The Density Dependence of Parasitism by Tephritid Fruit Flies

The density dependence of parasitism in host—parasitoid systems has received considerable theoretical and experimental treatment recently, but usually at the intergenerational or interseasonal level. Here we study the density dependence of parasitism within a season, using tephritid fruit flies as a model system for parasites. Our work is motivated by conflicting observations that individual Rhagoletis pomonella flies foraged with direct density dependence (Roitberg et al. 1982), but that a population of flies of the sibling species Rhagoletis cornivora showed either no relationship between parasitism and host density (density vagueness) or inverse density dependence (Borowicz and Juliano 1986). Thus, we want to understand how a direct response to density at the individual level can be translated to a lack of response or to inverse density dependence at the population level. To do this, we employ behavior—rich simulations as well as individual—based state variable models. Our results resolve the apparent conflict: individual behaviors may not translate to the same population level patterns due to (1) a lack of defined patch boundaries (from the parasite's perspective), (2) repeated visits by parasites to patches, (3) uneven rates of host acceptance, and (4) egg limitations. Finally, we show the importance of relating the experimental region to the rest of the world of the parasite and propose that future studies of host—parasitoid systems should monitor both host and parasite populations.