Effects of Intraspecific Competition and Host-Parasitoid Developmental Timing on Foraging Behaviour of a Parasitoid Wasp
Christelle Couchoux
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Saskya van Nouhuys
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S. van Nouhuys Department of Ecology and Evolutionary Biology, Cornell University
, Ithaca,
NY, USA
In a context where hosts are distributed in patches and susceptible to parasitism for a limited time, female parasitoids foraging for hosts might experience intraspecific competition. We investigated the effects of host and parasitoid developmental stage and intraspecific competition among foraging females on host-searching behaviour in the parasitoid wasp Hyposoter horticola. We found that H. horticola females have a pre-reproductive adult stage during which their eggs are not mature yet and they forage very little for hosts. The wasps foraged for hosts more once they were mature. Behavioural experiments showed that wasps' foraging activity also increased as host eggs aged and became susceptible to parasitism, and as competition among foraging wasps increased.
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Parasitoids demonstrate well the link between foraging behaviour (searching for
hosts) and fitness because each egg laid in a host represents a fitness increase for
the parasitoid (until Lacks clutch size is reached (Lack 1947)) . Therefore, they are
often used to test predictions of optimal foraging theory (Hubbard and Cook 1978;
Stephens and Krebs 1986; Wajnberg et al. 2006). But host-parasitoid interactions are
complex and a number of parameters must be taken into account to determine the
optimal foraging strategy (Mills and Wajnberg 2008; Corley et al. 2010). Factors
influencing parasitoid foraging behaviour include individual physiological state,
competition with conspecifics or other species, and host quality. The effects of those factors
interact with each other, increasing the complexity of optimal foraging strategies.
There is a strong relationship between physiological state of a parasitoid and its
hostsearching behaviour (Chan and Godfray 1993; Heimpel and Collier 1996). One important
component of the physiological state of a parasitoid is egg load, which has been shown to
have an impact on parasitoid foraging behaviour (Heimpel and Rosenheim 1995;
Heimpel et al. 1998; Babendreier and Hoffmeister 2002; Burger et al. 2004; Hardy
et al. 2013). Parasitoid egg production strategies such as resource allocation, total number
of eggs produced, number that are mature at a time, presence of a pre-reproductive adult
stage, and capacity to reabsorb eggs vary greatly among parasitoid species and depend on
a number of life-history traits such as body size, longevity, egg size, clutch size, and host
use type (idiobiont/koinobiont) (Jervis et al. 2001, 2008). Parasitoids can be categorized
according to the maturity of their eggs at adult emergence; pro-ovigenic wasps emerge as
adults with all their eggs mature, whereas in synovigenic wasps (most parasitoids) only a
fraction of the eggs are mature at the beginning of the adult life (Jervis et al. 2001). In
parasitoids egg load is an important life history trait because egg production is costly, but
egg limitation can be a constraint on lifetime reproductive success (Weisser et al. 1997;
Heimpel et al. 1998; Ellers et al. 2000).
A second factor that may influence parasitoid foraging behaviour is competition for
hosts. Competition occurs when an individual decreases the fitness of another by
increasing its own fitness while exploiting a common resource (Grover 1997). Competition may
occur among conspecifics or between individuals of different species. Limited availability
can be temporal or spatial and can concern a great diversity of resources such as food,
space, cover, territory, or mates. Resource competition can be indirect, when one
individual depletes the resource patch before another individual arrives, as in exploitation
competition. Or it can be direct interference competition, which may be resolved through
contests. These contests are mostly ritualized non-injurious agonistic behaviours
(Maynard Smith and Parker 1976; Briffa and Sneddon 2010; Kokko 2013).
In parasitoids, larvae can compete for space and food (Brodeur and Boivin 2004),
adults can experience local mate competition at emergence (Hamilton 1967) and also,
since the distribution of hosts is usually aggregative (Godfray 1994; Wajnberg 2006),
foraging adult females may face direct competition for hosts (Godfray 1994; Hardy
et al. 2013). Intraspecific competition then occurs when several individual parasitoids
of the same species exploit or attempt to exploit the same resource patch, sometimes
at the same time. The simultaneous presence of competitors in a host patch leads to
interference competition (Godfray 1994; Goubault et al. 2005). Mathematical models
predict that intraspecific direct competition can affect the strategies of wasps foraging
for hosts (Hassell and Varley 1969; van Alphen 1988) and studies have shown that
presence of competitors can lead to behavioural modifications (Field and Calbert
1998, 1999; Hardy et al. 2013).
In addition to the physiological state of the wasp (...truncated)