Density-dependent lifespan and estimation of life expectancy for a parasitoid with implications for population dynamics

Parasitoid lifespan is influenced by nutrient availability, thus the lifespan of parasitoids that rely on their hosts for nutritional resources (either via host feeding or by consuming honeydew) should vary with host density. We assessed the survival and reproduction of one such species,Aphelinus certus-a parasitoid of the soybean aphid,Aphis glycines-over a range of host densities using a laboratory assay. We found a positive, asymptotic relationship between host density and the lifespan and fecundity ofA. certusthat was supported by a traditional survivorship analysis as well as a logistic model. Parasitoids from this assay were also used to develop a wing wear index relating setae damage to parasitoid age. This index was used to estimate the life expectancy of field-collected parasitoids, which was shorter than the life expectancy of laboratory-reared female parasitoids. Finally, host-density-dependent parasitoid lifespan was incorporated into a coupled-equations matrix population model that revealed that decreasing the degree of host density dependence leads to higher equilibrium host densities and changes in the quality of equilibrium (e.g. stable limit cycles). These results detail the relatively unstudied phenomenon of host-density-dependent parasitoid lifespan and suggest that differences between laboratory- and field-determined parasitoid life expectancy have important implications for population dynamics and the biological control of insects.