Fenusa pusilla

Birch leafminer damage. Photo: Whitney Cranshaw, Colorado State University, Bugwood.
Scientific Name: 
Fenusa pusilla
Common Name: 
Birch Leafminer
Growing Degree Days (GDD's): 
190–290 GDDs; 530–700 GDD's, Base 50F, March 1st Start Date (Sources: Robert Childs, UMass Extension and Cornell Cooperative Extension.)
Host Plant(s) Common Name (Scientific Name): 
Black Birch (Betula lenta) *Rarely feeds on. Considered resistant.
European White Birch (Betula pendula) *Rarely feeds on.
Gray Birch (Betula populifolia) *Preferred host.
Paper Birch (Betula papyrifera) *Preferred host.
Yellow Birch (Betula alleghaniensis) *Rarely feeds on. Considered resistant.
Insect Description: 

This sawfly was first recorded in the US (from Europe) in CT in 1923. Adults are black and approximately 0.12 inches in length. Egg-laying female sawflies are attracted to fast growing, tender, spring foliage. Eggs are laid in these leaves singly. Once they hatch, larvae mine within the leaf. Larvae are flat. Young mines are small and individualized, but as larvae grow, the mines converge and become a large, hollowed-out blotch. A full grown larva can be 0.24 inches long. Once mature, the larva will chew its way out of the leaf and drop to the ground to pupate. Two-four generations are possible per year, depending on local temperatures. Subsequent generations of the birch leafminer in the same season may reinfest the leaves again if the host refoliates in the same year.

Damage to Host: 

Foliage of native white-barked birches are primarily impacted. Damage may be first apparent as dark greenish spots on the leaves. Young larvae create small, individual mines between the upper and lower surfaces of tender, new leaves. As the larvae grow, the mines converge and become one large, brown, hollow blotch. Larvae may be visible within these blotches if the leaf is held up to the light. Healthy trees can withstand several years of attack before showing signs of decline.

Monitoring: 

Monitor adults with yellow sticky cards to time sprays. First generation adults lay eggs when new leaves are about half emerged. It is most important to monitor for and manage the first generation of this insect each year. 

Cultural Management: 

When possible, plant resistant birches. Betula alleghaniensis, Betula grossa, and Betula lenta are thought to be highly resistant to the birch leafminer. Betula dahurica, Betula maximowicziana, Betula platyphylla, Betula schmidtii, and Betula costata show partial to intermediate levels of resistance to birch leafminer (Davidson and Raupp, 2014).

Natural Enemies & Biological Control: 

Native parasitoids in North America do attack the birch leafminer, however historically did not keep populations below damaging levels. Exotic biological control agents have been established, including Lathrolestes nigricollis and Grypocentrus albipes (ichneumonid wasps)  and one eulophid wasp (Chrysocharis nitetis), which have been reported to have reduced birch leafminer to barely detectable levels in areas of MA, CT, RI, NY, and PA. 

Lathrolestes nigricollis has become established and widely distributed (Van Driesche et al., 1997). Grypocentrus albipes, while established in a few locations, has been recovered much less frequently by researchers. L. nigricollis has resulted in decreases from 50 to less than 5% of first generation leaves being mined by the birch leafminer (Van Driesche et al., 1997). 

Chemical Management: 

Abamectin (NL)

Acephate (NL)

Acetamiprid (L)

Azadirachtin (NL)

Bifenthrin (NL)

Carbaryl (L)

Chlorantraniliprole (NL)

Clothianidin (NL)

Cyantraniliprole (NL)

Cyfluthrin (NL)

Deltamethrin (L)

Diflubenzuron (N)

Dinotefuran (NL)

Emamectin benzoate (L)

Fenpropathrin (NL)

Flonicamid+cyclaniliprole (N)

Gamma-cyhalothrin (L)

Horticultural oil (L)

Imidacloprid (L)

Insecticidal soap (NL)

Lambda-cyhalothrin (L)

Malathion (L)

Neem oil (NL)

Permethrin (L)

Pyrethrin+sulfur (NL)

Spinetoram+sulfoxaflor (N)

Spinosad (NL)

Notes: 

Chemical management may no longer be needed due to the successful biological control efforts for this insect. Manage adults prior to egg laying. Soil and trunk injection with imidacloprid, where applicable, has shown good results.

Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirchtin (injection, soil drench), chlorantraniliprole (soil drench), clothianidin (soil drench), cyantraniliprole (soil drench, soil injection), diflubenzuron (soil drench), dinotefuran (soil drench), emamectin benzoate (injection), 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: January 2024. To check current product registrations in Massachusetts, please visit: https://www.mass.gov/pesticide-product-registration .