Facultative mutualism between weedy vetches bearing extrafloral nectaries and weedy ants in California

Vetches (Vicia spp.) were studied in the San Francisco Bay Area of California in the spring of 1978. The stipular nectaries of the vetches are visited by the Argentine ant, Iridomyrmex humilis Mayr. The nectaries were removed to exclude ants in controlled experiments to determine if these ants protect the vetches from herbivores or seed predators. Plants with excised nectaries suffered substantially greater damage to their foliage than control plants, indicating that ants protect the foliage. There was no indication that ants protect the vetches from seed predators, but fruit set was substantially lower in plants with excised nectaries. Analysis of sugar and amino acid composition of extrafloral nectar served as a basis for feeding tests with Argentine ants by using artificial nectar solutions. Ants preferred sucrose and glucose solutions over fructose. They showed no preference for any one sugar mixture over another, nor did they exhibit differential recruitment to artificial nectar solutions containing only sugars or sugars and amino acids. VICIA SATIVA L. agg. (Fabaceae: Papilionoideae) comprises six or so specific segregates that vary nearly continuously (Hollings and Stace, 1978). At least two of the segregates, V. sativa sensu stricto and V. angustifolia L., have extrafloral nectaries. In vetches, these nectaries are on the stipules, and are often dark or reddish-purple. V. angustifolia also has very small, linear nectaries on the calyx teeth. Stipular nectaries secrete nectar from early in the ontogeny of a leaf until the leaf is mature; ants visit the nectaries even until the leaflets senesce and abscise. Ant visitation therefore accompanies flowering and fruit maturation at a given node. Extrafloral nectar is secreted when the plants are in sunlight (first noted by Darwin, 1900). The nectaries are visited by bees, flies, small moths, and ants. In the San Francisco Bay Area, ants are by far the most abundant visitors to these nectaries, especially Argentine ants, Iridomyrmex humilis Mayr (Formicidae: Dolochoderinae) (Fig. 1). Other species of ants visit vetch nectaries in areas of California where I. humilis is not found. The function of extrafloral nectaries has long been the subject of speculation and debate (as reviewed by Bentley, 1977). Most experimental 1 Received for publication 23 October 1978; revision accepted 5 June 1979. H. G. Baker, I. Baker, E. 0. Guerrant, and C. E. Turner provided valuable discussions and assistance in the course of this work. They also gave constructive criticism on the manuscript, as did G. W. Frankie, W. A. Haber, G. L. Hannan, J. Hickman, K. H. Keeler, C. Lumer, S. Y. Nose, G. E. Uno, and B. S. Yandell (who also provided statistical advice). All this is gratefully appreciated. studies of the ecological significance of extrafloral nectaries have been done with coevolved systems (i.e., with plants and ants native to the area in which they were studied). This experimental study involves two non-native, weedy species. Vicia sativa and V. angustifolia were introduced to California from Europe (USDA, 1960) as a forage crop for large mammalian herbivores, and have escaped from cultivation to become naturalized in grassy fields, along semidisturbed roadsides, and along residential streets in Berkeley and the Bay Area. Iridomyrmex humilis has been called a "tropicopolitan tramp" (Haskins and Haskins, 1965). It is native to Argentina and Brazil (Essig, 1926), and has spread successfully over a large portion of the southern United States, often displacing native ant species (Wheeler, 1910; Goetsch, 1957). Few species of herbivores are found on the vetches in the Bay Area (occasional geometrid larvae; alfalfa weevils, Hypera sp.; and black bean aphids, Aphis fabae). Why are there so infrequently herbivores on the vetches? Plants frequently escape their coevolved herbivores in a new habitat. However, one generally finds other insects on a vetch when that particular plant has no ants on it. This suggests that the ants protect the plants against foliage-eating insects, or ovule and seed predators. The specific epithet " sativa" means "edible," and these vetches are known to be palatable to cows, sheep, horses, and deer; perhaps what the vetches lack in generalized chemical defense against insect herbivores is compensated by ant protection. The preference of ants for various nectar constituents has received little attention. Ricks and Vinson (1970) did preference tests with the im-