and female egg-laying behaviour after being parasitized (Adamoet al.,
1995). However, it is unclear whether these changes are induced by
the parasitoid or are an indirect effect from the trauma of parasitism.
Changes in the movement or feeding of hosts have also been described
for a number of other parasitoids, but it is similarly unclear whether
these alterations have any adaptive significance (Godfray, 1994; Moore,
1995).
In contrast, there are a few studies that support the idea that some
parasitoid-induced changes in host behaviour have measurable con-
sequences for fitness. Observations of the aphid parasitoidAphidius
nigripesrevealed that hosts containing diapausing parasitoids move out
of the aphid colony to concealed locations, where the parasitoid larva
then consumes the aphid and pupates. Aphids containing non-diapausing
parasitoid larvae also tend to leave colonies, but generally move to the
underside of leaves rather than to concealed sites. Experiments with both
diapausing and non-diapausing parasitoids indicate that these alterations
in host behaviour result in reduced levels of hyperparasitism (Brodeur
and McNeil, 1989, 1992). Another example occurs among conopid flies
that parasitize adult bumblebees while flying in midair. The fly larva
takes many days to develop in the bumblebee’s abdomen, during which
time the host continues to forage (Schmid-Hempel and Schmid-Hempel,
1991). However, parasitized bees tend to visit different flowers and collect
more nectar than normal bees, which benefits the development of the
parasitoid.
An even more sophisticated change in host behaviour occurs in
the spider Plesiometa argyra after parasitism by the ichneumonid
ectoparasitoidHymenoepimecis sp. (Eberhard, 2000). During the first
few days after parasitism,P. argyraspins webs to capture prey that are
identical to non-parasitized spiders. On the day that the parasitoid larva
kills the spider, however, the spider builds a completely different type of
web, called a cocoon web. After the spider finishes spinning the cocoon
web, the parasitoid consumes the spider and then spins a pupal cocoon,
which hangs by a thread from the cocoon web. Manipulative experiments
reveal that changes in web-spinning behaviour are induced by chemical
factors secreted by the parasitoid larva. Cocoon webs support the
parasitoid cocoon more durably and prevent it from being removed by
heavy rains.Competitive Interactions between Larval Parasitoids
Beyond contending with the potentially hostile environment of the
host, parasitoid larvae also face threats from other parasitoids. These
include hyperparasitoids that oviposit directly into the parasitoid
larva, and intra- (superparasitism) or interspecific (multiparasitism)
competitors that oviposit into the same host and compete for available
host resources.140 M.R. Strand