DYNAMICAL EFFECTS OF HOST-FEEDING IN PARASITOIDS

1. Synovigenic parasitoids emerge as adults with fewer mature eggs than they can ultimately lay. They need to feed on host fluid or tissue to gain protein in order to mature additional eggs. The decision to host-feed or parasitize an encountered host depends in part on the current number of mature eggs that the parasitoid is carrying (its egg load), with host-feeding more likely at low egg loads. Protein gained from host-feeding may also be used for maintenance and to prolong the life of the adult parasitoid. Host-feeding often renders a host individual unsuitable for subsequent parasitism and thus can cause substantial host mortality. We investigated the dynamical effects of several aspects of host-feeding and synovigeny. 2. We found that host-feeding per se had no effect on the stability of interacting host and parasitoid populations. In addition, the decision to host-feed or parasitize an encountered host, based on the female parasitoid's current egg load, had no effect on stability. 3. The host and parasitoid equilibria were stabilized by a parasitoid mortality rate that was a decreasing function of egg load. 4. A drain of egg material gained from host-feeding for use in maintenance destabilized the equilibria, provided that either the parasitoid death rate or birth rate (via the probability of host-feeding given an attack) depended on egg load. An input of protein that could be used to produce eggs from a source other than the host had a stabilizing effect. 5. Results 3 and 4 arose in models that categorized the adult female parasitoid population according to each individual's egg load, and a female's behaviour was determined by her egg load. Analogous, but unstructured, models in which females responded to the mean population egg load gave results that were identical to the structured models, except when parasitoid survival depended on egg load.