I don’t have boxwood in my garden so I had to look up this pest.
I like Missouri Botanical Garden for their pretty thorough information, but I do look elsewhere for further details such as schedules and timing that is applicable for my specific area. You would need to do the same, but “when Weigela is in bloom” is a good phenological clue that might be useful in other areas.
They do mention the product you are using, but also present other suggestions for use at other times in the year. (Personally, I maintain as chemical-free as possible garden so I would try the organic approaches first. I might also try thoroughly blasting with home-made soap/oil/vinegar solution …possibly neem oil-based… when the adults emerge from pupa IF I felt like there were insufficient natural predators present and/or if I felt the predators on the boxwood might be sacrificed due to plentiful presence elsewhere in the garden to recover from their loss).
Take a look and let us know your thoughts :
Boxwood leafminer | Missouri Botanical Garden