Box 1 (continued)
more prevalent [22]. In such a situation, herbivores colonize an existing
lineage of plants well after its divergence, often specializing on hosts with
similar defences [5].
Reciprocally, the resulting diversification of host plant defences probably
supported further proliferation of feeding strategies in insect herbivores, in many
cases leading to their specialization. Although there are numerous exceptions, the
specialization of insect herbivores remains one of the main factors governing their
response to host plant defences in virtually all the systems where this was examined
(e.g., [7, 13–15]).
2
Roles of Insect Specialization in Their Response to Host
Plant Defences
Insect herbivores show a broad range of host specialization (Box 2). Specialization
to a limited set of hosts should allow herbivores to become well adapted to host
defences and vice versa feeding on multiple host species should come at a cost of
being maladapted to host defensive traits (e.g., [16]). Unspecialized insects are often
excluded from strongly defended plants [17–19]. For example, there are few generalist herbivores found on milkweeds, which are strongly defended by cardenolides
[17]. Generalist herbivores also strongly respond to unique or rare secondary
metabolites as predicted by feeding specialization and the biochemical barrier
hypotheses [20]. Indeed, plant lineages with a specialized defence often have highly
specialized herbivore fauna [18, 19].
Box 2
Herbivore specialization, defined as the number or diversity of host plant species
used, is one of the most widely used concepts in insect-plant ecology [14, 130].
Insect herbivores are traditionally classified as specialists or generalists. But
there is a broad continuum between the two – from herbivores feeding on a
single host (monophagy), through herbivores feeding on a limited set of hosts
(oligophagy), to herbivores feeding on multiple hosts (polyphagy). The definition of oligophagy and polyphagy is sometimes ambiguous. Most definitions of
specialization derive from simple counts of host species. However, most herbivores show some level of specialization when examined in detail. For example,
most insect herbivores feed on closely related hosts [131]. The majority of
herbivores in lowland tropical forests in Papua New Guinea seem to prefer to
feed on congeneric or confamilial tree hosts [132], for example. A similar trend
is apparent on a global scale and among several herbivore guilds [24].
(continued)
4 Differential Response of Herbivores to Plant Defence
79
more prevalent [22]. In such a situation, herbivores colonize an existing
lineage of plants well after its divergence, often specializing on hosts with
similar defences [5].
Reciprocally, the resulting diversification of host plant defences probably
supported further proliferation of feeding strategies in insect herbivores, in many
cases leading to their specialization. Although there are numerous exceptions, the
specialization of insect herbivores remains one of the main factors governing their
response to host plant defences in virtually all the systems where this was examined
(e.g., [7, 13–15]).
2
Roles of Insect Specialization in Their Response to Host
Plant Defences
Insect herbivores show a broad range of host specialization (Box 2). Specialization
to a limited set of hosts should allow herbivores to become well adapted to host
defences and vice versa feeding on multiple host species should come at a cost of
being maladapted to host defensive traits (e.g., [16]). Unspecialized insects are often
excluded from strongly defended plants [17–19]. For example, there are few generalist herbivores found on milkweeds, which are strongly defended by cardenolides
[17]. Generalist herbivores also strongly respond to unique or rare secondary
metabolites as predicted by feeding specialization and the biochemical barrier
hypotheses [20]. Indeed, plant lineages with a specialized defence often have highly
specialized herbivore fauna [18, 19].
Box 2
Herbivore specialization, defined as the number or diversity of host plant species
used, is one of the most widely used concepts in insect-plant ecology [14, 130].
Insect herbivores are traditionally classified as specialists or generalists. But
there is a broad continuum between the two – from herbivores feeding on a
single host (monophagy), through herbivores feeding on a limited set of hosts
(oligophagy), to herbivores feeding on multiple hosts (polyphagy). The definition of oligophagy and polyphagy is sometimes ambiguous. Most definitions of
specialization derive from simple counts of host species. However, most herbivores show some level of specialization when examined in detail. For example,
most insect herbivores feed on closely related hosts [131]. The majority of
herbivores in lowland tropical forests in Papua New Guinea seem to prefer to
feed on congeneric or confamilial tree hosts [132], for example. A similar trend
is apparent on a global scale and among several herbivore guilds [24].
(continued)
4 Differential Response of Herbivores to Plant Defence
79