Extrafloral nectaries indirectly protect small aphid colonies via ant-mediated interactions

When ant-tended aphids parasitize plants with extrafloral nectaries (EFNs), ant-mediated indirect interactions occur between the plants and aphids. In particular, when an aphid colony is small, the honeydew production of the colony is insufficient to attract ants, although EFNs can attract many ants. Under this condition, we asked whether a small aphid colony would gain indirect benefits from EFNs because ants would be attracted by the EFNs instead of the aphids, or whether the aphid colony would suffer strong predatory pressure from ants and/or other predators. To clarify the aphid-plant interactions, the protective services provided by two ant species, Lasius japonicus and Tetramorium tsushimae, for the survival of a small colony of Aphis craccivora were examined on the host plant, Vicia faba, with and without EFNs. More workers of both ant species were recruited to the plants with EFNs than to those without EFNs, and the larvae of a predatory ladybird beetle, Coccinella septempunctata, stayed for a shorter time and preyed on a smaller number of aphids on the plants with EFNs than on those without EFNs. In addition, no negative effects of EFNs, such as predation of the aphids by the ants, were found. Consequently, it is suggested that a small aphid colony gains indirect benefits from the EFNs of its host plant.