Which is the best strategy against foxglove aphid?

Abstract: Foxglove aphid [Aulacorthum solani (Kaltenbach) (Hemiptera: Aphididae)] is one of the principal aphid pests of greenhouse ornamental crops in North America. Increasing pesticide regulation and limited availability of efficacious pesticides that are compatible with biological control have increased the importance of developing effective biological control for this aphid. At the moment, biological control of A. solani mostly relies on the use of Aphidius ervi Haliday (Hymenoptera: Braconidae). Most of growers release this parasitoid using a preventative strategy, even if there are no aphids in the crop. The goal of this work is to compare a curative and a preventative strategy and recommend the best practices to control foxglove aphid in a protected crop. To emulate both strategies, parasitoids were introduced at 2 wasps/m2 in two greenhouse pepper crops with different aphid densities: (1) curative scenario with 20 A. solani per replication; (2) preventative scenario with 2 A. solani per replication. Aphids were released on the central plant of a group of nine plants, and groups were replicated nine times in the greenhouse. Each scenario also had three control groups of nine plants in cages to exclude parasitoids. In the short term, parasitoids were able to find aphids in both scenarios and aphids dispersed as a result of their defensive dropping behaviour. In the medium term, mummies were only observed in the curative scenario. A second generation of parasitoids continued to control the aphid population in the curative scenario, whereas in the preventative scenario, aphid populations increased exponentially because parasitoids did not establish. In conclusion, the curative strategy was more efficient than the preventative because more parasitoids established on the plants. It is still recommended to make multiple releases of parasitoids to ensure a population of A. ervi is established in the crop.