The effect of aphid density on intraguild predation of a soybean aphid parasitoid in the field

The effect of extraguild prey density on the strength of intraguild predation has not been sufficiently tested in a field setting, where population-level effects such as predator density-dependence or apparent competition can occur. We tested the strength of intraguild predation by predators in a soybean ecosystem against a parasitoid of the soybean aphid under differing densities of the extraguild prey, the soybean aphid, using an outplant design. Additionally, we assessed how intraguild predation against the parasitoid was affected by the surrounding field, and how this interacted with the effect of extraguild prey density, by spreading our study among many different soybean fields. We found that more predators aggregated to plants with higher extraguild prey density and that there was greater intraguild predation and less parasitoid success on these plants. Thus, extraguild prey can negatively affect an intraguild prey through apparent competition mediated by the intraguild predators. We also found that the level of intraguild predation in all treatments increased with the ratio of predators : prey in the surrounding field and decreased as the size of the surrounding plants increased. In conclusion, we showed that the field surrounding intraguild prey significantly affects intraguild predation, and testing the effect of extraguild prey in an open field led to an effect unobservable in the lab.