Yes, the adult life stage has already come and gone with this insect, which only has one generation per year. Boxwood Leafminer goes through a period of arrested development in summer as the weather warms, pausing feeding and maturation to resume in late summer and autumn. This is the main reason why a systemic insecticide (either acephate or a neonicotinoid like imidacloprid) will have little impact. (Acephate is also not recommended overall for several reasons, including pollinator protection, though it's more of a translaminar than a true systemic, meaning it can soak into the leaf tissue where applied. It does have some potential for systemic activity, though.)

Light trimming may be all that's needed for now, since it will physically remove the majority of the leafminer larvae, as they feed in the younger foliage near the branch tips. If even a light trimming will remove most of the plant's leaves, that suggests the plant has been over-pruned in the past, or might be in more shade than is ideal, where the inner foliage has already shed over time. (By itself, gradually losing inner leaves is normal for evergreens, since leaves have a finite lifespan and those are a shrub's oldest, but a light trimming should not ordinarily denude a boxwood.)

If you're concerned about leaf removal, can you share photos of the plant and what amount of foliage they have? We're only suggesting trimming off the outer few inches, though boxwoods that are otherwise healthy should be able to tolerate and rebound well even from a heavier pruning. If the plants are too spartan on the interior, then a rejuvenation pruning (which deliberately removes more branch length to force regrowth from the base) can help them recover aesthetically while also, for this year at least, taking care of the leafminer infestation.

If insecticide use is warranted, a lower-toxicity option using the active ingredient spinosad (also translaminar) may be effective enough to knock down the population of larvae in late spring or early summer, not long after they hatch. Avoid getting spinosad residues on flowers that bees may visit (which includes boxwood blooms, which are inconspicuous but which can be visited by bees).

Miri

Thank you for the additional information and photos. The 'Green Mountain' looks pretty good, though we do see the leafminer present, and you're right, that cultivar has moderate susceptibility to that pest. We don't think it needs pruning for shape, but you can do some light trimming if desired, especially if it overlaps with the effort to manually remove leafminer-infested tip growth. Don't prune or shear it too regularly, though, as this can generate very dense growth that increases vulnerability to infection, pest outbreaks, and premature inner leaf shedding. Boxwood can be pruned almost year-round, with the exception of avoiding autumn (August through October, which applies to most plants). A routine shaping prune would typically be done in late winter or early spring (March-ish), though in some years might be best delayed until mid-spring or so (maybe early April), since the resulting new growth (if produced too early in spring) could be injured by a late spring freeze. January might be fine, but otherwise is earlier than most gardeners would prune, in part because new growth won't cover-up the pruning cuts until a few months later.

When systemic insecticides are used for Boxwood Leafminer, they tend to get applied after the summer pause in development that the larvae go through, so August is a typical time frame for having a treatment applied. (Many such chemicals are restricted-use in Maryland, being neonicotinoids, and must be legally applied by a certified pesticide applicator. While others exist that are systemic that are not neonics., they can be hard to find or expensive, and may still be more practical to hire a professional to apply. Some are root-applied as a soil drench, but others can be sprayed onto the boxwood foliage for absorption.)

June does present an option for systemic treatment as well, especially for leaf-applied products, since the root-applied options will take longer to reach the foliage and take effect. Acephate would probably be a harsher systemic to try than some of the other, newer-chemistry systemics (like products containing the active ingredient abamectin), though research is still exploring its impacts on bees or other organisms. As with any pesticide, do not spray while the plant in question is blooming, and avoid drift of spray mist or drips onto nearby plants in bloom. If this can't be avoided (and blooms you suspect may be contaminated pruned off), then a systemic might not be appropriate for the situation.

The bronzing English-type boxwood is hard to diagnose, but it could be a reaction to general stress, such as limited root space, prior drought or over-saturated soil, or a mild nutrient deficiency. Soil acidity (pH) could be playing a role in the latter, though it's hard to know without laboratory soil test results. Boxwood is flexible as to soil pH, but prefers conditions a little on the acidic side. Eggshells don't really decompose well or influence pH unless finely crushed into a powder before composting, and even then, their impact on the pH of a compost pile is probably negligible unless a lot of them were added by volume. We can't rule-out early-stage Volutella Blight as a contributor to browning, but given it's been ongoing for five years, infected branches probably would have died back by now.

Hydrangea flowers that are capable of turning blue (not all cultivars do) do so under two overlapping conditions: acidic soil below a certain pH (around 5.5 or 6.0) and the presence of aluminum in the soil for roots to absorb. The aluminum forms part of the blue pigment, so acid soils without it don't result in blue flowers. There are different soil amendments used for hydrangea color changes, and we're not certain which product you're referring to since Espoma doesn't seem to list one with that exact name. Some are garden lime (which raises pH, thus making those flowers more pink), some are garden sulfur (lowering pH), and some are aluminum sulfate, which both lowers pH and provides aluminum. While none of these likely influenced the pH around the boxwood roots too greatly (especially if they are not immediately next to each other, or if the hydrangeas are only to one side of the boxwoods), too much aluminum for most plants (excepting hydrangea, which is more tolerant) can cause problems for roots. This is not to imply that aluminum toxicity is responsible for any current boxwood struggles, but it's something to keep in mind if you happen to re-treat the hydrangea soil in the future. Soil pH changes are gradual (and should be, to avoid stressing roots), and since our soils are not often deficient in aluminum, only acidification measures tend to be needed (if anything) to promote blue flowers on hydrangea.

The most leggy of the 'Elegantissima' boxwoods pictured (which otherwise look to be in decent shape) could have a rejuvenation pruning done if you don't want to wait for the lower side growth to fill in. This would entail cutting all stems back to (roughly) a foot off the ground and waiting for regrowth to fill back in, which will take a few years. A photo example of a rejuvenation-pruned boxwood is attached.

Boxwood roots stay relatively shallow compared to most other shrubs, with many of them just a few inches of the soil surface, and some boxwood experts caution that their roots should not be covered by more than an inch or inch-and-a-half of mulch because it reduces how much oxygen access they have. This presumably applies to soil as well, so surface-visible roots don't necessarily need covering if they have been growing at that depth for a while (remediating erosion and loss of soil exposing more roots over time is a reasonable exception). Here too, the topsoil you mention having already added back on top of the roots isn't necessarily the main factor in plant stress, but it's something to keep in mind going forward so it doesn't compound any other sources of plant stress. The groundcover plant visible under the more sparse individual in the photo might be depriving the plant of some resources, but it's hard to say, and at least it's keeping other weeds from doing the same and reducing the need for mulch.

Miri

Hello Bess,

Sticky traps might help you monitor for the presence of adult leafminer gnats next spring, but alone they will not trap nearly enough to be a control measure.

As for the potential hydrangea replacement(s), there are so many reblooming cultivars on the market these days that the Proven Winners branded line is only one option among many. Since reblooming varieties flower on both old wood (the first flowers of the season) and new wood (later growth), they are more forgiving of pruning timing and winter (or spring freeze) bud injury, and can bloom longer overall as well. The sulfur used probably contributed to the color change, albeit gradually, it seems. If the color drifts back into the purple and then pink range, another dose of sulfur could be used to nudge the pH back down to where the flowers turn blue. (It might not affect flowers until the following season, depending on when it's applied.)

Boxwoods are at least more forgiving of getting a bit dry than staying too wet.

A few regional soil testing lab candidates are included on the soil testing page we linked to earlier, but within that list, we don't really have a specific recommendation. We can say that a lot of residents who share test results with us for interpretation have been using the Univ. of Delaware lab, and we do find their results charts easier to read than most of the others.

Miri