1. Long-term control of insects by parasites is possible only if the parasite populations persist. Because parasite transmission rate depends on host density, parasite populations may go extinct during periods of low host density. Vertical transmission of parasites, however, is independent of host density and may therefore provide a demographic bridge through times when their insect hosts are rare.
2. The nematode Howardula aoronymphium, which parasitises mycophagous species of Drosophila, can experience both horizontal and effectively vertical transmission, relative rates of which depend, in theory at least, on the density of hosts at breeding sites.
3. A nine-generation experiment was carried out in which nematodes were transmitted either exclusively vertically or primarily horizontally. This experiment revealed that these parasites can persist and exhibit positive population growth even when there is only vertical transmission.
4. Assays at the end of the experiment revealed that the vertically transmitted nematodes had suffered no inbreeding depression and that they were similar to the horizontally transmitted nematodes in terms of virulence, infectivity, within-host growth rate, and fecundity. Thus, vertical transmission of H. aoronymphium did not appear to compromise the ability of these parasites to control Drosophila populations.