Propolis — "bee glue" — is a resinous cement bees make from tree and bud resins mixed with wax and enzymes. Colonies use it to varnish the inside of the hive, seal cracks, and shrink any opening smaller than their bee space (roughly 6mm). Beekeepers exploit this instinct with a plastic propolis trap: a screen of slots placed over the top bars. Slots narrower than bee space get relentlessly sealed with resin, while wider gaps are usually left alone or bridged with brace comb instead.
Propolis composition varies enormously by region because it reflects whatever resin sources — poplar, birch, conifer, alder buds — grow near the hive, which is why "poplar-type" and "Baccharis (green)" propolis have very different chemistries even though bees make both the same way.
A 3D propolis-trap simulation: bees seal a slotted screen wherever the gap is smaller than bee space, then chill the trap and flex it to flake off harvestable propolis.
Narrowing the trap's slot width below the ~6mm bee-space threshold makes far more of the screen "active", prompting foragers to seal it with resin. Warm propolis stays sticky; cold propolis turns brittle and flakes off cleanly when flexed.
Narrow the trap gap to recruit more sealing bees, raise colony activity to speed up deposition, then chill the trap and hit "Chill & flex" to harvest — watch flakes fall into the collection tray and track the grams collected.
Commercial propolis traps rely entirely on this bee-space instinct rather than any training — the same reflex that makes bees glue frames together is what fills the slots for harvest.