Adding floral nectar resources to improve biological control: Potential pitfalls of the fourth trophic level

The effects of floral nectar resources on ecosystem function were investigated by examining the consequences of increasing habitat complexity in field microcosms on the dynamics of a four-trophic-level community, consisting of lucerne (alfalfa), a herbivore (the pea aphid, Acyrthosiphon pisum), its parasitoid (Aphidius ervi) and a hyperparasitoid (Dendrocerus aphidum). The influence of buckwheat (Fagopyrum esculentum) flowers on the parasitism and hyperparasitism by A. ervi and D. aphidum, respectively, was compared with buckwheat-free treatments. Experimental units for this study were 1.8×1.8×2 m³ steel-framed cages covered with a fine mesh. Parasitism and hyperparasitism rates were significantly higher in the presence of flowering buckwheat. Parasitism rates by A. ervi were lower but not significantly, in the presence of D. aphidum in buckwheat and buckwheat-free treatments. A. pisum density was significantly reduced by A. ervi when buckwheat was present, but the density of the aphid was not affected by the hyperparasitoid. The parasitoid's potential to reduce the host population was, therefore, significantly influenced by the presence of floral nectar. Although hyperparasitism rates were significantly increased by buckwheat, this did not ‘cascade’ to the second trophic level, the pea aphid. However, before floral resources are deployed in agro-ecosystems to enhance biological control of pests, the influence of flowers on the second and fourth trophic levels should always be considered.