The pollen substitutes are necessary for honeybee colonies, especially when natural resources of nectar and pollen are unavailable or reduced. These are essential for development of young bees, rearing brood, reproduction and colony maintenance. In this study, the effects of three pollen substitute diets (black seed oil, gluten and vitamins) on food consumption rate, head width, length and fresh weight, as well as the hypopharyngeal gland acini volumes of Apis mellifera carnica worker were compared. Each diet was mixed with sugar and water in equal ratios. The highest food consumption rates of workers were recorded with control, vitamins, and gluten diets. Food consumption rates in winter and summer were significantly higher than in spring and autumn. Gluten- and vitamins-fed 12- and 18-days old workers showed significant increases in length, width and fresh weight of head as compared with control and black seed oil-fed ones. Gluten-fed workers exhibited significant increase in the volume of hypopharyngeal gland acini as compared with those fed other diets due to the high protein content. There was a significant correlation between food consumption rate and head fresh weight and the volume of the hypopharyngeal gland acini. This was an indicator of the degree of gland development. The acini volume significantly increased in both ages in winter and spring as compared with workers in summer, expect for those fed gluten diet. Histochemical study revealed that the midguts of vitamins- and gluten- fed workers exhibited increases in mucopolysaccharides with excessive merocrine secretions as compared with control and black seed oil-fed ones. The total protein content had a marked localization in midguts of gluten-fed workers as compared with workers fed the other diets. Gluten and vitamins pollen substitute diets were superior to black seed oil and are recommended for addition to the diets of honeybee colonies.
Key words: Apis mellifera, pollen substitutes, morphology, food consumption, hypopharyngeal gland, midgut histochemistry