ON THE CAPACITY OF MACROPARASITES TO CONTROL INSECT POPULATIONS

A graphical moder of the population dynamics of macroparasites and the ir hosts is developed. Three principal means by which the parasites ca n be regulated are considered: reduction in host density as a result o f parasite-induced host mortality, reduc tion in host density as a res ult of parasite-induced host sterility, and competition among parasite s within multiply-infected hosts. The means by which parasites are reg ulated has a major effect on the degree to which they can depress host population densities. In particular, a parasite that sterilizes its h ost is expected to reduce host density more than one that causes an eq uivalent decline in host fitness through increased mortality. A specia l case of the model is developed for herbivorous insects that, in the absence of parasites,are limited by larval food resources. Parasites t hat are regulated via parasite-induced host sterility will control the insect populations below the level set by larval resources if the thr eshold host density for the parasites (N-T) is less than the ratio of carrying capacity to net reproductive rate of the insects (K/R). Data are presented showing that all three means of parasite regulation, but especially parasite induced host sterility, can operate in Howardula aoronymphium, a nematode parasite of mycophagous Drosophila flies. Dat a from a field cage experiment show that, if these nematodes are regul ated primarily via reductions in host density due to this sterility, t he parameters N-T, K and R are such that Howardula is likely to play a n important role in controlling Drosophila populations. However, this conclusion must be tempered by the fact that these nematodes also caus e increased host mortality and experience within-host competition, mak ing the conditions for parasite control of the flies more stringent.