Food-dependent caste determination in the bumblebee Bombus pascuorum (Hymenoptera: Apidae)

For the first time the interdependence of caste determination and the food supply for larvae could be examined in colonies of Bombus pascuorum (Scopoli 1763) under controlled conditions in outdoor boxes. The development of larvae was observed in 7 colonies: 455 workers, 133 queens and 84 drones emerged out of 122 brood clumps. Larvae from 42 brood clumps were fed experimentally with different qualities and quantities of pollen. In none of the experimental colonies observed, the average feeding frequency of the workers brood clumps exceeded the feeding frequency of the queens. Regarding all colonies, a feeding ratio of 1 : 1,86 between worker and queen larvae could be registered. With a feeding frequency of <0.5 larval feeding/hour/day [Futt/h/d], workers were produced at 75% of the observed brood clumps. With >0.7 - <0.9 Futt/h/d, already 59% of queens, and from 1.2 Futt/h/d, only queens were produced, with one exception. Experimental feeding of larvae from 42 brood clumps proved a production of queens obviously dependent on the quantity of larval food. In an experiment which was based on a artificial feeding frequency of >0.5 Futt/h/d, it appeared that only queens were produced with one exception. Consequently, it is obvious to suggest a threshold value of food quantity inducing the production of queens in Bombus pascuorum colonies. Furthermore, the results shows no dependence of the caste determination on an egg or larva cannibalism, since these have never been observed in the course of this research.