Insect Attacking Bush

Question: Our regular gardener noticed that one of our front yard bushes was not thriving and found that some insect was attacking it.  I have enclosed photos of the bush, and closeups of the leaves that show insect damage, characterized by small black dots on the leaves.  I would appreciate you assessment of what is attacking our plant and how I can remedy it.
Thanks for you help.
Richard

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 08, 2023, 2:19 PM EDT
response: Hello Richard,  Although the photos are unfortunately hard to see clearly due to the small file size, these shrubs appear to be Boxwood and the damage due to both Boxwood Leafminer and Boxwood Mite. The dots appear to be the emergence holes in the leaf undersides where the adult leafminer flies emerged earlier this season. You can explore our boxwood diagnostic page for information about these and other commonplace boxwood ailments and how to manage them. Damaged leaves cannot heal and will eventually be shed by the plant, though not necessarily this year. New growth can help to gradually cover-up bare areas but given the naturally slow growth rate of boxwoods this might take a few years depending on how sparse or marred the current growth looks.  One option if you do not wish to use insecticides is to drastically cut back the branches to force the plant to regrow, taking care not to over-prune them by shearing them into dense, manicured shapes as they mature again, since this condition increases disease and pest vulnerability. Some boxwood varieties are inherently more resistant to some of the most prominent pests and diseases, though none are perfectly immune. Worst-case, if you don't want to treat these plants in future years and doing a renovation of drastic pruning is undesirable, then you can either replace them with one of the varieties of resistant boxwoods or with alternative shrubs.  Miri

responder: The Question Asker Replied June 08, 2023, 3:17 PM EDT
response: Thanks, Miri. Will horticultural oil or insecticidal soap spray work on both mites and miners?  Or should I use an insecticide?  If the latter, what one should I use?ThanksRichard On Jun 8, 2023, at 2:19 PM, Ask Extension <<personal data hidden>> wrote:﻿

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 08, 2023, 6:02 PM EDT
response: Either the hort. oil or insecticidal soap can help with mites but not well with leafminers (and only for the adults that are freshly-emerged or just starting to lay eggs, which they have already finished doing for this year). Instead, either a chemical-free approach by trimming off the branch tips that contain the leafminer larvae, whose feeding damage won't become evident until they're older later this year, or treating with a leaf-absorbed systemic pesticide will affect them given their relative shelter from sprays inside the leaf tissue.  Acephate is one systemic active ingredient that moves into leaf tissue after being sprayed on foliage, but it can be harsh on other non-pest insects contacted by the spray. (To be fair, boxwoods are non-native and don't tend to support many insects that aren't pests, and they are finished flowering awhile ago so are unlikely to interest bees.)  A neonicotinoid-class ("neonic") insecticide, to which several active ingredients belong, are another systemic option that is instead root-applied and absorbed (and gradually moved into the foliage in the plant's water-transport tissues). As of 2018 these are illegal for the general public to use in Maryland and must instead be applied by a certified pesticide applicator as they pose a hazard to bees and other pollinators. We don't keep close track of brand/product options nor do we recommend particular brands/products or sources, but a pesticide whose label information includes leafminers as one of the pests controlled and which is permitted for use on home landscape shrubs (they might not list boxwood specifically) should be suitable enough. That said, a quick-to-degrade contact-type of pesticide like the oil and soap options are among the lowest-toxicity candidates where they are useful. For the leafminer that is harder to treat, you might be able to use the active ingredient spinosad instead of the above systemics as this is also leaf-absorbed but lower-risk, as long as the label information matches the use you need it for (home landscape, leafminer pests, etc.).  Miri

responder: The Question Asker Replied June 09, 2023, 4:37 PM EDT
response: Thank you very much, Miri.  I appreciate your help. Richard

responder: Answered by the Home and Garden Information Center (HGIC), University of Maryland Extension. For timely gardening information and tips, subscribe to our Maryland Grows blog and quarterly newsletter. Follow us on Facebook, Instagram, and YouTube. Replied June 09, 2023, 4:50 PM EDT
response: You're welcome!