work of Vos (2001: chapters 4 and 5) on theC. glomerata–Pierissystem.
More than 100 years ago,C. glomeratawas introduced to the USA to
combatP. rapae.SinceP. brassicaeis absent in the USA,P. rapaeis the
main host for USC. glomerata. The distribution ofP. rapaein Europe and
in early and late season in the USA can be described by a Poisson distrib-
ution, whereas a more clumped negative binomial distribution applies in
the USA when host density is highest in midsummer. Vos (2001: chapter
4) showed that parasitoids of US and European origin behave differently
in the same experimental set-up. The difference between foraging
behaviour of US and EuropeanC. glomeratawas not a simple change in
one trait. All estimates for behavioural parameters, such as patch-arrival,
patch-leaving and travel-time decisions, were considerably different
between these populations. For instance, European parasitoids revisited
infested patches more often than US parasitoids, they had larger travel
times between patches and a higher tendency to leave after oviposition
than US parasitoids. These behavioural data from semi-field environ-
ments are used to parameterize a simulation model of C. glomerata
foraging behaviour in a large field with Brassicaplants. Vos (2001:
chapter 5) related the observed difference in foraging behaviour between
US and European parasitoids through simulations with different spatial
distributions ofP. rapaeto variation in lifetime reproductive success.
Reproductive success is not different between the US and European
strains when foraging on the (clumped) negative binomial distribution,
whereas parasitoids with ‘US’ behaviour obtain higher lifetime fitness
on a (more regular) Poisson distribution of hosts than ‘European’
parasitoids. All in all ‘US’C. glomerataseem to have flexibly adapted
their behaviour. This enables them to exploit a less clumped spatial
distribution of hosts.Conspecifics
The risk of superparasitism might lengthen the time spent by individual
wasps on a patch, since they can reduce their offspring’s mortality due to
superparasitism by a conspecific parasitoid. In situations where host
patches are scarce, many conspecifics are competing for limited resources
and superparasitism pays only if the time between two eggs deposited in
one host is short. In such cases, defending a patch against conspecifics
may be a beneficial strategy. The egg parasitoid Trissolcus basalis
frequently exhibits such defence behaviour (Field and Calbert, 1998). The
defence behaviour ofT. basalisis investigated with a combination of evo-
lutionary game-theory models and experiments. When two competitors
arrive simultaneously at the same patch, they first co-exploit the patch
together for some time. The time to the start of a contest is determined
by the size of the patch, the encounter rate with unparasitized hosts,
the encounter rate with conspecifics and the previous investment (Field
and Calbert, 1998). If the two competitors arrive at different times, theFlexibility in Host-search and Patch-use Strategies 55