I've already posted two BYGL Alerts this season on the non-native boxwood leafminer (Monarthropalpus flavus). You can read the Alerts by clicking on these hotlinks:
I'm posting this third Alert because boxwood leafminer activity is already evident on their namesake host in southwest Ohio. I've never before seen this much summer damage which does not bode well for 2020.
Shortly after boxwood leafminer females emerge in the spring, they mate and then use their sharp ovipositors to insert eggs between the upper and lower leaf surfaces. Adult emergence was the subject of my second boxwood leafminer Alert this season.
Once the eggs hatch, the resulting midge fly larvae (maggots) consume interior leaf tissue between the upper and lower leaf epidermis as they develop through the 1st and 2nd instar stages. These first-season mines appear as slightly raised "blister mines."
Winter is spent as 3rd instar larvae inside the leafmines. The larvae resume feeding in the spring and develop through a 4th instar stage. Typically, much of the leaf damage occurs in early spring with the ravenous larvae rapidly expanding their leafmines. Indeed, spring leaf damage was the subject of my first boxwood leafminer Alert this season.
However, I recently observed boxwood leaves with such a large number of summer mines that I suspect we could see damage by the end of the season as multiple mines coalesce to delaminate the upper and lower leaf surfaces. The damage will most certainly intensify next spring.
You should inspect boxwoods that are under your care. If you observe the level of leafmining activity that you see in the images I've posted labeled "Early July," you may want to consider making a soil-drench application of a systemic insecticide to kill the leafmining maggots before they produce additional damage. While some leaves may continue to delaminate, the application could help to reduce the overall impact.
The systemic neonicotinoids imidacloprid (e.g. Merit, Marathon, and generics) and dinotefuran (e.g. Safari or Zylam) are effective. However, imidacloprid takes longer reach a sufficient concentration in the leaves to kill the maggots compared to dinotefuran. As always, read and closely follow product label directions to maximize efficacy while minimizing non-target impacts.
As I noted in my previous BYGL Alerts, the best long term management strategy is to plant boxwoods that are less susceptible to the leafminer. A helpful research-based listing of the relative susceptibility of boxwoods was published in 2014 by the American Boxwood Society in their "The Boxwood Bulletin." Click this hotlink to access the publication: