Data from: Parasitoid wasps indirectly suppress seed production by stimulating consumption rates of their seed-feeding hosts

1. In parasitoid–herbivore–plant food chains, parasitoids may be
simultaneously linked with both herbivore hosts and plants, as occurs when
herbivores attacked by parasitoids continue to consume plants although
they are destined to die. This peculiar property may cause parasitoids to
confer a differential trophic cascading effect on plants than that known
for typical predators. 2. We hypothesized that larval koinobiont
parasitoids would confer an immediate negative effect on plant seed
production by stimulating consumption of their seed-predator hosts. We
tested this hypothesis in an alpine parasitic food chain of plant seeds,
pre-dispersal seed predators (tephritid fly larvae) and koinobiont
parasitoids using field observations, a field experiment and a microcosm
study. 3. We first compared observed seed production in (i) non-infected
capitula, (ii) capitula infected only by seed predators (tephritid flies)
and (iii) capitula infected by both seed predators and their parasitoids
in five Asteraceae species. Consistent with our hypothesis, seed loss in
the capitula with both seed predators and parasitoids was significantly
greater than in the capitula infested only by seed predators. 4. This
effect was replicated in a controlled field experiment focusing on the
most common parasitoid–seed predator–plant interaction chain in our
system, in which confounding factors (e.g. density and phenology) were
excluded. Here, we show that parasitoids indirectly decreased plant seed
production by changing the behaviour of seed predators. 5. In a microcosm
study, we show that larval parasitoids significantly extended the growth
period and increased the terminal size of their host tephritid maggots.
Thus, parasitoids suppressed plant seed production by stimulating the
growth and consumption of the fly maggots. 6. In contrast to the typical
predator-induced trophic cascade, we highlight the significance of
parasitoids indirectly decreasing plant fitness by stimulating consumption
by seed predators. Future studies on trophic interactions should consider
the net effect of both increased consumption by seed predators and their
death after development of parasitoids.