How large bees,BombusandXylocopa(Apoidea Hymenoptera) forage on trees: optimality and patterns of movement in temperate and tropical climates

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Abstract
Large bees, Bombus spp. (Apidae) of the temperate zone and Xylocopa spp. (Anthophoridae) (Hymenoptera) of the tropics forage upwards on inflorescences on the crowns of trees, laterally and upwards between inflorescences which are close together (< 15 cm), but downwards and laterally between inflorescences which are separated (> 20 cm). This pattern has been observed for large bees foraging on flowering trees of Aesculus hippocastanum, A. octandra (Hippocastanaceae), Robinia pseudoacacia and Cladrastis lutea (Leguminosae) from temperate North America and Tilia sp. (Tiliaceae) in Scandinavia and, in the tropics, one Peltopborum pterocarpum (Leguminosae) in Malaysia, Moringa oleifera (Moringaceae) in the Maldive Islands, and on Sarcotheca celebica (Oxalidaceae) in Indonesia. Upward foraging on inflorescences, whether upright or pendant, is consistent with well-known behaviour for bees on inflorescences of herbaceous plants. The downward movements between separated inflorescences may represent an exaggeration of the usual pattern exhibited by bees when they depart from high on one inflorescence and start low on the next one they visit. The observed patterns of bees foraging on trees for intra- and inter-inflorescence movements is energetically efficient. The former is likely because of the proximity of flowers within inflorescences and the greater ease and accuracy for bees making very short upward movements by flight or walking. The latter may be because the bee, starting high on a tree, gains mass as it accumulates its pay-load and downward flights are less energy-consuming than are upward or horizontal flights.