ecology of phasmids - KLUEDO - Universität Kaiserslautern

Adult female feeding preference & nymph performance 44
4 Preference of female adults and performance of larval
stages of M. diocles on single food plant species
4.1 Introduction
In tropical forests, herbivores consume up to 11 % of annual leaf production (Coley & Barone 1996).
Yet, despite high availability of plant tissue most herbivorous insects in the tropics are rare (Basset
1996, 1999; Basset et al. 1992, 1996; Barone 1998; Novotny et al. 2002b). In the initial debate about the
factors that keep insect herbivores at low densities, Hairston et al. (1960) suggested that natural enemies
were responsible for the state of affairs. Alternatively it was suggested that plants themselves keep
herbivores rare because they are poor food (McNeill & Southwood 1978; Janzen 1988), and that
evolution has favored selection of plant defenses and high plant biodiversity as response to herbivore
pressure (Ehrlich & Raven 1964; Janzen 1970; Connell 1971; Rhoades & Cates 1976; Coley et al.
1985). In the meantime there is consensus that both, bottom-up and top-down regulation mechanisms
maintain tropical herbivores at low densities and attention has shifted towards their relative roles (Pace
et al. 1999; Persson 1999; Polis 1999; Dyer & Coley 2001).
There is no doubt that most plants are poor food for particular herbivores, either because they are
nutritionally inadequate or because they are poisonous (Southwood 1973; Lawton & McNeill 1979;
Strong et al. 1984; Janzen 1988; White 1993). Low suitability of a plant is not necessarily a
consequence of herbivore selection, as for example high fiber and lignin contents are inevitable for plant
architecture (Hartley & Jones 1995). Regardless whether the evolution of poor quality of plant tissue
reflects herbivore pressure or functional traits, nutritional quality has been implicated as a major factor
in the debate about the roles in population regulation of herbivorous insects (House 1967; Onuf et al.
1977; Onuf 1978; McClure 1980). Limitations in the value of plant tissue as food reflect bottom-up
forces that influence herbivore performance determining larval growth rates, survival, adult size, and
fecundity (e.g., Häggstrom & Larsson 1995; Burghardt & Fiedler 1996; Barker & Maczka 1996;
Awmack & Leather 2002).
Plants are inadequate food because plant tissues contain low amounts of nitrogen and protein (McNeill
& Southwood 1978; Mattson 1980), sometimes they have lower water contents than animals (Scriber &
Slansky 1981), and they have tough leaves (Feeny 1976; Lowman & Box 1983; Nichols-Orians &
Schultz 1989). Plants also contain toxins, repellents, growth-inhibitors and digestibility reducing
compounds that can be present all the time, i.e. constitutive secondary metabolites (Rosenthal &
Berenbaum 1992). In addition, secondary metabolites may be induced in response to herbivore attack
including phenols (Niemelä et al. 1979; Rossiter et al. 1988; Haukioja 1990; Kogan & Fischer 1991;