Chemistry of a mating plug in bumblebees

In the bumblebee B. terrestris males transfer a mating plug into the queen' s sexual tract shortly after sperm transfer. The plug is a sticky, opaque s ecretion of the male accessory gland. In order to clarify the meaning of th e mating plug, we collected the plug substance directly from the male's acc essory gland and identified the chemical substances present with gas chroma tography. The main compounds found in the mating plug were four fatty acids (palmitic, linoleic, oleic, and stearic acids) and a cyclic peptide (cyclo prolylproline). Mixing the four fatty acids resulted in a similar sticky, o paque mass as found in natural plugs, indicating that cycloprolylproline is not necessary for the physical attributes of the plug. The function of the fatty acids may therefore be to build up a physical barrier, optimizing sp erm placement before the spermathecal duct or preventing sperm backflow Cyc loprolylproline, on the other hand, may influence female mating behavior so as to reduce her receptivity. In fact, peptides are known to reduce female receptivity in other insects. This would explain why queens of B. terrestr is are only singly mated, although multiple mating is beneficial during the colony cycle with respect to parasitism and fitness.