Envelope to turn an Effect on/off?

This seems like a real fundamental question and I'm having a mental block or something but I'm trying to find a way to turn an effect on and off. When I open the track envelope option for the effect (Sonitus:delay) there's a long list of options to control, but I'm not finding and on/off or bypass option. My end goal is that common effect of applying echo to the last section of a clip.

Thanks. I'll try that, although I suspect what I'll find when I turn the mix up is that the delay playback will include material ahead of where I want it to start.

Chappel - Thanks again. 

Rothchild - You got it in both shots.
1) I didn't know it was possible to add effects to clips!  That solution almost worked except for the fact that in this case the clip is a raw guitar track so the delay had the effect of altering the sound as it passed through the modeling effect; although that did create an interesting effect.

Either way, knowing about adding effects to clips may come in handy in the future.  I'd have prefered that solution if not for the raw track issue.
2) So I routed to a buss with just the echo effect and used the Send Envelope fader. 

Thanks!