This resinous substance is gathered from the buds or limbs of some trees. It is brittle in cold weather but so sticky in the warm weather that the workers apply it immediately to stop up cracks inside the hive to keep out the wind and rain, to reduce too large an entranceway, and to strengthen the combs at their juncture with the walls of the hive. Sometimes little critters like a field mouse will come into a hive as the weather gets cold. The bees will sting it to death and then seal up the corpse ( much like a mummy is wrapped up ) with the propolis.

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A lengthier dissertation on the subject from: http://www.innvista.com/health

PROPOLIS is often called "bee glue." It is the sticky resin that seeps from the buds of some trees and oozes from the bark of others. The trees that produce this resin are mainly conifers, which are evergreens that produce cones. Bees seem to prefer the resin from the poplar tree. There are only a few propolis-gathering experts in each hive. Bees of foraging age collect propolis only on warm days when the resin is soft and pliable. As the resin is gathered, it is blended with wax flakes secreted from special glands on the abdomen of the bee. The mixture is then kneaded or molded into a tiny ball and placed into the pollen baskets located on the legs of the bee. When the source is exhausted she flies to another area to gather until her pollen baskets are full. It may take an hour to fill her baskets. The same procedure is used in reverse when she takes her load back to the hive where the receiving bees help unload and store the substance. This procedure can take several hours.

Propolis is used to reduce the size of the entrance and to patch up holes or cracks. It is also used as an antiseptic, lining each cell and the interior of the hive. If another insect enters the hive, it is promptly killed and removed. If the body is too large to remove, it is covered with propolis to keep its contaminants from harming the hive.

Pliny, the Elder (79-23 BC) divided propolis into three categories: 1) commosis - referring to its use as a disinfectant; 2) pissoceros - referring to its use as a structural reinforcement; 3) propolis --referring to the reduction of the entrance to the bee's city or "polis." Pliny also describes the medicinal action of propolis on humans in the reduction of swelling, the soothing of pain, and the healing of open sores.

It is reported that the renowned Stradivarius (1644-1737 AD) handmixed his own propolis varnish to polish his handcrafted instruments. Having made only 1,116 stringed instruments, no one has ever been able to duplicate his workmanship or his recipe for this varnish.

During the Boer War (1888-1902 AD), propolis was mixed with petroleum jelly and used successfully to disinfect wounds. Before the days of antibiotics, propolis was used most often to combat infections. More recently, it has shown to be effective against bacteria resistant to penicillin, ampicillin, methicillin, streptomycin, chloramphenicaol, oxytetracycline, erythromycin, and sulfathiazole. It is also effective against E. coli and salmonella.

Used with alcohol, propolis has removed molds and fungi more efficiently and for a longer period of time than standard remedies. Propolis has antiseptic, antibiotic, antibacterial, antiviral, and antifungal properties. Scientists state that at least part of this can be attributed to the galangin, caffeic acid, and ferulic acid components. Other known components of propolis include: 55% balsam and resinous compounds, 30% beeswax, 10% ethereal and aromatic oils, 5% pollen, plus flavonoids, cinnamic acid, cinnamyl alcohol, vanillin, caffeic acid, tetochrysin, isalpinin, pinocembrin, chrysin, galangin, and ferulic acid. Propolis is said to have 500 times more flavonoids than the average orange.