1.2.1 Morphological Features for Physical Defense
Insect herbivores from all feeding guilds must make contact with the plant surface
in order to establish themselves on the host plant. It is therefore not surprising that
physical and chemical features of the plant surface are important determinants of
resistance. Epicuticular wax films and crystals cover the cuticle of most vascular
plants. In addition to their important role in desiccation tolerance, they also increase
slipperiness, which impedes the ability of many non-specialized insects to populate
leaf surfaces. The physical properties of the wax layer as well as its chemical composition
are important factors of preformed resistance (see M ¨uller this volume). Whereas
induced changes in wax production and surface chemistry have been observed, evidence
for a role of the cuticle and epicuticular waxes in induced resistance is still
scant.Wax biosynthesis and composition are knownto vary during plant development,
and the physico-chemical properties of the cuticle respond to changes in season and
temperature (M ¨uller this volume). Consideringthe ingenuity of plants in dealing with
their offenders, it would thus be surprising if regulated production of wax on the
leaf surface were not adopted to influence the outcome of plant-insect interactions.
Other components of the plant surface that serve a role in constitutive defense
include thorns and spines directed against mammalian herbivores, and hairs
(trichomes) which are effective against insects (Myers and Bazely 1991;
Schoonhoven et al. 2005). Non-glandular trichomes may serve as structural resistance
factors preventing small insects from contacting the leaf surface or limiting
their movement. Morphological and chemical resistance factors are combined in
glandular trichomes. Glands produce substances which may repel insect herbivores
or deter them from feeding (antixenosis), or immobilize them on the leaf surface.
Quite interestingly, trichome density in some plant species increases in response to
insect feeding, and therefore constitutes an inducible resistance trait. The defensive
role of trichomes is discussed in more detail by Dalin et al. (this volume).