Citation: Ndakidemi, B.J.; Mbega, E.R.; Ndakidemi, P.A.; Belmain, S.R.; Arnold, S.E.J.; Woolley, V.C.; Stevenson, P.C. Plant-Rich Field Margins Influence Natural Predators of Aphids More Than Intercropping in Common Bean. Insects 2022, 13, 569. https://doi.org/10.3390/ insects13070569 Academic Editors: Lucy Alford and Sacha Roudine Received: 14 April 2022 Accepted: 17 June 2022 Published: 23 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). insects Article Plant-Rich Field Margins Influence Natural Predators of Aphids More Than Intercropping in Common Bean Baltazar J. Ndakidemi 1, * , Ernest R. Mbega 1 , Patrick A. Ndakidemi 1 , Steven R. Belmain 2 , Sarah E. J. Arnold 1,2 , Victoria C. Woolley 2 and Philip C. Stevenson 2,3 1 Department of Sustainable Agriculture, School of Life Sciences and Bioengineering, The Nelson Mandela African Institution of Science and Technology, P.O. Box 447, Arusha 23218, Tanzania; ernest.mbega@nm-aist.ac.tz (E.R.M.); patrick.ndakidemi@nm-aist.ac.tz (P.A.N.); s.e.j.arnold@greenwich.ac.uk (S.E.J.A.) 2 Natural Resources Institute, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK; s.r.belmain@greenwich.ac.uk (S.R.B.); v.woolley291@gmail.com (V.C.W.); p.c.stevenson@greenwich.ac.uk (P.C.S.) 3 Royal Botanic Gardens, Kew, Kew Green, Richmond, Surrey TW9 3AE, UK * Correspondence: noebien@gmail.com Simple Summary: Field margin plants are important in providing resources for natural enemies (NEs) and improving biological control of crop pests. However, the use of field margin plants for biological control particularly of important common bean pests is understudied in smallholder farming systems of sub-Saharan Africa (SSA). We evaluated the potential of field margin plants with respect to intercropping systems in common bean fields to enhance the population of NEs of common bean pests. We observed a high assemblage of important NEs of common bean pests for some insect taxa with minimal impact of intercropping on NEs. Field margin plants could be managed to provide a wide range of resources to NEs and therefore biological control of common bean pests. Abstract: Field margins support important ecosystem services including natural pest regulation. We investigated the influence of field margins on the spatial and temporal distribution of natural enemies (NEs) of bean pests in smallholder farming systems. We sampled NEs from high and low plant diversity bean fields using sweep netting and coloured sticky traps, comparing monocropped and intercropped farms. NEs collected from within crops included predatory bugs, lacewings, predatory flies, parasitic flies, parasitic wasps, lady beetles, and a range of other predatory beetles; with the most dominant group being parasitic wasps. Overall, high plant diversity fields had a higher number of NEs than low-diversity fields, regardless of sampling methods. The field margin had a significantly higher number of lacewings, parasitic wasps, predatory bugs, syrphid flies, and other predatory beetles relative to the crop, but beneficial insects were collected throughout the fields. However, we observed marginally higher populations of NEs in intercropping than in monocropping although the effect was not significant in both low and high plant diversity fields. We recommend smallholder farmers protect the field margins for the added benefit of natural pest regulation in their fields. Keywords: natural enemies; predators; parasitoids; conservation biological control; field margin; Phaseolus vulgaris 1. Introduction Common bean (Phaseolus vulgaris L.) is one of the most important legume crops in sub-Saharan Africa (SSA) for the provision of proteins, vitamins, energy, and macronutri- ents [1,2] and due to its ability to fix nitrogen which contributes to soil fertility [2]. However, common bean production is constrained by insect pests [3]. One of the most serious is the black bean aphid Aphis fabae Scopoli which causes yield losses in the common bean of up to 90% in East Africa [4–7]. A. fabae also transmit plant diseases including bean common Insects 2022, 13, 569. https://doi.org/10.3390/insects13070569 https://www.mdpi.com/journal/insects