Foraging Strategy and Pollen Preferences of Andrena vaga (Panzer) and Colletes cunicularius (L.) (Hymenoptera: Apidae)

Andrena vaga (Panzer) and Colletes cunicularius (L.), both vernal ground nesting bees, were studied in the years of 1996-1999 in a lowbush heath near Cologne, western Germany. Both species are solitarily but nest gregariously and sometimes form large aggregations with thousands of nests. They are reported to feed strictly oligolectic on the genus Salix. We observed the daily foraging rhythms of both species and compared their foraging strategies. Colletes cunicularius started provisioning trips earlier in the morning, made more trips per day than A. vaga, and finished nest provisioning later in the evening. Colletes cunicularius burrowed even in the dark after 08.00 p.m. Andrena vaga collected pollen and nectar on different days each. One pollen day included 1 to 5 pollen trips. There was no clear correlation between the number of pollen trips and the occurrence of a subsequent nectar day. We found also no correlation between the occurrence of nectar-provisioning trips and weather conditions. Pollen loads of both species were analyzed qualitatively and quantitatively with a cell counter and two different hand-counting systems. Andrena vaga collected nearly twice as much pollen as C. cunicularius during one foraging trip. Cells and pollen loads of C. cunicularius contained large portions of other pollen types, mostly Rosaceae such as Prunus, Sorbus and Pyrus or Acer, Quercus and Ilex. Thus, C. cunicularius is not oligolectic as described in the literature. The percentage of pollen types other than Salix increased at the end of the flowering period of Salix, which indicates a resource restriction at the end of the season. The reproductive success of C. cunicularius measured by nest provisionment exceeded that of A. vaga, because of longer activity per day and digging activity in the evening.