Hello, Bill! Thank you for your question and for the excellent photos. I hope we can help you figure out what is happening to your lovely boxwoods and prevent further damage. Let’s see if we can narrow down the issue.

The most likely problems: winter injury, Boxwood Leaf Miner and resulting bird predation, mites (sometimes called spider mites), Boxwood and/or Volutella blight.

Winter Injury is characterized by faded leaves and is caused by extreme temperatures and sometimes salt. Since you wrote that your problem is worsening (I am assuming into summer) you might look for another cause.

This article describes difference between winter injury and boxwood blight.

https://www.canr.msu.edu/news/winter_injury_or_boxwood_blight

Boxwood Leaf Miner (and resulting bird predation) usuallly begins in the late spring and early summer. Small flying insects that resemble tiny mosquitoes lay eggs on the underside of boxwood leaves. If you look at the back of the leaf in an active infestation, you might see a small blister and the leaf may turn a copper color. After hatching the blister appears open and the leaf gradually browns, as the emerging pupae consume leaf tissue. The following two articles from OSU’s Buckeye Yard and Garden OnLine have photos and information that will help you decide if Boxwood Leaf Miners are your culprit and give advice for treatment & prevention. The second article has descriptions and photos of bird predation damage on boxwoods due to birds feeding on leaf miner pupae.

https://bygl.osu.edu/node/1461

https://bygl.osu.edu/node/1950

Mites (Eurytetranychus buxi) usually cause a stippling damage on boxwood leaves. They are hard to see, even with a magnifying lens. To detect them, you can hold a white piece of paper under the affected branch and tap it. If tiny black dots fall onto the paper, you might have boxwood mites. If you look carefully, you might even be able to see them move. This article, and the Clemson article cited later, have sections on boxwood mites and suggested treatments and preventatives. https://www.canr.msu.edu/news/boxwood_insect_pests

Boxwood Blight and/or Volutella Blight are diseases known to occur in boxwood plants. The following articles describe and show photos to help you diagnose. If you suspect your problem might be one of these diseases, you might want to send a specimen of your plant to a diagnostic lab for analysis. More information on submitting samples follows.

https://bygl.osu.edu/node/1359

https://bygl.osu.edu/node/288

Important: when pruning any plant with suspected disease, be sure to sanitize your pruners between cuts, or at least between plants to avoid spreading the pathogen to another plant or part of the plant. This is a good garden hygiene practice at any time, even when disease isn’t evident. You can use isopropyl alcohol, soap and water or a chlorine bleach solution. I favor the alcohol, as it is inexpensive and easy to carry in my gardening tote.

Submitting a sample for analysis (Note: we do not receive any profit from recommendations)

The Ohio State C. Wayne Ellett Plant and Pest diagnostic clinic will analyze physical or photographic samples. Fees start at $20. If you choose to send photos, be sure to include close ups of front and back of leaves as well as pictures of the whole plant and of affected sections. https://ppdc.osu.edu/submit-sample

MSU also has a diagnostic service. https://www.canr.msu.edu/pestid/

If you would like further reading, here is an article from Clemson that details various boxwood problems.

https://hgic.clemson.edu/factsheet/boxwood-diseases-insect-pests/

I hope this has been of help to you. If you need any clarification or have further questions about your boxwood issues, feel free to reply to this email and your response will come to me. Wishing you good luck with your boxwoods and happy gardening!

Hello, Bill!

I’m so happy that you found our response timely and helpful! And I am impressed that you implemented the advice so quickly. I am assuming that the “bugs” you are seeing are mites, either boxwood mites or two-spotted mites. That is most likely, but I can’t guarantee. We, at Ask a Master Gardener/ Ask Extension, have received numerous queries about boxwood mites lately, so you are in good company! (If you would like to attempt a close-up photo of the “bugs”, I would be happy to check it.) The following answer is based on the assumption that the critters you discovered are indeed mites.

There are several things you can do this season and next spring to help control the mite population on your boxwoods. The photos you sent lead me to believe that you will be able to save those lovely plants. 

Some thoughts: First, when a plant has symptoms of pests or disease, it is not uncommon to have more than one issue “going on”. Like humans, when a plant is weakened with one problem, it can be a target for other pests or pathogens. So, keep a close eye on your boxwoods and look for symptoms of other issues like leaf miners or blight. You can contact us, or me directly through this thread, if you see something. You might find it helpful to review some of the sites I sent previously to know what to look for.

Next thought, mites are not insects. They are more like spiders, and so application of an insecticide might not help. It may worsen the issue by destroying beneficial insects, such as ladybugs and lacewings, which prey on the mites. There are miticides that you can choose to use, but this is not the right time of year. More detail follows.

Suggestions: Believe it or not, the best treatment for mites is water. Mites thrive in a dry environment. (I have read, although have not been able to verify through a research-based site, that in a wet environment mites will swell and burst.) The first and most effective treatment is to spray your plants with a strong stream of water, focusing on the underside of the leaves where some mites hang out and lay their eggs. You will need to do this frequently, even daily if possible. One caveat: be sure that the roots of your plant are not in standing or saturated water, which could cause root rot if drainage is an issue. If you are still seeing mites (white paper test), your next step would be to treat with a horticultural oil. https://extension.unr.edu/publication.aspx?PubID=3029 and https://extension.umd.edu/resource/pesticide-profile-horticultural-oil

Treat in the evening because treating with oils in sunny conditions can lead to a sun scald of leaves. Repeat treatment every 10-14 days. Mites lay eggs for overwintering in September – October, so this is the time to act. (I use a concentrate and dilute it according to directions.)

Individually pruning affected branches will help reduce the population. Don’t compost the branches.

If you choose to use a miticide the time to apply is in early May. This is from MSU, cited below paragraph: “A more aggressive approach includes treating with abamectin (Avid), bifenthrin (Talstar), malathion or oxythioquinox (Morestan) during the first two weeks in May.” As always, be sure to follow all package directions. The following articles will give you more information about mites and miticide treatment in boxwoods:

https://www.canr.msu.edu/news/boxwood_insect_pests#:~:text=The%20boxwood%20mite%2C%20Eurytetranychus%20buxi,tiny%20white%20or%20yellow%20marks.

https://content.ces.ncsu.edu/boxwood-mite

Question about fertilizing your boxwoods: Before adding fertilizer, it is recommended that you complete a soil test. Too much fertilizer, specifically nitrogen, can contribute to mite infestations. You can submit samples to be tested https://ohioline.osu.edu/factsheet/hyg-1132

You can also use home tests available at most garden centers. I found this article about the reliability of home soil tests. https://extension.unh.edu/blog/2020/04/are-garden-soil-test-kits-good-alternative-lab-testing

A soil test will also determine pH level; boxwoods prefer a pH of 6.8-7.5.

If fertilizer is needed the best time to fertilize is late fall. It is best to apply the recommended amount of fertilizer just outside the dripline of the plant and on top of mulch.

Here are some sites about general care and fertilizing of boxwoods.

https://boxwoodsociety.org/abs_care.html

https://hgic.clemson.edu/factsheet/boxwood/

Sorry for such a long reply! I hope this is of help. If you have additional questions or need clarification on this response, I would be happy to have you contact me. Just reply to this thread.

Happy gardening, and best of luck to you and to your boxwoods!