Propolis — "bee glue" — is a resinous cement that honey bees make by mixing plant resins (gathered from tree buds and wounded bark) with wax and salivary enzymes. They use it to varnish the inside of the hive, seal cracks, smooth rough surfaces and embalm intruders they cannot remove. Bees strongly dislike open gaps of a certain width — the "bee space" — and will propolize them shut, a behaviour beekeepers exploit with a plastic or fabric propolis trap laid over the top bars: bees fill its slots with resin, and the whole sheet is then chilled until brittle and flexed to pop the propolis flakes free.
Propolis has documented antimicrobial, antifungal and antioxidant properties and has been used since antiquity — the ancient Egyptians used it in embalming, and modern research studies it for wound dressings, dental varnishes and natural preservatives.
Forager bees fly resin from a tree source to a hive-top mesh screen, gradually sealing its slots with propolis; freeze the trap once it's full to pop the flakes free and harvest them.
Propolis accumulation depends on forager numbers, hive temperature and — most interestingly — how close the trap's slot width sits to the "bee space" that triggers the strongest sealing instinct.
Adjust the forager-bee count, hive temperature and mesh gap width, watch the trap fill live, then click "Freeze & harvest trap" to chill and pop the flakes free — just like a real beekeeper would.
Propolis is soft and workable at hive temperature but turns hard and brittle when chilled — which is exactly why commercial harvesters freeze trap sheets before flexing the propolis loose.