Question on Xantid Swarm

[sa17dk]

Can my opponent bounce Xantid Swarm via boomerang or another spell when I declare my attack? I thought he couldnt because I thought the effect is immediately put on the stack, therefore he cant play boomerange in response to it. IF I'm correct, then they can only get rid of the XS on their turn right?

[Jebus]

Well, they can bounce it before you declare attackers.  They're last chance to do this is the Beginning of Combat step.  Be sure to make sure he knows what Phase/Step you are in.  If you skip a step without giving them a chance to play spells and abilities, you'll have to back up.

However, once you have declared it as an attacker, bouncing it is futile.  Its trigger is already on the stack and will resolve if the swarm is in play or not.