The factors which trigger propolis collection by bees are not completely understood. Hot weather is believed to be a necessary requirement as it softens the resins and makes them easier for a bee to manipulate in its mandibles. — Bernd König (1985) Plant Sources of Propolis, Bee World, 66:4, 136-139,

Contrary to propolis from the temperate zones, where poplars are its sole source, according to our results in Brazil there are more plants that bees could accept as sources of propolis and in different regions its chemical composition can differ. However, the criteria for choice of propolis plant sources by bees remain unclear. — Bankova, V., Boudourova-Krasteva, G., Popov, S., Sforcin, J. M., & Funari, S. R. C. (1998). Seasonal variations of the chemical composition of Brazilian propolis. Apidologie, 29(4), 361-367.

comment:
I remember reading something in detail about the criteria for choice of propolis sources, but I can't figure out where. It had to do with the physical properties of the substances, more than the phytochemical properties (which may be incidental, though fortuitous).

Peter L Borst

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