The impact of floral resources and omnivory on a four trophic level food web

Abstract

Omnivory is common among arthropods, but little is known about how availability of plant resources and prey affects interactions between species operating at the third and fourth trophic level. We used laboratory and field cage experiments to investigate how the provision of flowers affects an omnivorous lacewing, Micromus tasmaniae (Hemerobiidae) and its parasitoid Anacharis zealandica (Figitidae). The adult lacewing is a true omnivore that feeds on both floral resources and aphids, whereas the parasitoid is a life-history omnivore, feeding on lacewing larvae in the larval stage and floral nectar as an adult. We showed that the effect of floral resources (buckwheat) on lacewing oviposition depends on prey (aphid) density, having a positive effect only at low prey density and that buckwheat substantially increases the longevity of the adult parasitoid. In field cages, we tested how provision of flowering buckwheat affects the dynamics of a four trophic level system, comprising parasitoids, lacewings, pea aphids and alfalfa. We found that provision of buckwheat decreased the density of lacewings in the first phase of the experiment when the density of aphids was high. This effect was probably caused by increased rate of parasitism by the parasitoid, which benefits from the presence of buckwheat. Towards the end of the experiment when the aphid populations had declined to low levels, the effect of buckwheat on lacewing density became positive, probably because lacewings were starving in the no-buckwheat treatment. Although presence of buckwheat flowers did not affect aphid populations in the field cages, these findings highlight the need to consider multitrophic interactions when proposing provision of floral resources as a technique for sustainable pest management.