have been shown to learn colour better than pattern and pattern
better than shapes. The strong performance in shape-learning tasks
byM. croceipesmay be adaptive in dealing with the homochromatic
but multishaped environment in which parasitoids have to locate their
herbivorous hosts. Not only do plant species differ in their morphology,
but individual plant structures are usually also distinguishable by their
characteristic shape. However, the primary benefit of shape learning
probably lies in the fact that it could allow parasitoids to recognize
specific types of feeding damage.Host Finding
The distinction between patch finding and the subsequent stage of host
finding, where individual hosts are located within a patch, is not always
clear-cut. Long-distance cues are used during long-range orientation.
However, many of these cues may also be used at shorter range
(e.g. herbivore-induced plant cues, visual host-damage cues). Within the
gradual transition from long to short range, however, we often see a shift
from indirect cues, such as plant cues, to more direct ones, such as contact
chemicals directly derived from the host itself (e.g. frass, silk or other
excretions). These contact chemicals are perceived by taste receptors on
the antennae and tarsae, the stimulation of which generally elicits a strong
and distinct behavioural response in the parasitoid, such as changes in
walking speed and angles turned. The resulting intensified search of the
restricted area where the cue is perceived enhances the chance of locating
the host. As with patch finding, chemicals play a significant role,
but physical cues (both visual and mechanosensory) can also be
involved during the stage of host finding. Host-derived visual information
used by parasitoids includes host colour (Powellet al., 1998) and host
movement.Mechanosensory cues
Mechanosensory cues (sound and vibration) are commonly employed
by arthropods as a means of communication. Parasitoids also employ
mechanosensory communication as part of their courtship behaviour, but
in addition they use mechanosensory cues during foraging. They may
home in on sounds or substrate vibrations produced by their hosts during
various types of activity (Meyhöfer and Casas, 1999). Tachinid flies
represent a particularly well-evolved example of this type of orientation,
as they are specifically tuned in to the mating calls of their cricket hosts
(Ramsauer and Robert, 2000).
An entirely different category of vibrational orientation has been
described in pupal parasitoids. Pupal parasitoids of the genusPimpla
have been shown to track down hidden hosts through a highlyFlexibility in Host-search and Patch-use Strategies 47