BURGES has described the aggregation of the Khapra beetle, Trogoderma granarium1. Its mode of life helps this serious pest to survive in its niche in the granary. A beetle that lives on a stored product can develop normally in the dark, but when exposed to light its development is irregular and mortality high2. Both sexes secrete a substance that has a mutually pronounced effect3,4 and acts as a strong repellent for the red flour beetle, Tribolium castaneum5. We have investigated the structure and dual function of this substance.