My understanding is that trichogrammas eat bag worms and other caterpillar and worm-type pests.
I think you only need to release trichogrammas if you have one of those pests already.
So, if you don't have the pests, the trichogrammas don't stick around. (Same as ladybugs: no aphids, the ladybugs move on to where there is a food source.
I'm remembering this from reading one of Howard's books so maybe somebody who has first-hand experience will post.
Hope you get rid of whatever pest is interfering in your garden!
**Take time to stop and smell the flowers
(or... as my ladybug refrigerator magnet says
"take time to stop and eat