First rain, then sunshine and practically summer heat. The result is that Milan has been invaded by bees. Firefighters had to intervene several times to rescue citizens surrounded by swarms of pollinating insects. Swarming is a natural phenomenon in the life cycle of bees, in which part of a colony, led by the old queen, leaves the hive to establish a new colony. It is a mechanism of hive reproduction. The phenomenon is not dangerous to people: during swarming, bees take with them 50% of the hive's honey resources, substances that are useful for building the new base; they hardly pick a fight with their new "neighbors" and thus with people. Swarming is mainly a spring phenomenon, usually lasting two to three weeks, and depending on weather conditions. Occasionally, however, out-of-period swarming may occur in late summer, usually caused by health problems.