wind-tunnels or flight set-ups or under field conditions. Naïve animals
are often used as a control, or animals that have experienced different
conditioned stimuli are compared in the same bioassay. The conditioned
stimulus is often the odour of a food substrate of the host to which
the parasitoid initially shows little or no behavioural response (α- and
β-conditioning, respectively (see Vetet al., 1995)). Studies have also
shown sensitization, whereby responses to biologically relevant cues are
enhanced without reinforcement, through mere exposure of the animal to
the stimulus. For several species, it was shown that learned responses
wane without repeated positive reinforcement, whereby responses return
to those of the naïve state. The hymenopteran parasitoidLeptopilina
heterotomacan be used as an example, since it has been the subject of
many studies on both the mechanism and the function of parasitoid
learning (e.g. Papaj and Vet, 1990; Vet and Papaj, 1992; Vetet al., 1998).
This species attacks larvae of several drosophilid fly species, living in a
variety of microhabitats, such as decaying mushrooms, fermenting fruits
and decaying plant materials. Laboratory and field experiments have
shown thatL. heterotomafemales readily learn the odour of the micro-
habitat by associating substrate odours with the presence of hosts, and
strongly prefer these odours in choice situations. Female wasps were
marked and released in a forest where apple–yeast and mushroom baits
had been set out. An oviposition experience withDrosophilalarvae in an
apple–yeast or mushroom microhabitat had three effects. First, experi-
enced females were more likely to find a bait than naïve ones. Secondly,
for those females that found a bait, experienced females found it signifi-
cantly faster than naïve females (i.e. experience reduced travel times).
Thirdly, females who had experienced a particular microhabitat were
more likely to find that microhabitat than an alternative one, i.e. experi-
ence strongly influenced microhabitat choice (Papaj and Vet, 1990).
Experience with hosts in substrates affected movement in odour plumes
of each substrate. Using a locomotor compensator, it was shown that
females walked faster and straighter, made narrower turns and walked
more upwind towards the source in a plume of odour they were experi-
enced with, when compared with movement in odour plumes from the
alternative substrate (Vet and Papaj, 1992). Associative learning-induced
preferences disappear without repeated reinforcement after 72 h. Both
positive (finding hosts) and negative reinforcement (not finding hosts in a
substrate) play a role in habitat preference and substrate odour discrimi-
nation in this species (Vetet al., 1998). Learning is not restricted to but is
more pronounced in this generalist species, as compared with related
microhabitat-specialistLeptopilinaspecies. Microhabitat specialists have
a strong innate response to the odours of their natural substrates.
It is fascinating how well tuned parasitoid innate responses can be to
these complex odours.Leptopilina clavipes, for example, a parasitoid of
Drosophilalarvae living in decaying fungi, is only attracted to mushrooms
in a specific stage of decay, the stage likely to be infested with host larvae
(Vet, 1983). Simonset al. (1992) showed how microhabitat specialists canFlexibility in Host-search and Patch-use Strategies 43