Boxwood shrubs face a handful of persistent pests that cause distinctive damage, from blistered leaves to complete defoliation. The most common culprits are boxwood leafminer, boxwood psyllid, and boxwood spider mite, all of which have been around for decades. A newer and more destructive threat, the box tree moth, is now spreading across parts of North America and can strip a shrub bare in a single season. Knowing what to look for with each pest makes the difference between catching a problem early and losing a plant.
Boxwood Leafminer
The boxwood leafminer is the most widespread and damaging pest of boxwood in most regions. The insect responsible is actually a tiny fly, not a caterpillar or beetle. Adults are mosquito-like, yellow to orange-red, and only about 2.5 mm long. They live just a few days, during which they lay eggs between the upper and lower surfaces of new leaves. The larvae that hatch are small yellow maggots that spend most of their life feeding inside the leaf tissue.
Damage from leafminer starts in summer but usually goes unnoticed until the following spring. As temperatures warm, the larvae resume feeding and create obvious blisters on the undersides of leaves. Infested leaves look puffy or water-soaked, turn yellowish, and are smaller than healthy ones. A heavily infested plant takes on a thin, unhealthy look overall. By spring, the blisters develop a translucent “window” as pupae push through the lower leaf surface before emerging as adult flies, typically from mid-May through June (roughly when weigela blooms).
For control, a systemic insecticide applied as a soil drench reaches the larvae feeding inside the leaves and doesn’t require precise timing. Products containing imidacloprid, sold under names like BioAdvanced Tree and Shrub Insect Control, are effective. If you prefer a contact spray, the window is narrow: target the adults right as they emerge in spring, before they can lay eggs in fresh growth.
Boxwood Psyllid
Boxwood psyllids are tiny sucking insects, about 3 mm long, that resemble miniature cicadas. They overwinter as small, orange, spindle-shaped eggs tucked into bud scales. When new growth begins in spring, nymphs hatch and immediately start feeding on developing foliage. The nymphs are pale green with brown mottling and cover themselves in fluffy, white waxy strands that make them look almost cottony.
The signature symptom is cupped, distorted leaves at branch tips. As the nymphs feed, they cause new leaves to curl inward, which conveniently hides them inside the cupped foliage. Twig growth can also stall. While psyllid damage is mostly cosmetic on established plants, heavy infestations year after year will weaken a shrub and make it look ragged. Winged adults appear by early June and are less of a concern since they stop feeding on boxwood.
Spring is the critical treatment window. Spraying with insecticidal soap or horticultural oil while nymphs are active and exposed kills them without much harm to beneficial insects. Pruning out cupped, infested growth before nymphs mature into adults also reduces populations. Check your plants regularly in early spring when buds are breaking to catch them early.
Boxwood Spider Mite
The boxwood spider mite is so small (less than 0.4 mm) that it’s difficult to see even with a hand lens. Adults are greenish to yellowish brown and feed on both the upper and lower surfaces of leaves. They lay round, lemon-yellow eggs that are flattened on top and bottom. Unlike many spider mites, this species typically does not produce visible webbing, which makes infestations easy to miss.
Damage shows up as tiny scratch marks on the upper leaf surface, often described as “hen scratches” or fine flecking. From a distance, the whole shrub looks off-color, grayish, or bronzed. New growth is particularly vulnerable. If you suspect mites, hold a piece of white paper under a branch and tap the foliage. Tiny specks that move on the paper confirm their presence. You may also find cast skins or eggs on the undersides of leaves with close inspection.
Dormant oil sprayed in late winter or early spring, directed at the undersides of leaves, targets overwintering mites and eggs. During the growing season, light horticultural oils and insecticidal soaps work but require thorough coverage since they only kill on contact. A Belgian trial found that applying specialized miticides in April controlled 50 to 100% of developing winter eggs, so early spring treatment is the most effective timing regardless of which product you choose.
Box Tree Moth
The box tree moth is an invasive species originally from East Asia that has become a serious threat in Europe and is now establishing in parts of North America. Adult moths are white with a brown border around the wings, with a wingspan of about 1.5 inches. A less common dark variant is fully brown with small white spots on the forewings. The caterpillars are the destructive stage: black and green with green, yellow, black, and white longitudinal stripes.
This pest is far more destructive than the others on this list. Newly hatched caterpillars feed on the underside of leaves, then work their way through the entire plant as they grow. Leaves appear “peeled,” with only the midrib remaining. Under heavy feeding pressure, caterpillars will even strip bark from stems. Severely infested shrubs take on a brown, scorched appearance. The most recognizable sign is white webbing visible inside the canopy when you spread the branches apart. Green-black balls of frass (caterpillar droppings) scattered through the interior of the shrub are another telltale indicator.
For management, a biological pesticide containing Bacillus thuringiensis var. kurstaki (Btk), sold under names like Dipel, is effective against the caterpillars. Spinosad-based products like Conserve are another option. Both work best when caterpillars are small and actively feeding. Timing matters because the pesticide needs to be ingested by the larvae, so apply it when you first spot young caterpillars or fresh feeding damage rather than waiting until webbing is heavy. If you live in an area where box tree moth has been detected, regularly inspect the interior of your boxwood canopies for caterpillars, webbing, and frass.
How to Tell Which Pest You Have
Each pest leaves a different calling card. Blistered, puffy undersides of leaves point to leafminer. Cupped leaves at the branch tips, especially with white waxy threads inside, indicate psyllid. Fine scratch marks and a grayish overall cast suggest spider mites. And webbing inside the canopy with chewed or skeletonized leaves signals box tree moth caterpillars.
It’s worth noting that more than one pest can be present at the same time, particularly leafminer and spider mites, which share similar timing. A quick hands-on inspection in early spring, flipping leaves over and looking inside the canopy, will catch most problems before they become severe. Healthy, well-watered boxwoods tolerate moderate pest pressure better than stressed plants, so basic care goes a long way toward keeping damage in check.