How do I add CAPTCHA to first forum posts?

How do I add CAPTCHA to first forum posts? I have the module installed and I have CAPTCHA on comment replies, but not on the first forum posts. I have these CAPTCHA forms turned on:

* comment_form
* contact_mail_page
* contact_mail_user
* user_register

I have these turned off:

* comment_controls (I don't know what this does)
* user_edit
* user_login
* user_login_block
* user_pass

I don't see how to add a CAPTCHA to the forum thread starter. Any help?