If “Prickle Fairy” is changed to an Insect-Type monster with “DNA Surgery”, your opponent cannot attack.
Netrep Rulings:
If “Prickle Fairy” is turned to Insect-Type and there are only face up Insect-Type monsters on the player’s field, the opponent may not attack unless the attacker can inherently attack directly.
If a face-down Insect Type is attacked while “Prickle Fairy” is face up and active on the field, Damage Calculation will proceed normally.