Cost of strepsipteran macroparasitism for immature wasps: Does sociality modulate virulence?

An important factor modulating parasite virulence is the level of extrinsic mortality experienced by hosts. Where it is high, parasites are expected to grow or reproduce quickly to complete their lifecycle before their host is killed, whereas virulence is expected to be less under low extrinsic mortality, where growth/reproduction can be slower. A prominent example of a low mortality environment for parasites are immature social insects. Here we examined the cost of parasitism, i.e. virulence, experienced by larval and pupal stages of Polistes wasps following infection by endoparasitic Strepsiptera (under starvation conditions). We found that there was no difference in virulence between infected and uninfected individuals for the seven days following infection; either measured as host mortality or mass loss. Likewise, there was no observed cost of parasitism during the first seven days of the pupal stage of the host. Growth of the endoparasitic stages appeared the same between starved laboratory individuals and field caught samples. Strepsipteran parasites apparently enter a lag phase until the later stages of host pupal development, which we speculate reduces the negative impact of parasitism during the hosts' critical developmental stages. Our results highlight the need for further inquiry into the influence of sociality upon the evolution of parasite virulence.