predatory mites remain within a patch of prey only as long as the quality
of the patch remains high, as measured by the number of prey
encountered per unit time. The cost of an incorrect foraging decision,
such as leaving a patch of prey before it is depleted, is difficult to
determine, but is potentially much greater for parasites than for most
free-living animals. Host acceptance and infection usually result in a
physiological commitment on the part of the infecting parasite – often a
transition from an infective stage, suited to the outside environment, to
a parasitic stage, which is adapted to life inside a host (Viney, Chapter 6).
For parasitoid wasps that lay eggs in a host, the cost of laying an egg in an
inappropriate host is the loss of that potential offspring (Vetet al., Chapter
3; Strand, Chapter 7). For parasites that infect hosts themselves, the cost of
poor host choice is even more striking; a plant-parasitic nematode that
infects a plant that will not support its development and reproduction
will have no representative in the next generation (Robinson, Chapter 5).
On the other hand, encounters with hosts are probably rare events in an
infective-stage parasite’s life, so rejecting a suitable host may have an out-
come similar to that of infecting a poor host: no reproduction. Thus, this
scenario suggests that natural selection will act strongly on the abilities of
parasites to choose appropriate hosts.
Infection by a mobile infective stage occurs when the parasite
penetrates into the interior of the host, but where host finding ends and
host recognition begins is a grey area. For most parasites that actively
infect the host, the decision of whether or not to infect takes place outside
the host, and this limits the information available to the parasite. For
example, in entomopathogenic nematodes, the infective juvenile is not
likely to be able to enter a host and then leave to find another if the first
proves unsuitable, and the infective juvenile must decide on the basis of
cues available on the cuticle of the insect (Lewiset al., 1996). From these
cues, the parasite must gauge host suitability, host identity and whether
or not the host is infected by other parasites (Messina, Chapter 4).
Passive infection, accomplished by either an infective stage or an egg,
occurs when the infective stage is consumed by the host. In the tapeworm
Hymenolepis diminuta, the cysticercoid stage is embedded in the muscle
tissue of a beetle intermediate host, which is ingested as prey by the
definitive host, a vertebrate. The tapeworm begins development when
stimulated by conditions in the definitive host’s gut. Just how ‘passive’
this mode of infection is is sometimes debatable. The infection can alter
the behaviour of the intermediate host to make it more easily caught or
more apparent to predatory hosts (Combeset al., Chapter 1; Lafferty,
Chapter 8). Even in environmentally resistant infective eggs that initiate
development inside the host, development can be altered by conditions
that are experienced by the infective stage outside the host. Nematodes
in the genusStrongyloidesdevelop in the host’s stomach, but the route
to the stomach is influenced by temperature and moisture conditions
encountered by the infective stages outside the host (Viney, Chapter 6).
Parasites that are passively carried by a vector rely on the vector to makeDiverse Perspectives on Parasite Behavioural Ecology 341