My barrel cactus started having tiny white spots 2 months ago. Now, these tiny spots have become significantly larger and more new tiny white spots are developing.

No, I have not left my barrel cactus under the noon sun nor has it been under the hail.

Help me please....

http://i54.tinypic.com/sv1c8j.jpg

Looks to me like something has been munching on it. Any signs of slugs or some kind of caterpillar that could of got to it?

Yep, I concur. Definitely looks like scar tissue to me.

Thanks for your comments. No, I'm pretty sure those were not caterpillar bites nor eaten away by slugs. If you look closely at the picture you'd see some light yellow green discoloration.

These started out as yellow green discoloration, then eventually becomes small scars then grow bigger.

I also at first suspected insect bites therefore moved by barrel cactus indoor out of reach from insects but these spots continue to surface.