Trophically-transmitted parasites generally need to undergo a period of development in the intermediate host before
reaching infectivity. During this vulnerable period, manipulation of the host to reduce susceptibility to predation would be
advantageous for parasites, because it increases the probability of surviving until infectivity and thus the probability of
transmission. We tested this ‘predation suppression’ hypothesis in 2 parasite species that use copepods as first hosts: the
tapeworm Schistocephalus solidus and the nematode Camallanus lacustris. In a series of prey choice experiments, we found
that copepods harbouring uninfective, still-developing worm larvae were less frequently consumed by stickleback predators
than uninfected copepods. The levels of predation suppression were similar in the two parasite species, suggestive of
convergent evolution. Additionally, copepods harbouring 2 worms of a given species were not more susceptible to predation
than those with 1 worm, suggesting that excessive larval parasite growth does not increase host susceptibility to predation.
Our results support the idea that parasites can suppress intermediate host susceptibility to predation while uninfective, but
we also note that the available studies suggest that this effect is weaker than the frequently observed enhancement of host
predation by infective helminth larvae.