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.

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.

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.

Susceptible Hosts

• 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) and European white birch (B. pedula).

• Less susceptible species include river birch (B. nigra), black birch (B. lenta), and yellow birch (B. alleghaniensis).

Pest

Fenusa pusilla

Symptoms

• 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

Signs

• 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.

Life Cycle

• 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 into larvae 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 to start the cycle over again. There can be multiple generations depending on geography.

Distinguish from Look-alikes

• Bronze birch borer also causes leaves to turn brown.

• Birch trees in typical urban sites frequently develop symptoms related to abiotic stress. Wilting leaves, mineral deficiencies, and decline may be apparent on stressed birch trees regardless of the presence of the birch leafminer.

• Normally, damaged leaves from birch leafminer are scattered throughout the tree. Bronze birch borer damage begins in the upper 1/3 of the crown and works its way down.

• Leaves infested by bronze birch borer will be stunted, yellow, or show marginal chlorosis and begin to dieback.

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.

Early spring Transtect soil applications or foliar sprays of Orthene will provide acceptable same season control if they are properly timed.

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