Caste-specific differences in ecdysteroid titers in early larval stages ofthe bumblebee Bombus terrestris

Mounting evidence implicates ecdysteroids in queen-worker differentiation d uring the last larval instars of highly social insects. In the present stud y, we analyzed ecdysteroid titers in queen and worker larvae of the bumbleb ee Bombus terrestris from the second to the early fourth instar. B. terrest ris is of particular interest because caste is already determined in the se cond instar, presumably by a pheromonal signal emitted by the egg-laying qu een. Caste differences in the adults, however, are only expressed at the ph ysiological and not at the morphological level, except for the distinctly l arger size of the queen. In the second and third instar, ecdysteroid titers in queen larvae were generally higher than those of workers. These early c aste-specific differences, however, were abolished in the fourth instar. In the early fourth instar 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 ecdystero id and juvenile hormone titers suggests a synergistic action of these hormo nes in caste determination. (C) 2000 Elsevier Science Ltd. All rights reser ved.