Ants and extrafloral nectaries: no evidence for plant protection in Helichrysum spp. — ant interactions

Summary

Characterstics of Australian endemic Helichrysum bracteratum and H. viscosum suggest that foraging ants act as “guards” of developing flowerheads, protecting capitula from seed predators: (1) extrafloral nectar is secreted from leaves subtending the capitula and from bracts encircling the floral disc during pre- to post-flowering periods; (2) capitula are attended by ants; and, (3) encounters between ants and other capitula visitors, including predispersal seed predators such as Tephritis sp. (Diptera), can be frequent. In experiments to test the ant-guard hypothesis, exclusion of ants from plants increased abundance of other insects on the developing capitula. The difference between ant-access and ant-exclusion treatments was related to ant abundance on the access plants. These effects were statistically significant in spite of the large variation in insect activity between sites and through the season.

The increased abundance of insects on capitula following ant-exclusion did not, however, result in significant increases in the number of adult seed predators observed on capitula, the number of immature seed predators in capitula, or capitula damage as estimated between ant-access and exclusion treatments of either H. bracteatum or H. viscosum. Further, the ant-exclusion treatment on H. bracteatum had no significant influence on pollination as measured by seed set or on the degree of parasitism of Tephritis sp. by Megastigmus sp. Site and season most strongly affected numbers of immature seed predators and damage to capitula.

We discuss these findings in relation to the ant-guard hypothesis and suggest that generalization of the protection hypothesis to all plants with extrafloral nectaries is premature.