A warning page on Pangea tells about a horrible reaction to butter worm "juices" on a gecko's face...
http://www.pangeareptile.com/forums/...ad.php?t=36085
The pictures are no longer there, but the description is.
It seems that butter worms have some kind of caustic juice that burns when the juice squirts out onto the geckos' faces.
I'd avoid them at a ll cost.
I use wax worms.