The effects of pollen on the disease resistance of bees were tested by measuring different parameters (haemocyte levels, body fat levels, phenoloxidase activity). Haemocytes are cells in the haemolymph, the circulatory fluid in insects. They are involved in phagocytosis and parasite encapsulation, the latter also requiring activity of the enzyme phenoloxidase; body fats are the principal site for the synthesis of antimicrobial peptides.
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© INRA / Yves Le Conte
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As social beings, bees are not solely dependent on individual immunity but also on the functioning of the entire colony. For this reason, the researchers also analysed glucose oxidase activity as a parameter for social immunocompetence. This enzyme enables the synthesis of antiseptic products that are secreted in food for larvae and in honey. They thus contribute to sterilising food for the colony and consequently preventing contamination by disease at the group level.
In order to test the effect of the amount of protein (supplied by pollen) and pollen diversity on individual and social immunocompetence, groups of 80 bees were fed with monofloral pollens that differed in terms of their protein contents, while other groups received multiflower pollens that contained identical protein levels. A control group received no pollen. The experiment was repeated in five different colonies.
The results showed that the amounts of pollen consumed per bee and per day were the same in the different groups, whatever the type of diet. However, pollen composition exerted a significant effect on various immune parameters.
In terms of the individual immunity of the insects, no difference was observed between the different monofloral pollen diets. However, the multiflower pollen diet contributed to improved individual immunity.
The diversity of food resources contributes to improved colony immunocompetence
In terms of community immunocompetence, bees fed with monofloral pollen (even that containing the highest protein levels) produced fewer antiseptics (secreted in the food for larvae and in honey) than those fed with multiflower pollen. As a result, the colony became more susceptible to disease. This underlines the importance of a diverse diet to ensuring colony immunocompetence, which is necessary to supply the different amino acids arising from proteins or lipid compounds. The research team will be pursuing this work in order to identify the optimum mixture of pollen that enables the development of bee immunocompetence.
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© INRA / Yves Le Conte
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References :
Diet effects on honeybee immunocompetence
Cédric Alaux, François Ducloz, Didier Crauser and Yves Le Conte
Biology Letters. published online 20 January 2010 ; doi: 10.1098/rsbl.2009.0986
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