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.
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.
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.
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.
There are few known enemies of the boxwood leafminer on susceptible hosts.
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)
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.