ON WHICH DIET CAN WORKER HONEYBEES BE REARED INVITRO

The trophogenic basis of caste development has been studied in honeybee larvae mainly with respect to queen inducing factors. To date the alimentary conditions for worker development have not been analysed. Therefore we used in vitro techniques described by HANSER (1980) and REMBOLD and LACKNER (1981) to test the effect of 15 semi-artificial diets on caste differentiation. The aim was to investigate the nutritive conditions for rearing honeybee larvae in vitro into adult pure worker bees. Because the nurse bees feed 4th and 5th instar worker and drone brood on jelly diluted with honey and pollen, we used different diets for young (L2 and L3) and old (L4 and L5) larvae. The diet for young larvae was a mixture of 50.00% royal jelly, 36.25% aqua bidest, 6.25% fructose, 6.24% glucose and 1.25% Difco bacto-yeast extract. In the diets for old larvae the yeast extract was replaced with water. A stepwise reduction of the royal jelly content in the old larval diet resulted in an increase in mortality. A significant effect on caste differentiation was not observed (Fig. 1). Heat treatment of the royal jelly inhibited the production of queens. More worker bees were reared on such diets (Fig. 2a). Similar results were obtained by feeding larvae on diets without yeast extract (Fig. 2b). This indicates a more pronounced effect of food quality than food quantity on caste development. A simple and effective method to produce mainly (more than 90%) pure workers was found in a uniform supply of the larvae through all feeding stages with diet free of yeast extract. The yield of adult bees was approximately 50%. This feeding program can be applied for in vitro studies of pesticide effects on larval development as well as investigations of brood parasites like the Varroa mite. It is concluded that in the highly eusocial honeybees a strictly balanced feeding program was evolved only for workers. This enables the colony to produce large numbers of non-fertile individuals on the basis of a minimum quality food. Thus mass rearing of workers became effective with relatively low investment. By any quality improvement, caste differentiation is shifted towards queens.