consists of behavioural rules that are conditional on some characteristic(s)
of the habitat and some state(s) of the individual, still reflecting only part
of the total spectrum of parasitoids’ behaviours in varying environments.What factors affect leaving?
Patch-leaving decisions of parasitoids have frequently been studied
in an evolutionary context, focusing on the underlying mechanisms of
decision-making. For the parasitoidL. clavipes,for example, its tendency
to leave a patch has been studied on an artificial patch (Hemeriket al.,
1993). This tendency is shown to decrease when kairomone is present.
When the first encountered host is parasitized, the parasitoid’s leaving
tendency increases, whereas the opposite is true when it encounters
a healthy one, showing the parasitoid’s behavioural flexibility. For
arriving at these conclusions, a technique stemming from survival
analysis, namely, Cox’s regression model, is used (Cox, 1972). This
semi-parametric statistical model allows one to infer the effects of cues
and the (complex) interplay between different factors.
With the same statistical technique, a sequence of patch visits by a
parasitoid can be analysed. This was recently done for the parasitoid
C. glomeratain a semi-field set-up allowing the parasitoids to visit more
than one patch during the observation time of 1 h (Voset al., 1998).
Here, we use this analysis to illustrate the great behaviour flexibility of
generalist parasitoids. In Europe, the generalist parasitoidC. glomerata
prefers parasitizing larvae of the large white butterfly,Pieris brassicae,a
host with a clumped distribution (20–100 caterpillars on one leaf, but
clumps are rare). It also attacks larvae of the small white butterfly,Pieris
rapae, occurring in a more regular spatial distribution with low numbers
per leaf (one to five caterpillars on a leaf, but infested leaves are not rare).
Parasitoids of EuropeanC. glomeratawere observed in three different
environments: (i) with only larvae of the large white butterfly present;
(ii) with only larvae of the small white butterfly present; or (iii) with
larvae of both white butterflies present. This generalist parasitoid shows
substantial behavioural flexibility. If only the clumped hostP. brassicae
can be found, it uses information on previously visited numbers of host-
infested leaves and on ovipositions to determine when to leave. When
foraging forP. rapae, these cues have no effect on the leaving tendency,
whereas some (but not all) cues are used when both host species are
present. Thus, this parasitoid’s leaving rules are different for preferred
versus non-preferred hosts (Voset al., 1998).Host spatial distribution
The pay-off of different behavioural rules depends critically on the spatial
distribution of hosts across patches (Iwasaet al., 1981), exemplified by the54 L.E.M. Vetet al.