The importance of proline for the honeybee

Insects can cover their energy needs with carbohydrates, fats and amino acids. Many species have specialized in using one of them, some utilize one in special metabolic conditions and another under different circumstances. So some locusts use carbohydrates for short flights and switch to fats during longer flights. Some beetles use one special amino acid, proline, for their flight metabolism in a recycling process. So it was surprising to observe that honey bees who fly using carbohydrates as their energy source, have high levels of proline in their haemolymph. This could derive from their main food, nectar and honey, both containing proline in large amounts. However, not all honeys contain high amounts of this amino acid and our experiments showed that even artificially nourished bees, receiving diets free of proline, had very high levels of this amino acid in their haemolymph. This indicated, therefore, that the bees also produced proline to meet their own demand. Flight experiments showed that during the high energy demand caused by flying, small but significant absolute amounts of this amino acid were metabolized in addition to carbohydrate utilization. As honey bees have an enormous energy need during flight (40mW per bee) the energy derived from amino acids, even if small, may be an important contribution to the total energy expenditure. In a honey bee colony there are worker bees, which perform most flights. The proportion of proline of the total of amino acids is more than 2/3. Drones only fly during excellent weather conditions and during a certain period in the day. These flights are mating flights and if they are successful, the drone dies. The only function of the drones is mating and their amino acid and thus also the proline content is increasing during their development and then equally during the rest of their life. The latter amounts to about half of the total amount of free amino acids. Queens showed the most interesting correlation of amino acids as well as especially proline in their haemolymph. This caste, is usually maturing within the first days of life, performs several mating flights where she can have about 20 partners and then starts to lay eggs. Either she is never flying again during her future life of several years or she is swarming once. To be able to produce eggs that result in a colony population of some ten thousands, she has to be fed permanently. A queen shows an increasing amino acid content during the phase when she is developing and until she is mated. After mating, she has stable and high amino acid haemolymph content that consists of about 60% proline. If she is not enabled to lay eggs, this content is decreased. A lack of mating also causes a lower amino acid content. Thus the amino acid levels are correlated to certain ages and the functional status and the sex of the bees and are always high even if there is no actual need to perform flights