The temperature formula does work, I don't know it from memory though. I think they're not prolific. Where I live now, on the Oregon coast,
I've only heard the one, I caught it, put it in a big reptarium cage, and just before I closed the door- it hopped out. 3 days later I found it under
a blanket, and kept it for some time feeding it bramble and occasional misting.
Where I grew up, in the Willamette Valley, they were more common, but still not prolific. Now, in the right spot, a person could catch katydids by the handful.
Another fun insect to have, for their chirpings.