It's common for boxwoods to emerge from winter with yellow to brown foliage caused by a number of issues including winter injury and salt damage. However, a primary reason boxwoods are discolored this spring in many areas of Ohio is the heavy damage caused by the non-native boxwood leafminer (Monarthropalpus flavus) that was initiated early last season.
I posted an Alert in early July last season noting that populations of this tiny midge fly were unusually high in some parts of Ohio; they were the highest I'd ever seen in the southwest part of the state. Some plantings had virtually every leaf loaded with maggots. It was a portent of heavy leafmining symptoms that would appear this spring (Click this hotlink to read "Boxwood Leafminer 2020 Prediction": https://bygl.osu.edu/index.php/node/1342)
This non-native midge fly was accidentally introduced into North America from Europe in the early 1900s and is now common throughout Ohio. Adults emerge at around the time red horsechestnuts (Aesculus×carnea) and doublefile viburnums (Viburnum plicatum) are in full bloom (440 GDD). Except for their bright orange abdomens, the adults superficially resemble miniature mosquitoes.
The boxwood leafminer is grouped with the "gall midges" (family Cecidomyiidae, subfamily Cecidomyiinae); however, this taxonomic construct is not based on lifestyle. The flies do not produce galls; they are leafminers.
Females use their needle-like ovipositors to insert eggs between the upper and lower leaf surfaces of boxwood leaves. Each leaf may contain multiple oviposition sites with several eggs per site. These sites will become individual leafmines producing the blister-like leaf symptoms.
Eggs hatch in early summer and the resulting larvae spend the remainder of the season consuming interior leaf tissue as they develop through the 1st and 2nd instar stages. Winter is usually spent as 3rd instar larvae inside the leafmines. The larvae resume feeding in the spring and develop through a 4th instar stage. Much of the leaf damage usually occurs in early spring with the ravenous maggots rapidly delaminating the upper and lower leaf surfaces as they expand their leafmines.
Individual mines may turn reddish-green at this time of the year with heavily mined leaves turning from yellow to orangish-brown causing the leafmining damage to be mistaken for winter injury.
Damaging boxwood leafminer infestations can be suppressed through applications of neonicotinoids such as imidacloprid (e.g. Merit, Marathon, and generics) or dinotefuran (e.g. Safari or Zylam). However, applications should be delayed until AFTER boxwoods bloom to protect pollinators.
Boxwood blooms attract a wide range of pollinators; blooming plants can literally buzz with their activity. Delaying applications until blooms drop will result in some minor miner damage, particularly with imidacloprid because it is taken-up more slowly compared to dinotefuran. However, this is a small price to pay for protecting pollinators.
Plant selection provides a more long term solution by removing insecticides from the management equation. A helpful research-based listing of the relative susceptibility of boxwoods to the leafminer was published in 2014 by the American Boxwood Society in their "The Boxwood Bulletin" (Click this hotlink to access the leafminer information: http://www.boxwoodsociety.org/uploads/54_1_2014_Summer.pdf#page=9)
Boxwood blight caused by the fungal plant pathogenCalonectria pseudonaviculatais another boxwood challenge to consider when selecting replacement plants. I posted an Alert last season reporting this disease in southwest Ohio. The BYGL Alert also included links to online resources describing best management practices including selecting boxwoods that are less susceptible to the disease (Click this hotlink to read "Be Alert to Boxwood Blight": https://bygl.osu.edu/index.php/node/1359)