Notice the little skull shape on the wing.
The word Beelzebub translates as Lord of the Flies. Some take it to mean the devil himself but others call it variously: the prince of demons, prince of the flies, chief of the fallen angels (satan's deputy) or even a god. Safe to say, he's a biblical demon.
The nearest I got to him as an actual insect was as a god of the Canaanites, who represented him with the figure of a fly. But we don't need it to be literal. Beelzebub is never represented by a bee, as far as I can tell, and bee symbology tends to be used more for 'good omens'.
Whatever entymolgy you prefer, I'm settling for the tattoo as a symbolic representation of Beelzebub, Lord of the flies and Chief of the fallen angels.
Two eighteenth century magical textbooks, the Grimorium Verum and the Grand Grimoure, state that the supreme trinity of evil consists of lucifer, beelzebub and astaroth. When summoned by a magician, beelzebub appears in the form of a huge fly, allegedly.
Manson's fly tattoo might be the last one made for him by Albert Sgambati.
Related link to buzz off to: Beelzebub Demonology - A Gallery of Demons
You can also look out for the Dictionnaire Infernal
by J.A.S. Collin de Plancy, 1863. That's the illustrated sixth edition one but it was first published in 1818. J.A.S. stands for Jacques-Albin-Simon, if you wondered.
Many of its demon illustrations were republished in (Samuel Liddell Mathers alias) MacGregor Mathers' The Goetia: The Lesser Key of Solomon. I believe Aleister Crowley contributed to this work too.
Perhaps Manson has the above books in his collection.