Abstract
The yardlong bean Vigna unguiculata var. sesquipedalis (L.) Verdc (cv. Jyuroku-sasage) has extrafloral nectaries (EFNs) that attract ants. The efficacy of ants visiting EFNs of the yardlong bean as a guard against stink bugs that attack the legume seeds was evaluated by field observation and experiments. The number of ants visiting the EFNs at the base of a legume was not associated with the legume length; the EFNs attract ants during all stages of legume growth. Ant species included Formica japonica Motschulsky and Pristomyrmex punctatus Smith, with the former being predominant. Diurnal activities of ants (F. japonica) visiting EFNs and stink bugs (3 species) attacking the legumes were investigated during 3 days in summer. While the number of ants increased during the day and decreased at night, the number of stink bugs exhibited the reverse pattern. The frequency of stink bugs on legumes with ants was one-third of that seen on legumes without ants. We tested 4 treatments: (1) excluding ants from legumes, (2) applying honey to legumes, (3) aphid colony on legumes, and (4) control. I found that the number of stink bugs was the highest on the ant-excluded legumes and the lowest on the honey-treated ones. These results suggest that ants visiting EFNs serve as a guard against stink bugs that attack the legumes; the efficacy of this protection depends on the diurnal activities of the ants.
