2. Larval nourishment
Honey bee larvae do not feed on a readily available vegetal food; rather, they are fed by adult bees. In a colony, worker larvae are fed worker jelly according to their age by their adult sisters. This fact prevented artificial rearing for a long time. A larva is frequently inspected and progressively fed 135-143 times with worker jelly during its larval development (Lindauer, 1952; Brouwers et al., 1987). According to a rough estimation, one worker larva is fed a total of 59.4 mg of carbohydrates and 25-37.5 mg of protein (Hrassnigg and Crailsheim, 2005).
The composition of jelly fed to workers has been analysed and compared to royal jelly (food fed to immature queens, see Section 5.10. Royal jelly). Findings suggest that royal jelly and the jelly of worker larvae up to the age of three days are similar, at least regarding protein, sugar, and lipid content (Rhein, 1933; Brouwers et al., 1987). Pantothenic acid, for example, was found to be five times higher in royal jelly than in worker jelly; no other differences in vitamin content were reported (Rembold and Dietz, 1965).
Worker jelly is produced by the hypopharyngeal and mandibular glands of nurse bees (Jung-Hoffmann, 1966; Moritz and Crailsheim, 1987). Biochemical analyses show a broad variation among season, age of fed larvae, investigation methods and colony supply (Jung-Hoffmann, 1966; Brouwers, 1984). According to Kunert and Crailsheim (1987), worker jelly is composed of 39.2-53.3 % protein dry weight for larvae younger than four days. After that age, the protein content decreases to 15.7-26.1 % dry weight. Correspondingly, the sugar content increases from 7.8-19.6 % to 32.2-64.6 % dry weight after day four (Fig. 1). These findings were confirmed by Brouwers et al. (1987), who moreover described the decrease of lipids in the food of worker larvae with increasing age. Asencot and Lensky (1988) measured the fructose / glucose ratio of worker jelly and also confirmed the increase of sugar content with age.
Four- and five-day-old worker larvae are additionally fed an increasing, although still small, amount of unprocessed pollen (e.g. Rhein, 1933; Jung-Hoffmann, 1966). After day six, larvae cease feeding and larval cells are sealed by adult workers. During pupation, larvae make use of the material gained during larval nourishment for anabolism and catabolism. The metabolism of glycogen and lipids in particular plays an important role during metamorphosis (Hrassnigg and Crailsheim, 2005).
Fig. 1. Comparison of: a. protein and; b. sugar content of brood food of young (1-3 d) and old (4-6 d) worker larvae (Data summarized from Kunert and Crailsheim, 1987) and laboratory diet, both percentages of dry weight. Protein decrease and sugar increase are due to the increase in sugar added to diets (Aupinel et al., 2005). Range of content is determined by variation in royal jelly protein (Sabatini et al., 2009) and sugar (Brouwers, 1984) content. Protein and sugar in yeast were excluded from calculations.