Pollination, Predation and Seed Set in Linaria vulgaris (Scrophulariaceae)

Linaria vulgaris, a self-incompatible perennial herb, has a complex of adaptations related to insect-pollination. These include: (a) the production of conspicuous yellow flowers, each with a bright orange palate, arranged in vertical inflorescences of 15-20 flowers; (b) the production of multiple inflorescences by individuals. These flower in a staggered, but temporally overlapping, sequence suggesting a trade-off between maximum showiness and maximum blooming period; (c) the production of nectar containing 37% sugar, as sucrose equivalents. Despite these adaptations, however, Linaria vulgaris has low seed set when compared to most other self-incompatible perennials that have been studied. The success of ovules declined from ca. 1.6%o in early August (the height of the season for this species) to ca. 0.3% in early September, although individuals continued to produce new flowers until the end of September. The declining seed set occurred at a time when pollinator activity was declining. It is clear that predispersal seed predation by insect larvae was important in limiting seed set. Heavy predation of ovaries, however, made it difficult to assess the relative importance of pollinator availability as an additional limiting factor. INTRODUCTION Recently, there has been considerable interest in the features of flowering plants that make them attractive to pollinators. One of the questions investigated has been the extent to which seed set in self-incompatible plant species is pollinatoror predatorlimited. Schemske et al. (1978) showed that such limitation is a factor in six selfincompatible spring woodland herbs, which set fewer seeds under natural conditions that when hand-pollinated. Barkham (1980) suggested that seed set in Narcissus pseudonarcissus is severely pollinator-limited. Silander and Primack (1978), however, concluded that pollinator service was adequate to achieve complete seed set in Oenothera fruticosa. Zimmerman (1980a) reported that pollinator service was adequate to achieve a high level of seed set in Polemonium foliosissimum in the early part of its flowering season, but that pollinators were a limiting resource later in the season. Zimmerman (1980b) also reported that the seed predator, Hylemya sp., of P. foliosissimum caused high seed mortality in ovaries of flowers that had received a large number of pollinator visits and, as a result, set many seeds. This paper describes the pollination ecology of Linaria vulgaris Mill. (Scrophulariaceae). The research was designed to answer four questions: (1) How attractive are flowers of this species to pollinators; (2) What is the success rate for ovules of this species; (3) How available and effective are the pollinators; and (4) Is predation an important factor in limiting seed set? Linaria vulgaris is a rhizomatous perennial which is widespread along roads, railroad tracks and waste places throughout the United States. It is believed to have been deliberately introduced into eastern North America as an ornamental soon after the arrival of European settlers and has become a serious weed in some places, especially W of the Rocky Mountains. The flowers are strongly zygomorphic, yellow with a bright orange palate, and arranged in racemose inflorescences of 15-20 flowers. Each flower has a spur 15-20 mm long in which nectar collects.