From the Super Mario Wiki
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Bubble Dayzees are purple Crazee Dayzees that will walk a short distance and then blow out bubbles. They are in three levels of Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, and Yoshi's New Island, but not Yoshi's Island DS.
Yoshi can eat the bubbles (which are also produced by Barney Bubbles) and then spit them back out to destroy almost any enemy. He can also bounce off of them to reach greater heights, but the bubbles he touches will pop in the process. Once Yoshi blows them out he'll have to lick up another to keep the weapon. The bubbles can only run out if they all pop and the Bubble Dayzee is gone.