Buxus: Boxwood Leafminer

Problem
The leaves are puckered or blistered. The lower surface of the leaf is spotted yellow; the upper surface is green at first and then flecked brown and yellow. Leaves may drop prematurely. Growth is poor and the plant is rangy. Twigs may die back if the plant is infested more than 1 year. When the leaf is torn open, two or more small (1/8 inch), yellowish maggots or brownish pupae may be found between the upper and lower surfaces of the leaf.

Analysis
(Monarthropalpus buxi)

The boxwood leafminer is one of the most serious pests of boxwood. The larvae spend the winter in the leaf. When the weather warms in spring, they feed on the tissue between the leaf surfaces. In late April or May a tiny (1/10 inch), gnatlike, orange fly emerges from the pupal case inside the leaf. The emerging flies swarm around the plant in early morning, mating and laying eggs in the leaves. New blisters develop in midsummer from feeding by this next generation of larvae. When the weather turns cold, the larvae become inactive until the following spring.

Solution
Leafminer control is most effective when insecticides are applied just before eggs are laid in late spring. Spray with an insecticide containing acephate or lindane in late April or early May. If eggs are laid before a control is applied, an insecticide containing dimethoate applied in late June may kill young miners.