ecology of phasmids - KLUEDO - Universität Kaiserslautern

Predation mediated mortality & migratory behavior of nymphs 77
5 Predation pressure and its effects on survival and off-plant
migration of M. diocles nymphs
5.1 Introduction
Predation is considered an important determinant of the abundance and distribution of prey organisms
(Sih et al. 1985; Begon et al. 1996). To date, numerous studies have confirmed that predators and
parasitoids can have significant impact on insect herbivore populations (e.g. Chang 1991; Gomez &
Zamora 1994; Floyd 1996; Moran & Hurd 1998). Extensive analyses of life table data found natural
enemies as the most important source of mortality for holometabolous herbivorous insects (Cornell &
Hawkins 1995; Cornell et al. 1998; Walker & Jones 2001). Cornell et al. (1998) concluded that this
applies in particular for insects feeding externally on plants, starting out that the importance of natural
enemies of holometabolous herbivores increases from the early larval stage to the pupal stage. These
results emphasize the role of predation as a key factor in determining herbivore survival (Hairston et al.
1960; Walker and Jones 2001). Data on predation pressure on exophytic feeding hemimetabolous
herbivores mainly concentrate on Orthopterans. In general, natural enemies exert a significant impact on
populations of grasshoppers and population dynamics are most affected by nymphal mortality (Joern &
Gaines 1990 and references therein). Oedekoven and Joern (1998) showed that the youngest instars
experience the greatest losses and that predation from wandering spiders contributed significantly to
nymphal mortality. The majority of larval mortality however reflected other undefined sources.
According to Belovsky and Slade (1995) grasshopper nymphs may be more likely to suffer death from
starvation, while adults may be at greater risk from other factors such as avian predation. Thus specific
mortality causes of different stages remain uncertain and to my knowledge there is no data available that
assess the impact of predation on early life stages of phasmids.
Herbivorous insects face predation from a variety of natural enemies (Abrahamson & Weis 1997).
These undoubtedly influence how and where an herbivore feeds (Hawkins & Lawton 1987). In
response, prey organisms have developed a variety of mechanisms to escape from top-down pressure,
for example morphological mimicry, or behavioral and phenological escape (Sillen-Tullberg et al. 1982,
Kinsmann & Platt 1984, Damman 1987, Witz 1990). Migration patterns often reflect such predation
avoidance, either in prey moving within (Hopkins and Dixon 1997, Magalhães et al. 2002) or off
(Roitberg & Myers 1979) their host plants. Such behavior usually involves costs such as reduced
fecundity (Roitberg & Myers 1979) or larval growth (Heads 1986, Gotthard 2000). The latter may lead
to prolonged development times, increasing exposure to natural enemies during vulnerable immature
stages (Feeny 1976, Benrey & Denno 1997). In consideration of these costs, predator avoidance via