Collection on spring pollens by bombus terrestris queens. Assessment of attractiveness and nutritive value of pollen diets

International audience

English. Analysis of the crop content of 44 queens of Bombus terrestris captured in spring over 5 weeks revealed that pollens ingested were collected from 15 plant species. Prunus and Salix accounted for 62% of the total, by volume. Through feeding tests performed with queenless micro-colonies, consumption and nutritive value of Prunus pollen was compared to those of two other pollens: Salix, also frequently collected by queens and Taraxacum which was never found in their crop. If honeybee loads of Prunus and Taraxacum were presented to workers, Prunus consumption was 2.5 times higher than that of Taraxacum. When honeybee loads were mixed with syrup to make pollen pastes of pure Prunus or Taraxacum and blends containing 10%, 50% and 90% of the other pollen, pure Prunus paste was consumed 4 to 12 times more than pastes with 50% to 100% Taraxacum. The nutritive value of the three pure pollens was estimated by the comparison of their effect on the reproductive success of queenless micro-colonies composed of three callow workers. Prunus, Salix and Taraxacum pollen resulted in similar effects on worker survival and egg clump deposition. Taraxacum pollen significantly affected ovary maturation estimated by the delay of the first eggs laid (10.6 days) compared to Prunus and Salix (7.6 days). Taraxacum did not enable production of males whereas Salix and Prunus resulted in male offspring production (8 and 19 males per micro-colony respectively). Prunus resulted in significantly higher performances than Salix in terms of oophagy (7.6 % versus 38% of egg clumps eaten), whereas Taraxacum resulted in 88% oophagy which is significantly higher than oophagy with Salix. Therefore, oophagy would be the more discriminant criterion when pollen nutritive value has to be tested comparatively. Protein rates were 27%, 20% and 17% in Prunus, Salix and Taraxacum, respectively. Protein efficacy ratios (number of males produced / weight of protein consumed) were 10.1, 6.6, and 0, respectively. These figures suggest that an amino-acid deficiency or inadequate balance affected the nutritive value of Salix and especially Taraxacum, compared to that of Prunus.