ecology of phasmids - KLUEDO - Universität Kaiserslautern

Predation mediated mortality & migratory behavior of nymphs 87
My results need to be discussed with regard to two major problems involved in conducting field
experiments. My design of exclosures could have caused microclimatic changes leading to differential
mortality. I think matching proportions of intrinsic mortality in both the field and the migration
experiment in greenhouses do enforce my results and suggest that influence of microclimatic differences
in the exclosures was negligible. Further, in my study disappearance of nymphs in control plots (54 %)
could just reflect emigration, and emigration then was higher at night as phasmids are nocturnal. I
presented evidence that emigration of nymphs most likely did not affect the presented results, because
my searching area was well above the average distance of 47.53 cm nymphs moved between checking
the plots. Similar movement ranges in phasmids have been reported previously (0.55 m d -1 , Willig et al.
1986), and phasmid nymphs were shown to behave philopatric while adults represent the dispersal stage
(Willig et al. 1986, 1993). In addition, similar proportions of migrating nymphs in both treatments
indicate that migratory behavior was observed adequately.
5.4.2 Is off-plant migration mediated by natural enemies or by resources?
The initially stated hypothesis that migration behavior of M. diocles nymphs would reflect predator
avoidance could not be supported by observations in the field experiment. Preys are expected to invest
in predator avoidance tactics when predation risk is sufficiently high (Charnov et al. 1976, Lima &
Bednekoff 1999, Luttbeg & Schmitz 2000). Experimental studies show that prey display migratory
predator avoidance based on the risk imposed by predators (Snyder & Wise 2000, Venzon et al. 2000).
This may not apply for the presented study system. I showed that nymphs of M. diocles suffered
significant pressure from the third trophic level, but I did not find the expected increase in migratory
activity of nymphs in controls.
Increased migratory activity of nymphs in the field as compared to nymphs in greenhouses may reflect
odor-released migration. For example, herbivorous mites migrate in-between their host plant as a
specific reaction to odors of their predators (Magalhães et al. 2002). The general observation that part of
M. diocles nymphs moved in both the field and the greenhouse experiment may be intrinsically
triggered as a consequence of enemy free space. This has been shown for herbivores moving in non-
feeding periods to patches of reduced predation risk (for a review see Beredegue et al. 1996). However,
migration may be of minor importance for predation avoidance in phasmids, because they show a
variety of behavioral and morphological adaptations to escape from predators, such as chemical defense
(e.g. Eisner 1965; Carlberg 1985; Eisner et al. 1997), acoustic and color display (Robinson 1968a,
1968b, 1969), as well as camouflage and mimicry (Bedford 1978).
As an alternative to top-down related explanations of migration, data indicated that emigration may be
attributed to ‘plant size’ (in terms of leaf number). Mean number of healthy leaves per host plant was
significantly lower for emigrating nymphs than for sessile or remigrating nymphs. Plant size may reflect