PARASITE-INDUCED MORTALITY IN MYCOPHAGOUS DROSOPHILA

Reductions in host survival due to parasitism have immediate effects o n the population dynamics of both parasites and hosts. Despite its pot ential importance, parasite-induced host mortality has rarely been stu died experimentally under natural conditions. We conducted a series of field releases of Drosophila putrida and Duosophila neotestacea that had been parasitized in the laboratory by the nematode Howardula aoron ymphium. In 11 out of 12 independent tests, the proportion of flies th at were parasitized declined significantly through time, suggesting th at parasites cause host mortality. Independent tests showed that paras ites have no detectable effect on dispersal of flies and that motherwo rms do not disappear from surviving hosts, thus supporting the conclus ion that parasites cause significant host mortality in the field. Host mortality rate increased with worm burden, significantly so in four i ndependent tests. This is a possible mechanism for density-dependent r egulation of these parasite populations. Parasite-induced mortality wa s considerably greater among flies that had been released in the field than among those kept concurrently in the laboratory. Thus, laborator y or clinical trials may not provide an accurate assessment of parasit e virulence in natural populations. These results support a central as sumption of many recent ecological and evolutionary models, i.e., that parasites are harmful to their hosts under natural conditions.