Plant tolerance
Importantly, the aforementioned immune-mediated
pathway, along with other related mechanisms driving
bottom-up effects of plants on parasitoids (see Figure 1),
assumes that resistance traits are the only means for
defending against herbivory. A second scenario that
would bypass these resistance–parasitoid relationships
altogether emerges when considering plants that prefer-
entially invest in a tolerance-based strategy. Tolerance of
herbivory includes mechanisms of regrowth after damage,
enhanced plant vigor, or patterns of allocation [35,36],
which could improve host health by providing more or
better food resources after damage [37]. The combination
of increased tolerance and decreased resistance due to
trade-offs between these two defenses can lead to high
densities of herbivores [38] and their associated predators
[37]. Thus, plant populations that support larger or faster
growing herbivore populations would also be predicted to
support larger populations of parasitoids. However, the
tri-trophic impacts of plant tolerance of herbivory cascading
up to parasitoid fitness are virtually unexplored.