maintenance metabolism, and blood meals, which will increase reproduc-
tive output but may shorten lifespan (Koella, 1999). This is exemplified
by mosquitoes, where feeding drive will be matched to the minimum
meal size required to mature a batch of eggs (Clements, 1992) and trade-off
decisions will be made with regard to egg-batch size/mortality risk
(Anderson and Roitberg, 1999). For the parasite too, a conflict exists
between maximizing host contact by increased vector biting and mini-
mizing vector mortality by decreased biting. The parasite’s success is thus
constrained by a conflict between increasing transmission by increasing
biting and decreasing mortality by decreasing biting. However, the opti-
mum balance between host contact (biting) and mortality risk may not be
the same for vector and parasite. This concept has been explored in a
model of the transmission success of malaria sporozoites as a function of
mosquito biting rate (Koella, 1999; Schwartz and Koella, 2001). Koella’s
model predicts that selection pressure will increase parasite transmission
by changing the compromise between vector biting rate and vector repro-
ductive output such that parasite success is maximized. If the trade-off
positions postulated in this model also pertain to other haematophagous
insects, we should expect many vector-transmitted parasites to evolve to a
situation where they alter some aspect of biting behaviour. Evidence will
be presented below to demonstrate that there are indeed many examples
of parasites that do change both the host-seeking and the biting behaviour
aspects of the blood-feeding behaviour of their vectors in ways that intu-
itively suggest that transmission to the vertebrate host will be enhanced.
Despite the obvious importance of vector blood-feeding behaviour to
parasite-transmission dynamics, there is a major lack of quantitative data
on the consequence of parasitic-induced alteration of this life-history
trait. In particular, few studies have demonstrated that changes in vector
feeding actually do increase parasite transmission. Yet verification of
increased parasite success is fundamental to our understanding of the
evolutionary significance of parasite-induced alteration of vector blood
feeding.
If and when changes in behaviour are observed, are these non-
adaptive pathological consequences of infection or has the parasite
directly manipulated the host? Many authors have argued the importance
of distinguishing between these options if we are to understand the
evolutionary significance of host behavioural changes (Minchella, 1985;
Moore and Gotelli, 1990; Horton and Moore, 1993; Hurd, 1998). Poulin
(1995) identified key indicators of adaptive manipulation. Paramount
among these was the need to demonstrate fitness benefits for the parasite.
Natural selection will clearly favour a parasite that is able to alter the
behaviour of its vector such that its transmission success is enhanced
compared with that of its conspecifics. In addition, Poulin (1995)
suggested that parasite-induced changes in host behaviour that are
adaptive are likely to be complex, to function precisely to enhance trans-
mission, to have arisen by chance and to have evolved several times in
different taxa (see also Moore and Gotelli, 1990; Poulin, 1998).Parasite Manipulation of Vector Behaviour 261