I think what I would do is use Misty Fuse to fuse the crocheted butterflies to the background first. Place a sheet of parchment paper over the crochet when fusing so none of the fusible gets on your iron, and iron from the back of the fabric. (You could put a sheet of parchment paper over the fabric too, just to make sure that no fusible gets on the iron, but it would be unlikely.) Misty Fuse is extremely light and fine and will not be obvious even in the holes of the crochet. After that I would sew around the outside edges of the butterfly, either by hand or machine, to secure it. I would add extra stitching around the body and around the wings near the body, but I would not sew around every hole.
Google Misty Fuse to find websites with examples of how it has been used.
This is definitely not outside the realm of quilting, but how are you planning to use the finished item? If it will be a bed quilt, you would want to take extra care when washing and drying as the crochet will be more delicate than most quilts.