On the capacity of macroparasites to control insect populations.

A graphical model of the population dynamics of mac- successful biological control show that parasites can have roparasites and their hosts is developed. Three principal means by which the parasites can be regulated are considered: reduction in a major effect on their host populations (Myers et al. host density as a result of parasite-induced host mortality, reduc- 1954), there are numerous cases of failed biological con- tion in host density as a result of parasite-induced host sterility, trol programs (Strong et al. 1984; Stiling 1993). It is far and competition among parasites within multiply-infected hosts. from clear what conditions are required for parasite reg- The means by which parasites are regulated has a major effect on ulation of host populations. A reductionist approach, the degree to which they can depress host population densities. In based on understanding the basic biology of host-parasite particular, a parasite that sterilizes its host is expected to reduce interactions, could provide an a priori indication of the host density more than one that causes an equivalent decline in host fitness through increased mortality. A special case of the likelihood of control (Waage and Mills 1992). model is developed for herbivorous insects that, in the absence of This article aims to identify some general features of parasites, are limited by larval food resources. Parasites that are host-parasite associations that determine the degree to regulated via parasite-induced host sterility will control the insect which parasites can depress the density of their host pop- populations below the level set by larval resources if the threshold ulations. I first present a simple graphical method for ex- host density for the parasites (N T) is less than the ratio of carrying ploring the population dynamics of interactions between capacity to net reproductive rate of the insects (K/R). Data are hosts and macroparasites. This shows that the mecha- presented showing that all three means of parasite regulation, but especially parasite-induced host sterility, can operate in Howardula nism by which the parasites are regulated can have a ma- aoronymphium, a nematode parasite of mycophagous Drosophila jor effect on their host populations. I then develop a flies. Data from a field cage experiment show that, if these nema- more specific model of directly transmitted macropara- todes are regulated primarily via reductions in host density due to sites of insects and determine under what conditions the this sterility, the parameters NT, K, and R are such that Howardula parasites will depress host densities below carrying capac- is likely to play an important role in controlling Drosophila popu- ity, which is determined by larval food resources. Finally, lations. However, this conclusion must be tempered by the fact I consider a specific host-parasite association involving that these nematodes also cause increased host mortality and expe- rience within-host competition, making the conditions for parasite mushroom-feeding Drosophila flies, and the nematode control of the flies more stringent. parasite Howardula aoronymphium and ask whether these nematodes play an important role in controlling popula-