Abstract: This study is the first exploration of ecological relationships between ants, aphids, and black-eyed pea (Vigna unguiculata). Black-eyed pea produces nectar in extrafloral nectaries (EFNs) on leaf stipules and below inflorescences. EFNs generally attract ants that consume EFN-nectar and protect plants from herbivores (Koptur et al. 2015). We explored the indirect defensive relationship of ants on black-eyed pea to compare growth and reproduction of plants in four treatments: (1) Ants + Active EFN (control), (2) Ants + Sealed EFN, (3) Ants Excluded + Active EFN, and (4) Ants Excluded + Sealed EFN. We hypothesized that plants benefit from ants decreasing herbivore damage on plants if EFN- nectar is available, but plants lose this benefit if they maintain EFN-nectar when ants are absent. We did not anticipate high aphid populations in summer 2022; this altered ecological dynamics in our field plot and allowed us to explore additional interactions. Evolution of aphid-tending behavior in ants (Offenberg 2000) permits aphids to parasitize ant-plant mutualisms by stealing phloem sugar and protection from ant “bodyguards” (Katayama et al. 2013). After ants (Camponotus spp.) established themselves in our field plot, aphid numbers increased rapidly, and we observed aphid-tending behavior by ants. In addition to herbivores, the abundant aphid population attracted a variety of predatory insects. The presence of ants had a negative effect on shoot and fruit weight; means for ant exclusion treatments were significantly higher than means for ant inclusion treatments. The adverse outcomes for plants with ants most likely relate to high energetic costs of sugar removal by ant-tended aphids. The lowest means for shoot and fruit weight occurred in plants with Ants + Sealed EFN. We expect that aggressive aphid-tending behavior by ants is likely in this treatment, since ants received no EFN-nectar. Interaction between ants and nectar effects on shoot and fruit weight showed a trend for less severe negative effects of ants when nectar was available. Ants may reduce the intensity of aphid-tending behavior on plants that supply EFN-nectar rewards. Ants can be allies or enemies to EFN-bearing plants; more research is needed to study shifting interactions as ecological communities change. Vigna unguiculata provides high nutrition ( > 25% N) under stressful climate conditions around the world, particularly in Africa. This study provides new insights about ant-plant ecological dynamics, and suggests potential agroecological approaches to increase global food security in regions threatened by negative climate change impacts (Jones et al. 2017).
