Extrafloral nectaries exhibit dual ecological functions in a plant from the Brazilian Cerrado

Extrafloral nectaries (EFNs) are nectar-secreting plant glands not related to pollination. Several not mutually exclusive hypotheses explain the ecological function of EFNs. We focused on the (1) protection hypothesis, which predicts the attraction of natural enemies by the extrafloral nectar, providing indirect defense to plants against herbivores, and the (2) ant-distracting hypothesis, which predicts the distraction of ants away from sap-sucking herbivores secreting sugary rewards (honeydew). We aimed to test both hypotheses simultaneously to understand the relative ecological roles of EFNs in a model plant from the Brazilian Cerrado. We experimentally manipulated plant groups according to the presence or absence of EFNs and hemipterans during two years of field study (2018 and 2019). We found some support for both hypotheses. Extrafloral nectaries reduced the damage caused by herbivores by attracting predatory ants, although plants with both EFNs and hemipterans showed the highest herbivory reduction in 2018. The presence of neither EFNs nor hemipterans was associated with increased fruit production. The honeydew, which had higher sugar concentrations than the extrafloral nectar, was used by ants for longer periods. However, more than 70% of the ants were observed feeding on the extrafloral nectar instead of honeydew. We suggest that the relatively high quantity of EFNs can compensate for their lower quality and still reduce the number of ants attending and protecting hemipterans, ultimately mitigating plant damage. In summary, EFNs may fit multiple functions depending on the ecological context.

Automated summary

Extrafloral nectaries (EFNs) are nectar-secreting plant glands not related to pollination. They function as indirect defense by attracting predatory ants to protect plants from herbivores and distracting ants away from sap-sucking herbivores. This study found that EFNs can reduce plant damage by attracting predatory ants, but their effectiveness is enhanced when combined with the presence of sap-sucking herbivores.