Boxwood Leafminer

The Boxwood Leafminer (Monarthropalpus flavus) is a significant pest of boxwood plants (Buxus spp.). This insect is a species of gall midge and is considered the most serious insect pest that attacks boxwoods.

Host Plants

Boxwood Leafminer primarily targets species of boxwood plants. These evergreen shrubs are known for their dense, attractive foliage and are commonly used in landscaping and horticulture.

While all boxwood species can be affected by the leafminer, certain varieties may show varying levels of susceptibility. Boxwood Leafminer heavily attacks common boxwood (Buxus sempervirens), round leaf boxwood, Buxus roduntifolia, and balearica boxwood (Buxus balearica). 

Regions impacted

The Boxwood Leafminer is native to Europe but has become established in many regions across the globe where boxwoods are grown. In the United States, it is widespread from coast to coast, where it can cause significant damage to boxwood shrubs.

Description

• Adults: The adult form of the Boxwood Leafminer is a small, orange-yellow fly that’s only about 1/8-inch long. Its wings are transparent with a slight brown tint. It emerges beneath infested boxwoods in late April to early May, or around the time when boxwood plants are in bloom. This is when the adults mate and the females lay their eggs in the leaves of boxwoods.
• Larvae: The larvae are small, yellow maggots. These larvae hatch from the eggs and start feeding on the interior of the boxwood leaves, causing them to blister. The larvae are the primary damaging stage of this insect’s lifecycle. The feeding damage from the larvae creates visible blisters or mines on the leaves.
• Pupae: In late summer, the larvae transform into pupae within the leaf. They overwinter in this state and transform into adults in the spring.

Life Cycle

The Boxwood Leafminer has a complete metamorphosis life cycle, including egg, larval, pupal, and adult stages. This cycle takes one year to complete.

• Egg Stage: Adult female boxwood leafminers emerge in late April to early May, typically when boxwoods are in bloom. They lay their eggs on the undersides of the boxwood leaves, inserting them into the leaf tissue using a saw-like ovipositor. Each female can lay up to 29 eggs per leaf.
• Larval Stage: Once the eggs hatch, the larvae begin to feed on the interior of the leaves. This feeding activity causes the leaves to form blisters or “mines”. This larval feeding stage lasts throughout the summer.
• Pupal Stage: In the fall, the larvae transform into pupae within the leaf mines where they overwinter. This stage is typically completed by late October or early November.
• Adult Stage: The pupae metamorphose into adult flies in the spring. These adult flies cut exit holes from the leaf mines and emerge to mate and start the next generation.

The life cycle of the Boxwood Leafminer is synchronized with the growth of its boxwood host, and the timing can vary depending on the local climate and the specific variety of boxwood. Regular monitoring of boxwoods can help detect the presence of leafminers, and timely intervention can prevent significant damage.

Damage and Detection

Boxwood Leafminers cause significant damage to boxwood plants during their larval stage when they feed on the leaves from the inside, creating visible blisters or mines.

Damage: The feeding damage appears as blisters on the undersides of the leaves. As the larvae continue to feed, these blisters can become more pronounced and may turn yellow or brown. In severe infestations, this can lead to premature leaf drop and overall reduction in plant health. The continuous stress can make the boxwood more susceptible to other diseases or pests. In extreme cases, entire plants can be defoliated.

Detection: The blisters on the leaves are the most obvious signs of a Boxwood Leafminer infestation. These blisters are typically more noticeable on the underside of the leaves. If you hold an infested leaf up to the light, you may see the larva moving inside the blister. Adult flies are another sign of infestation, although they are only present for a short period in the spring.

It’s worth noting that some varieties of boxwood may show more significant symptoms of leafminer damage, while others may appear to be more resistant.

Prevention and Control

Preventing and controlling Boxwood Leafminer infestations can involve several strategies, including cultural practices, biological controls, and chemical treatments.

Cultural Practices

• Choose boxwood varieties that are resistant to leafminers:

  • Buxus microphylla var. japonica: Often called Japanese Boxwood, this variety is more heat-tolerant than some other types of boxwood and generally grows in a rounded shape. It can reach up to 6 feet in height and spread, but it grows relatively slowly and can be kept smaller with pruning.

  • Buxus microphylla ‘Green Pillow’: This is a low-growing, compact boxwood that typically reaches only about 1 foot in height but spreads up to 3 feet wide. Its common name comes from its soft, rounded shape that resembles a green pillow.

  • Buxus microphylla ‘Grace Hendrick Phillips’: Known for its small, densely packed leaves, this variety grows in a rounded form and can reach up to 3 feet in height and width. It’s often used for low hedges or as a border plant.

  • Buxus microphylla var. sinica ‘Franklin’s Gem’: A compact and rounded shrub, ‘Franklin’s Gem’ has a mature height of around 1-2 feet. It’s known for its small, bright green leaves and good winter hardiness.

  • Buxus sempervirens ‘Pendula’: Also known as Weeping Boxwood, ‘Pendula’ has a unique cascading growth habit with long, arching branches. It’s a larger variety that can reach up to 5 feet in height.

  • Buxus sempervirens ‘Argenteo-varigata’: This variety features dark green leaves with creamy white edges, adding a pop of color to landscapes. It can grow up to 5 feet tall and wide.

  • Buxus sempervirens ‘Handworthiensis’: This variety is characterized by its narrow, columnar growth habit. It can reach up to 6 feet in height but stays relatively narrow, often used for hedging or topiaries.

  • Buxus sempervirens ‘Pyramidalis’: As its name suggests, ‘Pyramidalis’ grows in a natural pyramid shape. It’s a larger variety, reaching up to 15 feet in height.

  • Buxus sempervirens ‘Suffruticosa’: Also known as English Boxwood, this is a slow-growing dwarf variety with dense foliage, often used for low hedges or edging.

  • Buxus sempervirens ‘Vardar Valley’: Known for its resistance to winter bronzing, ‘Vardar Valley’ has a spreading habit and grows up to 2-3 feet in height and 4-5 feet wide.

  • Buxus sempervirens ‘Justin Brouwers’: A compact variety with a naturally rounded shape, ‘Justin Brouwers’ has small, dark green leaves and grows up to 2 feet (60 cm) in height and width.

  • Buxus harlandi ‘Richard’: This variety is characterized by its long, glossy leaves and distinctively textured bark. It’s more heat- and drought-tolerant than many other boxwoods and can grow up to 4-6 feet tall.

  • Buxus sinica var. insularis ‘Nana’: A slow-growing dwarf variety, ‘Nana’ is known for its small, shiny leaves and compact habit. It typically grows only about 1-2 feet (30-60 cm) tall.

• Provide proper care: including regular watering and fertilizing, can also help boxwoods resist and recover from leafminer damage.
• Prune: Prune the leaves before adult emergence or right after egg-laying in May to decrease leafminer numbers.
• Pinch: Pinch leaves firmly to eliminate maggots in infested leaves when possible.

Biological Control

Several natural enemies of the Boxwood Leafminer have been identified, including green lacewings and spiders. Encouraging these beneficial insects in your garden can help control leafminer populations.

Chemical Control

Insecticides can be used to control leafminer infestations, but timing is important. Ideally, insecticides should be applied just after the adult flies emerge and before they lay their eggs. Consult with a local extension service or a professional to identify the most effective and environmentally friendly options for your area.

Remember, always follow all label directions when using any pesticide, and consult with a local professional or your local extension service for the best advice on managing pests in your particular situation.

In all cases, regular monitoring of boxwoods is critical to catch any infestation early and manage it effectively before it can cause significant damage.