Boxwood Leafminer

Boxwood Leafminers,infested with this leafminer develop blisters on the lower leaf surface. Infested leaves are usually smaller, off-color and drop sooner than healthy leaves. Heavily infested boxwoods usually have sparse foliage and poor color. Shrubs are generally not killed by leafminers.

This is the most serious insect pest that attacks boxwood. The leafminer is the larva immature form of a small, orangish mosquito-like fly. These flies are less than 1/8-inch long and can often be seen swarming around boxwoods in the spring.

The adult female fly inserts eggs with her ovipositor egg laying structure into new boxwood leaves through the leaf’s upper surface. When the larvae hatch, they feed inside the leaf, creating a mine. Larvae are orange and about 1/8-inch in length. They overwinter survive the winter in the leaves. Adults emerge from the leaves the following spring, just after new growth occurs on boxwoods. There is one generation per year.

Life Cycle:This pest overwinters as a larva in the leaf blister. During the spring the leaf blister develops a translucent window. In early spring larvae molt into the resting stage called a pupa. During May, this stage wriggles through the blister and protrudes from the lower leaf surface, and adults emerge. Adults mate soon after emerging. Mated females deposit eggs in new foliage by thrusting a curved needle-like ovipositor through the lower surface of the leaf.

The eggs may be seen easily, especially on new growth, by holding the leaf up to the light. After laying an average of 29 eggs, the tiny female fly dies. The eggs hatch in about 14-21 days into young larvae. The larvae continue to grow and feed in the leaf through the remainder of the summer. One generation of this pest occurs each year in Pennsylvania.

Damage: The larval stage of this pest can cause extensive damage to foliage. Injury to the host plant is caused by this stage feeding between the upper and lower leaf surfaces. Feeding produces a blister on the leaf, which may or may not become discolored. Heavily infested leaves may contain several leaf blisters, and the entire leaf may become swollen. Infested leaflets often drop prematurely resulting in unhealthy, ragged plants with occasional dead twigs. Most varieties of boxwood are attacked by this pest.

Biological controls: There are few known natural enemies of the boxwood leafminer. Severely shearing the foliage before adult emergence or after egg laying ends will reduce the overall population. Plant resistant varieties.

In The parasitoid Cirrospilus coachellae has been reported preying on leafminers. This parasitoid is common, particularly during late summer and early fall. Predators also appear to be important biological control agents of leafminers. The predacious mites, the Yuma spider mite and a Tydeus sp. have been observed feeding on citrus leafminer larvae though the leave’s upper epidermal layer.

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