Description: Boxwood leafminer is the most serious insect pest that attacks boxwood. A beautiful row of boxwoods can suddenly turn yellowish in color and decline in health and appearance with its leaves taking on a deformed shape. The culprit is the larva of a small, orangeish mosquito-like fly. These flies are less than 1/8-inch long and can be seen swarming around boxwoods in spring, just after the boxwoods have put out their new growth. After laying eggs in the leaf tissue, the flies die.
Symptoms: Tiny white maggots hatch and feed inside the leaf. As they grow, the maggots become bright yellow. Their feeding causes the formation of blisters on the lower leaf surface, where the maggots develop for about a year. The telltale signs of infestation are oval, water-soaked swellings on the underside of the leaf. Very early the next spring, the blisters form a thin, translucent spot called the window, and the maggots develop into orange pupae. Fully developed pupae wriggle through the windows and hang from the underside of the leaf. Soon adult flies emerge from the pupae to begin a new generation.
Action: Contact insecticides are most effective against this pest when adults have emerged and before they can lay their eggs. Systemic insecticides are effective when leafminers are present inside the leaf.
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