flies swarming in early summer
Archaic seasonal word for the flies that appear in great numbers at the start of summer. Not used in modern everyday speech; encountered mainly in classical literature or poetry.
五月蠅 was used as a summer season word in old waka and haiku.
From 五月 “the fifth lunar month” and 蠅 “fly,” with voicing of はえ to ばえ; it denotes the flies that swarm in early summer.