ecology of phasmids - KLUEDO - Universität Kaiserslautern

Predation mediated mortality & migratory behavior of nymphs 88
both food availability and quality. As first instar nymphs consume minimal quantities of leaf tissue per
day (pers. obs.) food in general was available and hence the described emigration pattern seems not to
be related to resource availability. I suggest that nymphs left their host because of significant bottom-up
pressure in terms of reduced suitability of the host plant. Leaf traits such as nutrient or toxin content can
be plant size related. Plants with lower leaf area suffer relatively higher damage by larval feeding and
consequently are at higher risk of reduced fitness (Marquis 1992). The greater impact of herbivory on
small plants should have selected for constitutive defenses that decline during development (Coley et al.
1985; but see Kearsley & Whitham, 1989). Plant size was shown to positively affect larval performance
of insect herbivores (Price 1991; Teeder & Tammaru 2002) and according to the plant vigor hypothesis,
bigger plants or plant modules are more vigorous and preferred by herbivores (Price 1991). Both
increased constitutive defenses and reduced plant vigor in smaller plants result in diminished food
quality. Poor quality food is known to decrease survivorship (e.g., Lill & Marquis 2001) and
development (Damman 1987) of herbivores and as a consequence larva may move on to a new host
(van Dam et al. 2001). The presented relation of emigration and plant size supports this view but further
research is needed to evaluate whether smaller individuals in Piper marginatum are of minor quality as
food a source for M. diocles nymphs.
If reduced suitability was the motivation to leave a host plant, there are still costs involved in searching
for a new host. First the herbivore may not encounter a suitable new host in time and it is deprived of
food while searching. Very young caterpillars, for example, often have trouble finding neighboring food
plants (van Dam et al. 2000) and temporal deprivation of food can retard development more strongly
than feeding on an induced low quality food source (van Dam et al. 2001). Second, while searching, the
herbivore runs risk of being attacked by enemies (Price et al. 1980). In contrast, staying can result in
higher enemy-caused mortality as development is decelerated (slow growth/ high mortality) (Feeny
1976; Clancy & Price 1987). Hence bottom-up factors can affect mortality directly through starvation
and indirectly through a feedback loop to predation.
5.4.3 Who are the predators?
In my study, disappearance of nymphs was significantly higher at night. I showed that disappearance to
a high extent may have reflected predation. As a consequence, I concluded that birds, a prominent group
of predators on herbivorous insects (e.g., Marquis & Whelan 1994; Floyd 1996; Murakami 1999) seem
to be of no importance as mortality source of M. diocles nymphs. In accordance with Belovsky et al.
(1990) I suggest that arthropods are the principal predators on immature individuals. During the day,
nymphs of M. diocles mainly hide on the underside of leaves. They huddle against the leaf midrib with
their morphology perfectly blending in with the substrate (pers. obs.). Due to this camouflage, they may
prevent predation during the day from visually searching natural enemies. At night, when moving from
their hiding location to a feeding site, they are most vulnerable to attack. Nocturnal activity is a common
behavioral adaptation to decrease predation risk. Nonetheless, many predators search for prey at night. I