ecology of phasmids - KLUEDO - Universität Kaiserslautern

Adult female feeding preference & nymph performance 46
males considering the relatively higher reproductive investment (Boys 1978; Bernays & Chapman
1994).
Explaining the effects of food quality on herbivores one has to differentiate between generalists and
specialists. According to biochemical coevolution, herbivore generalists should be more susceptible to
qualitative toxic chemical plant defenses than specialists (Ehrlich and Raven 1964; Feeny 1976; Cornell
& Hawkins 2003). Thus generalist herbivores may have complex diets to meet nutritional requirements
while avoiding plant defenses (Belovsky 1984). Specialized herbivores that overcome such plant
defenses by detoxifying or even sequestering plant compounds should be less affected. Consequently
specialists such as mono- or oligophagous herbivores may particularly respond to physical defenses
such as leaf toughness and water content.
The study presented here assessed the impact of interspecific variation in food quality due to physical
and chemical defensive leaf traits on preference and performance of the walking stick Metriophasma
diocles.
As shown before (see Chapter 2 and Berger & Wirth 2001) M. diocles feeds exclusively on plants from
two different plant families, the Piperaceae and Araceae. According to recent phylogenetic analyses
these plant families are not closely related (Judd et al. 2002). The Piperaceae are phylogenetically
separated from other dicots, which may involve differences in phytochemistry (Judd et al. 2002). In
fact, Piper species are characterized by a wide array of phenolic compounds (Sengupta & Ray 1987;
Baldwin & Schultz 1988; Parmar et al. 1997; Dyer et al. in press), many of them deterring insect
herbivores (Parmar et al. 1997 and references therein). Likewise the Araceae as monocots are distinct
from dicots in many features. Araceae were characterized by the (probably) universal presence of
tannins (Grayum 1990), they contain calcium oxalate crystals, cyanogenic compounds, and sometimes
alkaloids, all known to deter herbivores (Judd et al. 2002 and references therein). Although M. diocles
seems to be able to detoxify such toxic or deterrent plant chemicals (cf. Chapter 2), the phylogenetic
distance of Araceae and Piperaceae does not allow to conclude on a strict coevolutionary scenario
(sensu Ehrlich & Raven 1964; Strong et al. 1984). Such specialization on distantly related hosts is best
described as disjunct oligophagous (Chapter 2; Bernays & Chapman 1994). Considering M. diocles as
herbivore specialist, I hypothesized that preference and performance of this species would rather be
explained by physical leaf characters than by content of chemical compounds.
In particular, (1) I assessed interspecific differences in food quality of M. diocles host plant species on
the base of leaf toughness, water content, specific leaf weight (SLW), tannin and total phenol contents.
By the use of feeding trials, (2) I ranked host-plant species according to female adult preference and (3)
surveyed nymph performance (sensu survival and growth) on single plant species. (4) By correlation
analysis I examined the relation of defensive leaf traits to adult preference and nymph performance. In