Chemistry of a mating plug in bumblebees

Abstract

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 secretion of the male accessory gland. In order to clarify the meaning of the mating plug, we collected the plug substance directly from the male's accessory gland and identified the chemical substances present with gas chromatography. The main compounds found in the mating plug were four fatty acids (palmitic, linoleic, oleic, and stearic acids) and a cyclic peptide (cycloprolylproline). Mixing the four fatty acids resulted in a similar sticky, opaque 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 sperm placement before the spermathecal duct or preventing sperm backflow. Cycloprolylproline, 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. terrestris are only singly mated, although multiple mating is beneficial during the colony cycle with respect to parasitism and fitness.