Boxwood leafminer: How to identify and control this boxwood pest

The boxwood leafminer is a common pest of boxwood shrubs (Buxus spp.). If you have brown, blistered leaves on your boxwoods, this little critter could be to blame. The boxwood leafminer was introduced to the North American continent from Europe in the early 1900s and is now found in nearly every region of the United States and southern Canada. In this article, I’ll share more about this pest and offer tips for controlling it without synthetic chemical pesticides.

What does leafminer damage look like on boxwoods?

Like other species of leafminers, the boxwood leafminer (Monarthropalpus flavus) feeds by “mining” out the juicy green tissue in between the upper and lower surface of the leaves of boxwood plants. This results in several distinct symptoms.

1. Active infestations appear as puffy blisters with an orange-ish tint on the bottoms of the leaves and pale light green or yellow speckles on the upper leaf surface.
2. Later in the growing season, damaged leaves turn brown on the top and bottom, and may even drop from the plant in large quantities.

If you see either of these symptoms on your boxwoods, it’s time to take a closer look to confirm the presence of this pest using the info found in the following sections.

Description of boxwood leafminer

Adult boxwood leafminers are small orange flies in the order Diptera (which means they have two wings). They look a lot like tiny orange mosquitoes, though they do not bite animals or feed on blood. Instead, each adult boxwood leafminer fly lives for only about a day. During their brief time as adults, they breed and lay eggs and do little else. You’ll only see the adults for a period of about two weeks. Though their time as adults is short-lived, they make their presence known. In mid-spring, the adults create a cloud of tiny flies around the plant as they emerge, breed, and lay eggs. If you’re a bug nerd like I am, boxwood leafminers are in the family Cecidomyiidae. They are a type of gall midge.

Boxwood leafminer larvae, on the other hand, are seldom seen by gardeners – unless you know what to look for and are willing to peel apart a leaf. The larvae are miniscule (3 mm) yellow to orange maggots that spend their whole larval life-stage inside of a single leaf. Sometimes these larval miners live by themselves in a leaf, but there can be up to 8 maggots sharing a single leaf. If you suspect an infestation, peel open a leaf in the early spring and you’ll see the maggots inside (see photo below).

Lifecycle of the boxwood leafminer

The lifecycle of this Insect is really interesting (you can hear my enthusiasm for them in the video embedded below). Let’s start the description of their lifecycle in the spring, when the adults are active.

Like other flies, the boxwood leafminer passes through four life stages:

• Adult: Adult leafminers emerge from their pupal cases, often found protruding from the undersides of boxwood leaves in the spring (see photo below). They breed, and then the females lay 20-30 eggs in new leaves.
• Eggs: The eggs are deposited into the boxwood leaf using needle-like ovipositors on the female flies. The ovipositor is strong enough to break through the leaf tissue to insert the eggs. Eggs take between 10-14 days to hatch.
• Larvae: Boxwood leafminer larvae (called a maggot) exist for about 8 to 9 months. They hatch in spring and feed on the leaf tissue. They grow larger as the season progresses throughout the summer. When cold temperatures arrive, they pupate and overwinter inside the leaves as pupae.
• Pupae: In spring, when the weather warms, the pupal leafminer becomes active again and chews a small hole through the underside of the leaf. The adult fly emerges from that hole shortly thereafter, often leaving its pupal skin behind (see photo below).

What plants are affected?

Boxwood leafminers are host specific. This means they only affect one species of plants, Buxus. Unfortunately, almost all species of boxwoods are susceptible, though there are a few species and cultivars reported to be less susceptible (see a list of resistant selections later in this article). These leafminers will not attack other species of plants.

Why is there a crackling sound coming from my boxwoods?

One easy way to diagnose an infestation of boxwood leafminers is to listen to your plants. This is how I discovered these little boxwood pests on my own plants a few years ago. I was weeding nearby when I heard popping and crackling coming from my boxwood bushes. They sounded like a bowl of Rice Krispies! You’ll hear this sound only for a week or two in the spring because it is the sound of the pupae chewing their way through the leaf bottoms before emerging as adults. If you see the blisters on the leaves in the spring, put your ear up to the shrub and listen. It’s a very distinctive sound!

Use pruning to control this pest

Properly timed pruning is the easiest way to control boxwood leafminers. This can be done at two different times.

1. Early spring: If you see blistered leaves with a hint of orange on the bottom leaf surface, or if you hear the popping and crackling sound when you put your ear up to the shrub, but you have not yet seen any adult flies, it’s time to prune. Using a clean, sharp pair of pruners, cut out as many of the branches with blistered leaves as you can. This is a very short window of opportunity that you don’t want to miss. If you wait too long, the adults will be able to emerge and begin to breed.
2. Late spring: Another possible pruning time is late spring, after the adults have laid their eggs. If you opt to prune boxwoods at this time, you’ll hopefully be removing as many egg-infested leaves as possible. Trim off the outermost 6 inches or so of leaves as that’s where the egg-laying is heaviest.

Disinfect your pruners with a spray disinfectant before moving from one bush to another to prevent the spread of potential pathogens like boxwood blight. And throw your trimmings into a garbage bag and send them to the landfill or bury them. Do not compost them or the flies will emerge, and your problem will not be solved.

How to use yellow sticky cards for boxwood leafminer

Another way to manage infestations is through yellow sticky cards. I do this every spring and it seems to help reduce egg laying. In mid-spring (which is late April in my Pennsylvania garden), when I start to hear the crackling sounds, I put up yellow sticky cards. I hang them from stakes so they sit in between the bushes or about 6 inches above the tops of the shrubs. Like many other common plant pests, boxwood leafminer adults are attracted to the color yellow. They fly to the sticky card and get trapped on its surface.

I then dispose of the sticky cards when the 2-week breeding window is over. It is not a 100% reduction in egg laying, but it sure does make a huge dent. Combining sticky cards with proper pruning is just as effective as the chemical products recommended by some extension services for managing this pest (if not more-so).

Why you should not use systemic pesticides for boxwood leafminer

You will often see systemic insecticides, such as imidacloprid and dinotefuran, recommended for use against boxwood leafminers. These insecticide products are in a class known as neonicotinoids and are translocated throughout the vascular tissue of a plant. This means they reach the protected larvae inside of the leaves. But it also means they end up in the pollen and nectar of plants where they can negatively affect the pollinators who visit them. While they are not showy, boxwoods do bloom. And boxwood flowers are pollinated by bees, flies, and other insects. For this reason, I do not recommend using systemic products of any sort on your boxwoods (or anywhere else in the garden, for that matter).

Sometimes Spinosad-based products are recommended to help control boxwood leafminers by spraying in the spring, just as the adults are about to emerge. Spinosad, though organic, can be harmful to bees as well, so be sure to follow label instructions and only apply in the evening when bees are not active (or, better yet, skip it entirely and opt for pruning and yellow sticky cards instead).

Beneficial insects that eat boxwood leafminer adults

There are a handful of beneficial predatory insects that feed on adult boxwood leafminer flies, including spiders, green lacewings, damsel bugs, dragon flies, and praying mantids. These good bugs are also harmed by the use of pesticides, which is yet another good reason for skipping them. Encourage these predators by planting lots of flowering plants in your garden with varying bloom times and flower shapes.

Boxwood varieties are resistant to boxwood leafminer

There are a handful of boxwood species and varieties with a known resistance to boxwood leafminers. According to a 2019 report by researchers Thurn, Lamb, and Eshenaur, boxwood leafminer-resistant varieties include

1. Buxus sempervirens ‘Pendula’, ‘Suffruticosa’, ‘Handworthiensis’, ‘Vardar Valley’, ‘Pyramidalis’, ‘Argenteo-varigata ‘ and ‘Justin Brouwers’.
2. Buxus microphylla var. japonica and var. sinica ‘Franklin’s Gem’
3. Buxus microphylla ‘Green Pillow’ and ‘Grace Hendrick Phillips’
4. Buxus harlandi ‘Richard’
5. Buxus sinica var. insularis ‘Nana’

To see these pests in action and hear what they sound like, check out this video I shot in my own garden:

Other common boxwood problems

If your symptoms don’t match with those described above, there are two other common pests of boxwood aside from the boxwood leaf miner. These include:

• Boxwood spider mites (Eurytetranychus buxi): Also called the boxwood mite, they cause tiny white speckles on the leaf surfaces which often occur in small lines. Extremely tiny.
• Boxwood psyllids (Cacopsylla busi): Causes curling and cupping of the new leaves at the tips of the shrubs. Fuzzy white psyllid nymphs are often found inside the curled leaves in spring. American boxwood (Buxus sempervirens) is most susceptible.

Unlike boxwood leafminers which are protected by being inside of the leaves, these two pests are easily controlled with insecticidal soap or horticultural oil.

Beautiful boxwoods

Despite the potential problem of boxwood leafminers, boxwoods are still very worthy plants to grow. They remain among my favorite evergreens for the landscape. Boxwoods are generally deer resistant and low-maintenance plants. If you’re planting new boxwoods, be sure to select resistant varieties.

For more on growing beautiful shrubs, check out these articles:

• Rose pests and how to control them
• Dwarf flowering shrubs for the landscape
• Low growing shrubs
• Hydrangeas and deer
• Shrubs with dark foliage
• Low maintenance shrubs

Pin this article to your Landscaping board for future reference.