herbivores, in general we expect synergism between
tolerance and parasitoid success if tolerance increases
the likelihood that parasitoids complete development
within the host. Therefore, we predict that plant traits
involved in parasitoid attraction will co-occur with those
mediating tolerance. This parasitoid-tolerance syndrome
would resolve some of the conflict surrounding theories
underlying plant–carnivore mutualisms, which, despite
the antagonistic role plant resistance can have on natural
enemy bodyguards through negative impacts on herbi-
vores, have exclusively emphasized resistance as the only
option for plant defense [43]. As an alternative, tolerance-
based defenses, especially those involved in vigorous
plant regrowth after damage, may have mutualistic effects
on herbivore natural enemies through increased herbi-
vore performance. As an initial step in determining the
relative co-occurrence of different defenses, studies could
test for simple genetic correlations between parasitoid-
enhancing traits and tolerance mechanisms within and
across different plant species.
Resistance and tolerance also fundamentally diverge in
their evolutionary implications for host–parasitoid relationships.
Tolerance, although potentially imposing some
selection on herbivores for plant preference [44], should
not impose strong selection on herbivores for adaptations
against plant defense (e.g., detoxification enzymes involved
in countering some plant resistance traits). However,
models of resistance and tolerance against
pathogens suggest that an increase in host tolerance
may inadvertently select for increased virulence through
reducing the costs associated with increased pathogenici-
ty, for example, host death which reduces pathogen
survival (e.g. [45]). If a similar phenomenon occurs in
plants, increased tolerance in plant populations could
select for increased herbivore damage, through decreas-
ing the costs associated with high damage, that is, reduced
plant regrowth capacity or host plant death. However, the
eco-evolutionary feedbacks associated with plant resistance
versus tolerance and herbivore–parasitoid interac-
tions have never been theoretically or empirically tested.
Likewise, the genotypic or phenotypic correlations between
insect adaptations to different plant defense strategies
and those involved in vulnerability to parasitoid
attack are completely unknown. For instance, if the
physiological mechanisms leading to detoxification of
ingested plant toxins are negatively correlated with insect
immunity, we would expect a trade-off that results in
decreased resistance to parasitoids in host populations
evolving on highly resistant plants. This outcome
assumes that plant–herbivore evolutionary dynamics affect
the ecology of herbivore–parasitoid interactions. Another
possible outcome is that plant defense shapes the
coevolutionary trajectory between parasitoids and their
herbivorous hosts. Strong directional selection to counter
plant resistance traits is predicted to reduce herbivore
genetic diversity at least of specialists, for example [46];