Parasite‐Induced Mortality in Mycophagous Drosophila

Abstract

Reductions in host survival due to parasitism have immediate effects on the population dynamics of both parasites and hosts. Despite its potential importance, parasite—induced host mortality has rarely been studied experimentally under natural conditions. We conducted a series of field releases of Drosophila putrida and Drosophila neotestacea that had been parasitized in the laboratory by the nematode Howardula aoronymphium. In 11 out of 12 independent tests, the proportion of flies that were parasitized declined significantly through time, suggesting that parasites cause host mortality. Independent tests showed that parasites have no detectable effect on dispersal of flies and that motherworms do not disappear from surviving hosts, thus supporting the conclusion that parasites cause significant host mortality in the field. Host mortality rate increased with worm burden, significantly so in four independent tests. This is a possible mechanism for density—dependent regulation of these parasite populations. Parasite—induced mortality was considerably greater among flies that had been released in the field than among those kept concurrently in the laboratory. Thus, laboratory or clinical traits may not provide an accurate assessment of parasite virulence in natural populations. These results support a central assumption of many recent ecological and evolutionary models, i.e., that parasites are harmful to their hosts under natural conditions.