defensive chemicals, include mainly large froghoppers or cicadas. This is not a
coincidence as small herbivores would not be able to efficiently suck on xylem due
to its negative pressure. On average, xylem suckers are thus larger than phloem or
leaf suckers [89]. Small herbivores also often respond more strongly to physical
defences. This is well illustrated by small, freshly hatched caterpillars, which
sometime have troubles with chewing on tough, mature leaves. Small chewing
herbivores also have problems cutting through sclerophyllous veins of grasses
[76]. Other physical traits which probably also affect small herbivores more than
large ones are trichomes [17]. Trichomes possess various functions and show high
morphological variability. Glandular trichomes may serve for secreting defensive
secondary metabolites [3]. Simpler, nonglandular trichomes serve mainly as
mechanical protection. They prevent small herbivores from reaching the surface
of the plant, make their movement more difficult, and increase their chance of
falling. This makes herbivore feeding less efficient and increases predation risk
[37, 92]. Trichomes also prevent females of small herbivores from ovipositing their
eggs on the leaf surface [93]. On the other hand, females of some specialized
herbivores can use trichomes to get a better grip on the plant, enhancing oviposition efficiency [94].
Different herbivores can also have different conditions in their guts, which largely
affects how they process their diet and what traits of the host affect them. One such
example is the response of caterpillars to tannin content and activity. Tannins
represent a diverse group of phenolic compounds that are broadly distributed
among plants [95]. It was proposed that one of the main defensive values of tannins
in terms of anti-herbivore protection results from their ability to precipitate proteins
in guts of herbivores under low pH. Such a mechanism is known in the case of
mammalian herbivores, in which some tannin groups reduce apparent N digestibility
[96]. The protein precipitation activity is especially high in procyanidins (condensed
tannins) [97], which have been shown to affect food selection in beavers, for
example [98]. Many of the previous studies on insect-plant interactions focused
primarily on this group of tannins when interpreting herbivory by insects. However,
most caterpillars tend to have alkaline mid-guts [99]. Several studies have shown
that ability of procyanidins to precipitate proteins is limited in such conditions [95,
100, 101]. Condensed tannins thus probably serve simply as indigestible matter,
lowering overall feeding efficiency in caterpillars [59, 102]. From the perspective of
anti-caterpillar protection, groups of tannins other than procyanidins may be more
important. These include ellagitannins, which show high oxidative activity. Recent
results suggest that tannin oxidative activity tends to have much more pronounced
effects on caterpillar community composition and diversity than does tannin protein
precipitation capacity [7, 103]. The oxidation of ellagitannins in caterpillars’ mid-gut
can facilitate nucleophilic reactions with proteins and the formation of highly
reactive hydroxyl radicals. In other words, the products of tannin oxidation can
damage nutrients in the gut lumens of insect herbivores or produce cytotoxic effects
in their tissues [95, 104]. The nutritional stress may be especially important as a form
of defence against some herbivores specialized on high tannin content, such as
4 Differential Response of Herbivores to Plant Defence
89
coincidence as small herbivores would not be able to efficiently suck on xylem due
to its negative pressure. On average, xylem suckers are thus larger than phloem or
leaf suckers [89]. Small herbivores also often respond more strongly to physical
defences. This is well illustrated by small, freshly hatched caterpillars, which
sometime have troubles with chewing on tough, mature leaves. Small chewing
herbivores also have problems cutting through sclerophyllous veins of grasses
[76]. Other physical traits which probably also affect small herbivores more than
large ones are trichomes [17]. Trichomes possess various functions and show high
morphological variability. Glandular trichomes may serve for secreting defensive
secondary metabolites [3]. Simpler, nonglandular trichomes serve mainly as
mechanical protection. They prevent small herbivores from reaching the surface
of the plant, make their movement more difficult, and increase their chance of
falling. This makes herbivore feeding less efficient and increases predation risk
[37, 92]. Trichomes also prevent females of small herbivores from ovipositing their
eggs on the leaf surface [93]. On the other hand, females of some specialized
herbivores can use trichomes to get a better grip on the plant, enhancing oviposition efficiency [94].
Different herbivores can also have different conditions in their guts, which largely
affects how they process their diet and what traits of the host affect them. One such
example is the response of caterpillars to tannin content and activity. Tannins
represent a diverse group of phenolic compounds that are broadly distributed
among plants [95]. It was proposed that one of the main defensive values of tannins
in terms of anti-herbivore protection results from their ability to precipitate proteins
in guts of herbivores under low pH. Such a mechanism is known in the case of
mammalian herbivores, in which some tannin groups reduce apparent N digestibility
[96]. The protein precipitation activity is especially high in procyanidins (condensed
tannins) [97], which have been shown to affect food selection in beavers, for
example [98]. Many of the previous studies on insect-plant interactions focused
primarily on this group of tannins when interpreting herbivory by insects. However,
most caterpillars tend to have alkaline mid-guts [99]. Several studies have shown
that ability of procyanidins to precipitate proteins is limited in such conditions [95,
100, 101]. Condensed tannins thus probably serve simply as indigestible matter,
lowering overall feeding efficiency in caterpillars [59, 102]. From the perspective of
anti-caterpillar protection, groups of tannins other than procyanidins may be more
important. These include ellagitannins, which show high oxidative activity. Recent
results suggest that tannin oxidative activity tends to have much more pronounced
effects on caterpillar community composition and diversity than does tannin protein
precipitation capacity [7, 103]. The oxidation of ellagitannins in caterpillars’ mid-gut
can facilitate nucleophilic reactions with proteins and the formation of highly
reactive hydroxyl radicals. In other words, the products of tannin oxidation can
damage nutrients in the gut lumens of insect herbivores or produce cytotoxic effects
in their tissues [95, 104]. The nutritional stress may be especially important as a form
of defence against some herbivores specialized on high tannin content, such as
4 Differential Response of Herbivores to Plant Defence
89