Increasing floral diversity for selective enhancement of biological control agents: A double-edged sward?

Abstract

Floral resource subsidies can have differential effects on insect herbivores compared with the herbivores’ natural enemies. While the nectar of many plant species enhances parasitoid fitness, it may also increase damage by herbivores. This may occur as a result of enhanced herbivore fitness or by enhancing fourth-trophic-level processes, possibly disrupting a trophic cascade as a result. The responses of different arthropod guilds to different floral resource subsidies were compared using Plutella xylostella (Hyponomeutidae), its parasitoid Diadegma semiclausum (Ichneumonidae) and data from two other published herbivore–parasitoid systems. These were Dolichogenidea tasmanica (Braconidae) and its host Epiphyas postvittana, and Copidosoma koehleri (Encyrtidae) and its host Phthorimaea operculella. The parasitoids and hosts in the three systems exhibited differential responses to the nectar sources. The differential response was not explained by morphology, demonstrating that physical access to nectaries alone does not determine the potential of flowers as a food source. For some flowering plants, enhancement of herbivore and parasitoid fitness occurred. Other flowering plants, such as buckwheat and phacelia, conferred a selective enhancement on parasitoids by increasing only their fitness. More effective conservation biocontrol may be achieved by the provision of selective floral resources. Attempts to ‘engineer’ agroecosystems to enhance biological control require an extensive knowledge of the ecology of the herbivore, its enemies and their interactions with potential resource subsidies.