There are pros and cons to each of the different management options for boxwood leaf miner. This MSU Extension article provides some great information about the insect with details and treatment options.  The timing for management using contact insecticides would be right around the time that the adult flies are laying eggs in the spring. This timing coincides with blooming of Weigela bushes and is also about the same time that the boxwoods will be producing their own flowers. Boxwood produces small inconspicuous flowers along the tips of last years shoots (see attached photo). This is the ideal timing for an insecticide application targeting the adult leaf miners. However, applying contact insecticides to a blooming boxwood is a sure-fire way to kill bees and other pollinators. Fortunately, it is pretty easy to identify infected leaves in the spring (the article shows great photos to help with this). Pruning off the most affected branches and destroying them by fire or sending them away with your landfill waste can help control their populations. This is also a much safer approach for your pollinators and other beneficial insects. Another option is removing the flowers before applying the insecticide which will prevent you from poisoning flower-visiting insects.