independently in Thysanoptera, Lepidoptera, and mainly in Hemiptera, which
include most sucking herbivores [3]. The haustellate mouthparts are formed from
several components, including maxillary and mandibular stylets, which serve for
piercing plant surface. There are three main guilds of sucking herbivores feeding on
aboveground plant parts – phloem suckers, xylem suckers, and leaf suckers
[42] (Fig. 4). The different food source influences their level of specialization and
host preference.
Phloem suckers feed on phloem fluids. This guild includes mainly aphids, scale
insects, and most leaf hoppers. Their diet is high in primary metabolites such a
sugars, while it is relatively low in nitrogen. Excess sugars are often excreted to
attract ants, which protect the phloem sucker from predators. Phloem is also low in
secondary metabolites. Hence, when the relative ratio of nutrients and defensive
secondary metabolites are compared, phloem may be a more favorable food source
than leaf tissue in some cases [3]. Xylem suckers include mainly cicadas and
cercopoid froghoppers. These herbivores have specialized on an even poorer diet,
the xylem fluids. Xylem is ca. 200Â lower in nitrogen than Angiosperm leaf tissue
[3, 86]. However, it is almost devoid of defensive secondary metabolites. Due to the
avoidance of chemical defences, both phloem and xylem suckers are often primarily
affected by nutrient content and physical characteristics of the host, such as thickness
of the surface cuticle [3, 87]. Although there are some notable exceptions, both
guilds show relatively low levels of specialization on the whole [42]. This is in sharp
contrast to leaf suckers. Leaf suckers include specialized Heteroptera [88] and some
cicadellids, which evolved from their phloem-feeding ancestors [89]. These herbivores suck on the content of individual leaf cells. Although they can avoid some
compartmentalized secondary metabolites, they are probably still exposed to much
higher content of defensive compounds than phloem- and xylem-sucking herbivores.
This may be one explanation as to why this guild includes mainly highly specialized
herbivores [42, 89].
In addition, sucking herbivores, such as aphids or some leaf hoppers, can
suppress plant defences [65]. Aphids inject saliva containing suppressor proteins
in the host tissue while feeding [90]. This has been shown to affect mainly Ca
2+
signaling in the attacked tissue [91]. Suppressing Ca
2+ signaling has pronounced
cascading effects on induced responses to damage. Indeed, sucking herbivores have
been shown to induce a weaker response of their hosts than leaf-chewing insects
[65], probably allowing them to partially avoid indirect induced defences.
6
Roles of Insect Morphology and Physiology in the
Response to Host Plant Traits
The response to host plant defences is also driven by morphologic and metabolic
characteristics of insect herbivores, some of them being unrelated to their specialization. For example, small herbivores tend to respond to host defences differently
than large ones. As outlined above, xylem suckers, which can feed on a diet low in
88
M. Volf
include most sucking herbivores [3]. The haustellate mouthparts are formed from
several components, including maxillary and mandibular stylets, which serve for
piercing plant surface. There are three main guilds of sucking herbivores feeding on
aboveground plant parts – phloem suckers, xylem suckers, and leaf suckers
[42] (Fig. 4). The different food source influences their level of specialization and
host preference.
Phloem suckers feed on phloem fluids. This guild includes mainly aphids, scale
insects, and most leaf hoppers. Their diet is high in primary metabolites such a
sugars, while it is relatively low in nitrogen. Excess sugars are often excreted to
attract ants, which protect the phloem sucker from predators. Phloem is also low in
secondary metabolites. Hence, when the relative ratio of nutrients and defensive
secondary metabolites are compared, phloem may be a more favorable food source
than leaf tissue in some cases [3]. Xylem suckers include mainly cicadas and
cercopoid froghoppers. These herbivores have specialized on an even poorer diet,
the xylem fluids. Xylem is ca. 200Â lower in nitrogen than Angiosperm leaf tissue
[3, 86]. However, it is almost devoid of defensive secondary metabolites. Due to the
avoidance of chemical defences, both phloem and xylem suckers are often primarily
affected by nutrient content and physical characteristics of the host, such as thickness
of the surface cuticle [3, 87]. Although there are some notable exceptions, both
guilds show relatively low levels of specialization on the whole [42]. This is in sharp
contrast to leaf suckers. Leaf suckers include specialized Heteroptera [88] and some
cicadellids, which evolved from their phloem-feeding ancestors [89]. These herbivores suck on the content of individual leaf cells. Although they can avoid some
compartmentalized secondary metabolites, they are probably still exposed to much
higher content of defensive compounds than phloem- and xylem-sucking herbivores.
This may be one explanation as to why this guild includes mainly highly specialized
herbivores [42, 89].
In addition, sucking herbivores, such as aphids or some leaf hoppers, can
suppress plant defences [65]. Aphids inject saliva containing suppressor proteins
in the host tissue while feeding [90]. This has been shown to affect mainly Ca
2+
signaling in the attacked tissue [91]. Suppressing Ca
2+ signaling has pronounced
cascading effects on induced responses to damage. Indeed, sucking herbivores have
been shown to induce a weaker response of their hosts than leaf-chewing insects
[65], probably allowing them to partially avoid indirect induced defences.
6
Roles of Insect Morphology and Physiology in the
Response to Host Plant Traits
The response to host plant defences is also driven by morphologic and metabolic
characteristics of insect herbivores, some of them being unrelated to their specialization. For example, small herbivores tend to respond to host defences differently
than large ones. As outlined above, xylem suckers, which can feed on a diet low in
88
M. Volf