Monarthropalpus flavus (buxi)

Boxwood leafminer mine in boxwood leaf. Photo: Tawny Simisky
Scientific Name: Monarthropalpus flavus (buxi)
Common Name: Boxwood Leafminer
Growing Degree Days (GDD's): 350–600 GDD's (adults), 1200-2400 GDD's (larvae), Base 50F, March 1st Start Date. (Source: Cornell Cooperative Extension)
Host Plant(s) Common Name (Scientific Name)
American boxwood (Buxus sempervirens) *Attacks most cultivars.
Harland boxwood (Buxus harlandii)
Japanese boxwood (Buxus microphylla) *Attacks most cultivars.
Insect Description:

This insect entered the United States with the introduction of new species and cultivars of boxwoods prior to 1910. It can be found throughout the United States wherever boxwood is grown. Overwinters as a partially developed larva in the leaves. The larvae become active in the warming spring and will mature into an orange pupae by late April. Adults emerge by mid-May and are tiny (2-3 mm.), leaving behind a pupal case that may cling to the leaf following adult emergence. Adults are yellow-orange and look like gnats (flies). The female fly will lay approximately 29 eggs, inserted deeply into the upper surface of the current season's leaves. Females die within a matter of hours following laying their last eggs. Eggs hatch and larvae begin to mine the leaves and grow slowly through the summer. Many larvae may be found in the same mine and are protected from contact insecticides. There is one generation of boxwood leafminers per year.
Damage to Host:

English boxwood (Buxus sempervirens 'Suffruticosa') is rarely attacked by this fly. Foliage of susceptible species and cultivars may be heavily mined. Undersides of boxwood leaves will have yellowed mines present. Infested leaves may be yellow, spotted, and drop prematurely. Plants appear sparse and foliage is no longer dense, as in a healthy boxwood. Frequent, persistent attack by this pest may lead to branch dieback and weakened plants more susceptible to abiotic stressors or disease. Boxwood leafminer infestation might make plants grown in colder climates more susceptible to winter kill. 
Monitoring:

Boxwood leafminer adult emergence may be monitored with yellow sticky cards in the nursery and landscape. Alternatively, shake bushes beginning in late April to detect flying adults. During the spring and summer, while scouting, check the undersides of older (previous year's) boxwood leaves to detect signs of infestation. Mines in the current season's leaves may not become noticeable until the fall. 
Cultural Management:

When installing boxwood in a landscape, use resistant varieties. 

Highly resistant: Buxus sempervirens ‘Handsworthiensis’ and Buxus sempervirens ‘Vardar Valley'.

Resistant/least susceptible: Buxus sempervirens ‘Memorial’, Buxus sempervirens ‘Pendula’, Buxus sempervirens ‘Pyramidalis’, Buxus sempervirens ' Suffruticosa’, and Buxus sempervirens ‘Argenteovariegata'. (Source: Cornell Cooperative Extension.)

Additional reports of varying levels of resistance in boxwood varieties are available from private nurseries specializing in growing boxwood.
Natural Enemies & Biological Control:

There are few known enemies of the boxwood leafminer on susceptible hosts.
Chemical Management:

Abamectin (NL)

Acephate (NL)

Acetamiprid (L)

Azadirachtin (NL)

Bifenthrin (NL)

Carbaryl (L)

Clothianidin (NL)

Cyantraniliprole (NL)

Cyromazine (NL)

Diflubenzuron (N)

Dinotefuran (NL)

Fenpropathrin (NL)

Flonicamid+cyclaniliprole (NL)

Gamma-cyhalothrin (L)

Horticultural oil (L)

Imidacloprid (L)

Insecticidal soap (NL)

Lambda-cyhalothrin (L)

Malathion (L)

Neem oil (NL)

Permethrin (L)

Pyrethrin+sulfur (NL)

Spinosad (NL)
Notes:

Manage adult females prior to egg-laying. Larvae will be protected from contact insecticides inside their mines. As such, foliar-absorbed active ingredients or systemic insecticides are most effective for the immature life stages of boxwood leafminer.

Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), clothianidin (soil drench), cyantraniliprole (soil drench, injection), diflubenzuron (soil drench), dinotefuran (soil drench), imidacloprid (soil drench), and neem oil (soil drench).

Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.

Note: Beginning July 1, 2022 neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: September 27, 2024. To check current product registrations in Massachusetts, please visit the MA Department of Agricultural Resources Pesticide Product Registration page and click on "Search Pesticide Products Registered in Massachusetts - Kelly Solutions".