Cropping systems and field margin vegetation shape hoverfly-mediated control of bean aphids in smallholder farming systems

Le résumé fourni par la source

Hoverflies (Syrphidae) are important biological control agents in agriculture because their larvae prey on soft bodied insects such as aphids, while adults also contribute to crop pollination. In many smallholder farming areas, the prevalent hoverfly species are not well-described, nor are the factors associated with their population dynamics and potential role in aphid suppression understood. Here we aimed to identify aphidophagous hoverfly species in maize-bean intercrops and bean monocrop fields, assessing the influence of field margin floral resource diversity on hoverfly and bean aphid abundance in two districts of Malawi. The study was conducted between May and August 2021 across 330 farmer managed bean fields in Lilongwe and Salima Districts. Aphid incidence and abundance were assessed through systematic visual inspection of randomly selected bean plants, while hoverfly larvae were sampled from aphid-infested plants and adults were collected using sweep netting in the observational field. A total of 760 hoverflies belonging to eight species were collected, with five being predatory. Data analysis showed that district, bean cropping system, field margin floral resource diversity and bean growth stage significantly affected both aphid incidence and abundance. Maize-bean intercropping was significantly associated with lower bean aphid incidence and abundance compared with monocropped bean fields. Fields with high field margin floral resource diversity showed lower aphid incidence and abundance than those with low floral resource diversity. The flowering bean stage was associated with both high aphid incidence and abundance. Hoverfly larvae and adult populations were highest in Lilongwe District compared with Salima District and in fields with high field margin floral resource diversity. Hoverfly larvae and adults were significantly more abundant in maize-bean intercrops than in bean monocrop fields and both life stages recorded highest abundance during the flowering bean stage. This study shows that hoverfly abundance was negatively correlated with aphid incidence and abundance in smallholder production systems. Maize-bean intercropping, combined with diverse flowering field margin vegetation was linked to lower aphid populations through enhanced hoverfly presence. Incorporating locally available flowering plants into field margins is a practical, low-input strategy for strengthening natural pest regulation and reducing reliance on synthetic pesticides.

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