Flower strips adjacent to greenhouses help reduce pest populations and insecticide applications inside organic commercial greenhouses

Flower strips can play an important role in agro-ecosystems by supporting populations of pests' natural enemies, thereby enhancing biological control. However, few studies have considered enhancing habitat for natural enemies around greenhouses. We conducted a two-year field experiment to (i) identify potential flowering species enhancing natural enemy populations but not pest populations; and (ii) evaluate how the presence of flower strips adjacent to greenhouses helped reduce pest abundance and insecticide use by attracting natural enemies inside greenhouses. We tested six flowering species in monofloral plots placed in flower strips adjacent to greenhouses and measured pest and predator abundance in monofloral plots but also on eggplants as well as eggplant yield and insecticide use inside greenhouses. All flowering species attracted more pests and predators than strips of naturally occurring weeds.Cosmos bipinnatusandBorago officinalishosted high predator abundance and low pest abundance. Conversely,Tagetes erectaandVerbena x hybridahosted intermediate predator abundance but high pest abundance, andCirsium setosumandCentaurea cyanushosted lower predator and pest abundances. Overall, both predator and pest numbers were higher at high flower density. Pest abundance was reduced by 43% in greenhouses adjacent to flower strips compared with control greenhouses, while predator numbers were 20 times higher, and insecticide use was reduced by 34%, but yields remained unchanged. Flower strips around greenhouses are therefore a promising, economically viable strategy to enhance pest control and to reduce insecticide use, and mixtures of flowering species in flower strips should be further tested to enhance the diversity of the predator community.