The swarming season of Japanese honeybees is said to be from the end of April to June (Matsuura, 1969), from the end of March to May (Okada, 1990) or from April to June (Sasaki, 1999).
In the northeast part of Osaka Prefecture where I have been doing the research, honeybees swarm from both keeping nests and wild nests even in July. I call this the summer swarming, because the bees swarming in July may come from a nest which have already been swarmed in the spring. Therefore, some of Japanese honeybees swarm twice a year, in spring and in summer. In 2001, according to my research, the summer swarming in Moriguchi city occurred on July 1st, 17th and 20th from the keeping nests, and on 27th from a wild nest. Especially, the ones on 17th and 20th were from the nests which had been swarmed in the spring.
The summer swarming is different from the spring swarming in some points. First of all, in the summer swarming, the number of honeybees swarming is very large. It is said to be half the number of honeybees in a nest leave to swarm in spring. This means that the number of swarming honeybees is larger than the number of remaining bees in summer. The mechanism relating to the number of bees, to leave or to remain, has not yet been analyzed. I think that the summer swarming may give us a suggestion to solve this mystery.
The second different point is that colonies gathering on swarm luring boards take long time to be compact in summer. It is probably because of the high temperature. In the spring swarming, honeybees on a beeball once compact each other. And then, usually in the next day, they take off to the new place. If you get a compacted colony and put it in a wooden box or something, they will make the new nest in there. However, in the summer swarming colonies, they usually escape from wooden boxes. It looks that their excited state at the time of swarming lasts forever. I would try to measure the temperature of honeybees to compare the physiological condition of the spring swarms with the summer swarms.

巨大な分蜂群 丸木をくりぬいた巣より分蜂 (2001.7.1)	分蜂直後の巣内 入口の開いた王台が５、閉じた王台が１存在 (2001.7.1)

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