80
M. Volf