New bioassays to evaluate diet in Bombus terrestris

Traditional bioassays of nutritional requirements in Apis mellifera are based on parameters which cannot be used in Bombus terrestris (ie hypopharyngeal gland development, longevity of caged honey bee workers and the surface of sealed brood). It is then necessary to create an original method to compare nutritional quality of several bumblebee diets. The new method proposed here is based on the evaluation of reproductive productivity of queenless colonies and of the size and longevity of male offspring. For this experiment, 5 newly emerged worker bumblebees were kept queenless in small boxes divided in 2 compartments. The workers were reared with their offspring and were fed with test diets and sugar syrup. Measured parameters were: male longevity; radial length of male right wings; time before each male offspring; number of male offspring within 100 d; and time before first cocoon. A complementary method was necessary to compare the palatability of the bumblebee diets. It is based on comparison of the different kinds of feeding by colonies. As an evaluation of the methodology, the authors have tested the nutritional quality of 2 mixtures of pollens: the first mainly composed of sunflower (Helianthus annuus L) pollen (low protein concentration: 13%), and the second of rape (Brassica napus L var oleifera (Moench) Delile) pollen (high protein concentration: 22%) (table I). The longevity of males, the radial length of their right wings and the productivity of queenless colonies were greater (respectively, 36.47 ± 14.17 d, 3.72 ± 0.30 mm and 34.75 ± 8.70 males) in the colonies fed with rape pollen diet than in the colonies fed with sunflower (respectively, 28.81 ± 11.25 d, 3.43 ± 0.30 mm and 24.25 ± 8.41 males) (table II). Moreover, colonies of bumblebees ate more rape pollen (49.41 g) than sunflower pollen (27.06 g) (table III). This new bioassay showed great differences between both diets and is thus well suited to study nutritional requirements of bumblebees