Early flowers of Bartsia alpina (Scrophulariaceae) and the visitation by bumblebees

Phenology and insect visitation of early flowers of Bartsia alpina, a perennial herb, in a subalpine population in northern Sweden, were investigated to find causes for low seed set in early flowers. Bumblebees are the only visitors of B. alpina; they collect pollen and nectar. Flower phenologies of several bumblebee visited species and their nectar standing crops were measured. Flowering of Bartsia started simultaneously with Astragalus frigidus while Pedicularis lapponica was nearly out of flower. Early blooming flowers of Bartsia received fewer visits than later blooming ones. The large volumes of nectar with a high sugar concentration in early flowers indicate that bumblebees neglect or fail to discover early blooming flowers of Bartsia. Early in the season seven bumblebees species visited Bartsia but later on Bombus pascuorum was dominant. This species was also an important visitor of Astragalus. A rather low percentage of the pollen loads contained only Bartsia pollen, 27-28%, during early and peak flowering of Bartsia. Corbicular loads of B. pascuorum, the most frequent visitor of Bartsia, often contained Bartsia but also contained Astragalus pollen and to a lesser extent Ericaceae pollen (probably of Andromeda polifolia). The large variation in composition of corbicular loads indicated that bumblebees were not faithful to Bartsia only and that several plant species were visited by the same individuals. Bumblebee visitation of Bartsia and Pedicularis during a single trip may result in heterospecific pollen deposition which may lower seed set in Bartsia. Bumblebee visitation of Bartsia and Astragalus during a single trip may not result in heterospecific pollen deposition on Bartsia flowers due to the deposition of Bartsia pollen on the dorsal and Astragalus pollen on the ventral side of the body of bumblebees. Low seed set of early blooming flowers of B. alpina was probably due to low visitation rates by bumblebees. In addition, seed set of early flowers may be low due to heterospecific pollen depositions but this also may occur later in the season if the composition of corbicular loads is considered. After artificial heterospecific pollinations seed set decreased as compared with pure pollinations.