Mounting evidence implicates ecdysteroids in queen/worker differentiation during the last larval instars of highly social insects. Since models on social insect polymorphism predict that the expression of morphological caste differences should depend on caste determining signals occurring in earlier larval stages, we now focussed on these instars. Bombus terrestris is of particular interest because caste is already determined in the second instar, presumably by a pheromonal signal emitted by the egg-laying queen. Caste differences in the adults, however, are only expressed at the physiological and not at the morphological level, except for the distinctly larger size of the queen. In the present study, we analyzed ecdysteroid titers in queen and worker larvae from the second through to the feeding period of the fourth (last) instar. Pronounced cycles of caste-specifically modulated ecdysteroid titers were apparent in the second and third but were abolished in the fourth instar. In feeding-stage fourth instar larvae we could detect two small ecdysteroid peaks, with the one preceding the cocoon-spinning phase presenting the characteristics of a pupal commitment peak. The synchrony of caste differences in ecdysteroid and juvenile hormone titers suggests a synergistic action of these hormones in caste determination and provides endocrine evidence for separate mechanisms acting in caste determination and caste differentiation in social bees.

Index terms: ecdysteroids, caste development, bumble bee, Bombus terrestris

Copyright: The copyrights of this abstract belong to the author (see right-most box of title table). This document also appears in Session 13 – INSECT PHISIOLOGY, NEUROSCIENCES, IMMUNITY AND CELL BIOLOGY Symposium and Poster Session, ABSTRACT BOOK II – XXI-International Congress of Entomology, Brazil, August 20-26, 2000.