To get rid of them, remove the affected leaves.

To stop future infestations of additional is a different matter.

Once the eggs hatch and the leaf miner is inside the leaf there is nothing you can do that will stop them, except squish the wee buggers. There are some pesticides that if applied at the proper time could, possibly stop the adults form layuing the eggs that hatch into the leaf miners.
For the most part the damage done by leaf miners is cosmetic and not harmful.

See my posts in this forum on Ilex meserveae. Some of this applies. Boxwood Leaf Miners are tough because of the short window that the adults are active. If you wish to treat the adults you might want to try using degree days to time the adult hatch. Boxwood leaf miner adults hatch between 448 700 degree days. Around here UNH has volunteer degree day calculators and you might find that a university around you does the same, timing is critical. If you can't use degree days, the adults are active around the time Weigela florida is in flower. Look for small orange flies. Also removing infected and fallen leaves will help. I am sending you to a link below, this is somewhat old information though and does not include the Conserve mentioned in the Ilex post . If you want to know more about degree days type something like "predicting insect stages using degree days into your browser".

Here is a link that might be useful: Boxood Leaf Miner Info

The use of phenology (using the timing of life-cycle events to predict the behavior of plants and animals) and indicator plants to help determine the timing of a specific pest outbreak isn't mentioned as much here as it should be. I used to be very involved in the practice and need to get back into it. It's a valuable tool in IPM, that's for sure!

An additional comment about controlling boxwood leaf miners (and other species, too): some control can be achieved by the well timed application (timing IS everything) of a horticultural oil or of Neem oil.

But you do have to have a good idea, as IPM man mentions, of the actual emergence of the adult insect. Please remember that the larvae overwinter in those infested leaves, so some pruning might be in order.

Call your extension office to find out more about using phenology or degree days to determine the timing of any product application. If the local office is of no help, insist that they provide you with the number of someone at UConn who is able to provide you with the information you need.

The attached link has some information about these subjects; scroll down towards the bottom of the fact sheet.

Here is a link that might be useful: FYI

Systemic insecticides such as imidacloprid may help.

I don't have Horticultural Oil listed for this pest in my literature and the Neem products I have listed are Margosan-0, Azatin XL and Neemazad. I don't know if these are available to the homeowner. This does not mean that Horticultural Oil or regular Neem oil will not work, information changes.
Some other plants that are in bloom at hatch time are:
Kousa dogwood, Cranberry bush, Beauty bush, and Mountain Laurel.

Taz Responded as I was typing my last post.
True any of the Neonicotinoids like Merit will knock these things for a loop. I generally try to start with the least toxic approach first and move to these if I don't get control.

Thanks to everyone for kindly responding to my post! We drink well water here so I'm very unwilling to apply pesticides, did only once or twice on the boxwood and it didn't even work, apparently wrong timing. Is Neem oil organic? I'm thinking to trim 2/3 of leaves away to reduce the population, I don't care to decrease the size of the plant as it will grow back very quickly, but then I don't want to trim too much to accidentally kill the plant. What's the maximum amount of leaves that I can trim away? I noticed that for each branch, the leaves on the lower portion are not infested (neither are the tip top and too bad they have to go), so by sacrificing mid-upper portion of a branch, I will have got rid of 80-90% of the larvae, if I do it now, they will never have a chance to become adults, I'm just not sure if I will kill the plants by trimming so much. What do you think?

Neem oil is organic. Spinosad is derived from soil microbes and is organic also but not all formulations have been registered as such. Entrust is a Spinosad formula that is a federally approved product for use on organic produce but it is not registered for landscape use. It is hard to tell how much you can safely prune without seeing the plants. Usually up to 1/3 is safe if the plants are healthy. yes you can do this now.
BTW Arsenic, Nicotine, Cyanide, and Strychnine are all organic.

I think that IPM man is trying to make a point, egrety, that the term 'organic' can be a difficult one to nail down. It truly means so many different things to different people.

In the true chemical sense, the term 'organic' refers to substances with carbon atoms as part of their structure. Others use the term to indicate substances that are natural (not made-made). To some, the word means something that was once living. Lots of folks use it to indicate (so called) non-chemical pesticides, (so called) safe pesticides, etc. What do YOU mean when you use the word "organic'?

Regarding the pruning question...luckily, boxwood is one of the many plants that respond very well to hard pruning, and can even be cut all the way to the ground in one fell swoop in what is called 'rejuvenation pruning'. Rejuvenation pruning is often more desirable than shearing or stubbing, which may result in deformed shrubs.

Whether or not you want to take on the task is a matter of personal choice. Shrubs that have been cut all of the way back usually grow back robustly and evenly, often being far more attractive than ever. However, it may take awhile, depending on how tall you wish your plants to be. And plants aren't going to grow as rapidly in Woodbridge as they might in a warmer climate.

And then there's the very real possibility (a likelihood) that the leaf miners will return again to wreak havoc on your recovering boxwood. It is very difficult to control a pest when you are providing them with THE plant they prefer above all others.

Please do some research about the efficacy of neem oil. You'll learn that not only does it provide a barrier that may repel the adults from laying eggs and even reduce their life span and egg laying ability, but it also affects the life cycle of the larvae. It is considered an IGR (insect growth regulator) for those pests that feed on neem treated plant tissues.

But be mindful of the fact that even a plant based, organic product such as neem still needs to be used with care. (This is true for anything you might use!) When used properly and according to label directions, it is safe and highly effective for the control of a wide variety of plant feeding pests...without providing a significant problem for beneficials. It also has some excellent disease fighting qualities, too.

Rhizo thanks for the further clarification. Yes I was just making a point, not trying to be rude of push Spinosid. There is often a lot of confusion about what is organic and what is safe. Rotenone for example is organic but quite toxic and a broad spectrum product that kills many predators. The point is that one should always research what they use whether it is organic or not.

Yep, there are so many misconceptions and misunderstandings about the use of lots of these products. It's always good to remind people that even the most seemingly benign product can be harmful if not used properly.

egrety, did you get the answers you needed? You have a difficult problem, that's for sure.

Thank you both so very much! I do have a big bushy mountain laurel that I can use as a guide to the timing of adults' emergence, so perhaps I should trim now, and then spray with Neem oil as soon as the Mountain Laurel is in bloom, to hopefully kill the remaining ones. rhizo, thanks for pointing out they can always come back, I hope if this approach works, I will know what to do next time around. Also, since they can come back anyway, I will not pursue a more radical cut (to the ground for example), but I love learning about things like rejuvenating trimming, so thanks! Also really appreciate your cautioning on the use of word "organic", whenever I see "organic" label, I assume the product must be totally free from any man made chemical and therefore can be applied on edible plants, and therefore safe on ornamental ones -- such is my logic. I always willingly pay double or higher prices whenever I see "certified organic" mark, be it on milk, eggs, vegetables, grains, fruits ... anything, I always go for organic if it's an option, quietly swallowing the extra cost thinking it's worth it since it's for health, am I being misled? Sorry I do realize that I'm off topic now, but you are really giving me an education on the subject that makes me reconsider the concept that I have long accepted without giving it a careful thought. Really, how should an educated consumer go about judging an "organic" label case by case?

Well, the 'organic' label simply means that what you are consuming is either grown under organic methods or is approved by national and/or state organizations to be used on those crops. Anyone who SELLS such produce must be Certified Organic. Inspections are made, certain chemicals and practices off limits, etc.

But you need to understand that some of the chemicals we use in our organic practices, though they may be natural in composition, range from outright dangerous (as in those mentioned by IPM man earlier and plenty of others still available on shelves) to very benign AS LONG AS THE LABEL DIRECTIONS ARE FOLLOWED.

For example, insecticidal soap has long been a trusted and trusty chemical, whether commercially produced OR made in the kitchen sink. The commercial products are approved for certified organic use. It can be safely on edibles. BUT, you don't want to spray it willy-nilly (that's a technical term) all over your flowering plants when bees or other pollinating insects are visiting....or 'just in case'. Insecticidal soaps should be considered a broad spectrum organic pesticide. It can harm the good guys right along with all of those aphids. Make sense?

The smartest thing you can do when getting ready to use a product is to be mindful of a few things: you must be certain about the identification of the pest, disease, etc.; you must know the ID of the plants you are trying to protect or cure; and you need to read the label carefully.
Just as your shampoo bottles have labels on them (and warnings), so do these 'organic' pesticides.

It all boils down to a little bit of time on your part, I guess. You've already taken the time and trouble to find some solutions to your problem. You know, now, how important timing is in the control of leaf miners. You've learned that no matter how careful you might be in the future, that boxwood leaf miners will zero in on boxwood. Carefully timed spraying will probably be the norm for as long as those boxwood live. And you've learned some good chemical suggestions that are approved for organic use (horticultural oil, spinosad, neem oil). AND you've learned about rejuvenation pruning!

Go forth and kill some leaf miners.

Thanks a lot, rhizo!

Using Merit. Studies show Boxwood leaf minor love Merit. It's candy for them. Use other products, Neem, Oil.
In fact, Merit and leaf minor on all plants are poor choice, Besides being a Bee killer.