Sugar and pollen supply enhances aphid control by hoverflies in strawberry

Larvae of many hoverfly species (Diptera: Syrphidae) are efficient aphid predators, while the adults feed on pollen and nectar. Strawberry crops can be infested by several aphid species, such as the strawberry aphid Chaetosiphon fragaefolii (Cockerell) and the potato aphid Macrosiphum euphorbiae (Thomas), both of which can cause economic damage. Protected cultivation of strawberry seems to be ideal for introducing biological control agents that depend on flower resources, but the nectar and pollen provided by the crop may not be the most suitable for all flower-dependent insects. In this study, we evaluated the biocontrol potential of hoverflies released preventively or curatively against the potato aphid in strawberry in two separate greenhouse experiments and the effect of adding sugars and pollen on their control efficacy. In the curative experiment, Eupeodes corollae (Fabricius) and Sphaerophoria rueppellii (Wiedemann) were released and supported with flowering plants of Fagopyrum esculentum Moench (buckwheat). Both species reduced the aphid numbers significantly compared to the control treatment. In the preventive experiment, we tested three syrphid species, Episyrphus balteatus (De Geer), S. rueppellii, and E. corollae, in the presence and absence of additional food sources (sugar solution and pollen). All three syrphid species reduced the aphid population to a similar extent, but only in the presence of additional food sources. This study shows that hoverflies perform better in strawberry crops when adults have access to different supplementary food sources.