A landscape-scale study of bumble bee foraging range and constancy, using harmonic radar

1. Bumble bees play a vital role in the pollination of many crops and wild flowers, and plans for their conservation require a knowledge of the dynami cs and spatial scale of their foraging flights, which are, at present, poor ly understood. 2. We investigated the foraging range and constancy of two colonies of bumb le bees Bombus terrestris L, on a mixed arable farm using harmonic radar, w hich has a unique capability to record the trajectories of insects flying a t low altitude in the field. 3. Foraging bees were fitted with lightweight radar transponders and tracke d as they flew to and from the nest to forage. The resulting tracks gave in formation on length, direction and straightness of foraging routes. Superim position onto a map of the foraging landscape allowed interpretation of the bees' destinations in relation to the spatial distribution of forage. 4. Outward tracks had a mean length of 275.3 +/- 18.5 m (n = 65) and a rang e of 70-631 m, and were often to forage destinations beyond the nearest ava ilable forage. Most bees were constant to compass bearing and destination o ver successive trips, although one bee was tracked apparently switching bet ween forage patches. Both outward and return tracks had a mean straightness ratio of 0.93 +/- 0.01 (n = 99). The bees' ground speeds ranged from 3.0 m s(-1) to 15.7 m s(-1) (n = 100) in a variety of wind conditions. 5. The results support the hypothesis that bumble bees do not necessarily f orage close to their nest, and illustrate that studies on a landscape scale are required if we are to evaluate bee foraging ranges fully with respect to resource availability. Such evaluations are required to underpin assessm ents of gene flow in bee-pollinated crops and wild flowers. They are also r equired when making decisions about the management of bees as pollinators a nd the conservation of bee and plant biodiversity.