Native to Europe, the birch leafminer, Fenusa pusilla, first appeared in Connecticut in 1923. This pest is known to feed on most species of birch trees, although susceptibility varies. The birch leafminer feeds on the tissue between the upper and lower surfaces of newly emerging leaves producing large dead blotches and eventual leaf loss. Severe infestations can lead to damage of nearly every leaf by mid-summer. Continued leaf loss year after year can lead to a weakened tree, decline, and greater susceptibility to secondary insect and disease pests including bronze birch borer.
First generation feeding by birch leafminer larvae occurs early in the growing season. Proper application timing is essential for optimal management of this insect. Soil applications of Xytect should be applied in late summer or early fall in the previous year to ensure the Xytect has ample time to translocate within the crown in sufficient titers to control the larvae. Xytect 10% as a tree injection treatment for trees that cannot be treated with foliar sprays or other systemic options can be applied in early spring. Early spring Transtect soil applications will provide acceptable same season control if they are properly timed.
Other Treatment Practices
- Promote health and vigor with proper irrigation, mulching, proper pruning and prescription based fertilization practices.
- Adequate water is a key factor in maintaining stress free plants. A slow, deep watering event once every few weeks during dry conditions will help maintain soil moisture levels and minimize stress.
- Mulch is very beneficial for all trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 3 inches of wood chips or shredded bark under the drip line can have a very beneficial effect by holding in moisture and promoting healthy fibrous roots.
- Cambistat can be also be used to promote tree health and reduce the incidence and severity of stress mediated diseases and insect problems on trees growing in urban areas.
The first generation must be controlled making spring applications challenging. Xytect soil applications are applied in fall to kill adult and larval leafminer the following spring. Xytect and Transtect soil applications will provide protection against bronze birch borer. Transtect soil applications applied immediately after soils thaw in the spring can provide adequate control. However, Transtect may not translocate quick enough in the tree if soils abruptly thaw and temperatures increase quickly in the spring.
Signs of Damage
Early symptoms of birch leafminer include small serpentine mines on new leaves in the spring of the year. These mines eventually overlap and appear as blistered, translucent blotches. Entire leaves can turn completely brown and paper-like by mid-summer and fall off. Normally, damaged leaves are scattered throughout the tree. Eventually the entire tree can look scorched or blighted.
- Individual leaves develop small, serpentine mines progressing to translucent blotches and wilting
- Entire leaves turn brown, paper-like, and eventually drop off
- The whole tree looks scorched or blighted from a distance with foliage at the top turning brown first
- Complete defoliation is possible
- The adult is a four winged black fly-like insect about ¼” long (3mm)
- The larva is flat, white, and ½” long (6mm) at maturity and can be easily seen when holding the leaves up to the light
Photo: Brian Kunkel
University of Delaware, Bugwood.org
Photo: Whitney Cranshaw
Colorado State University, Bugwood.org
Trees At Risk
Birch leafminer feeds in the leaves of most birches including:
- Paper birch (B. papyrifera)
- Gray birch (B. populifolia)
- Erman Birch (B. ermanii)
- Asian White Birch (B. platyphylla)
- Monarch Birch (B. maximowicziana)
- European white birch (B. pedula)
Less susceptible species include:
- River birch (B. nigra)
- Black birch (B. lenta)
- Yellow birch (B. alleghaniensis)
- Mature larvae over-winter in debris in the soil.
- Adults emerge and mate as the leaves begin expansion in the spring.
- The female adults insert their eggs in soft newly developing foliage.
- The eggs hatch 7-10 days later (190-290 GDD) and feed for 2-3 weeks before dropping to the ground to enter the soil and pupate.
- A second generation of adults appears 15-20 days later (530-700 GDD) to start the cycle over again.
- There can be multiple generations depending on geography.
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.