From the laver and wood book on bats:
Len Newbery, one of the most influential personalities in the batmaking industry in the 1970s shares our sentiments on Butterfly Willow. "The timber of the 'salix alba caurulea' is distinguished by a bright orange stain which is known in the trade as 'Butterfly' stain'. When these stains appear on the blades of bats, players often reject such bats as thinking that the stains are faults or knots, whereas they are in fact the hallmark of the finest quality cricket bat willow."