Chemical control is difficult because the application must be timed with the emergence of the adult flies. An application of an insecticide spray when the adult flies emerge (this corresponds to when weigela is in bloom) can reduce populations. Insecticide sprays containing bifenthrin (Talstar), carbaryl (Sevin), cyfluthrin (Tempo or Bayer Advanced Vegetable & Garden Insect Spray), or malathion are among the recommend materials that can be applied to control the adult flies. Imidacloprid, a systemic insecticide (Merit or Bayer Tree and Shrub Insect Control) will control the larvae inside the leaves and does not require precise timing. This product should be applied in mid to late April. Be sure to read and follow all instructions and safety precautions found on the label before using any pesticide.
Yes leaf miners could kill a plant, or disfigure it permanently. Because timing is so important and if you have a lot of boxwoods, a visit from a Certified Arborist would give you peace of mind by handling it professionally. Find one or more Certified Arborists in your zip code at www.treesaregood.com Go to the Find an Arborist tab. Arborist evaluations are not expensive and where you can learn exactly how to handle the situation, or he/she could handle it for you. Read more here: https://www.canr.msu.edu/news/boxwood_leafminer_a_serious_pest_of_a_favorite_landscape_plant