The holly leafminer, Phytomyza ilicis, is one of at least seven species of leaf-mining flies of holly (Ilex spp.). It feeds mainly on English/European holly, Ilex aquifolium. Heavy infestations may mine every leaf on a plant, which may cause it to drop leaves, reducing the aesthetic appearance until growth begins next spring. There is one generation per year.|
Emerging over 6 weeks in the spring, adult females are present for a brief time after new leaves develop on plants. The female lays eggs into the leaves in early to late spring, normally on leaf undersides and in the midvein of individual leaves. The female pierces the leaf epidermis with her sharp ovipositor and then inserts an egg into the leaf's mesophyll layer. Egg-laying can create a noticeable green blister on the leaf. Eggs hatch into yellow larvae or maggots, 1.5 to 2.0 millimeters long when full-grown. Larvae create narrow, winding mines as they feed between the leaf surfaces. These mines are translucent or white at first but turn brown. Larval feeding can create blisters or blotches that are very evident. As the larva feeds, the mine enlarges until the larva is ready to pupate in late fall to winter. Larvae overwinter in the leaf mine.
Plant injury results not only from larval feeding but also from adult feeding. This occurs when the adult female punctures the leaf with her ovipositor. The wounds created allow plant fluids to flow out, which are consumed by the female. These wounds leave small, round, deep fissures, visible from both sides of the leaf. Pest-control materials recommended include acephate (Orthene) and spinosad (Conserve). Apply in late May or early June when leaf mines first appear. These materials are effective against holly leafminer because they have translaminar properties, meaning they penetrate the leaf surface; the active ingredient then resides within the leaf tissue, where larvae feed. As a result, these materials last longer than typical contact pest-control materials, and their efficacy is not influenced by rainfall.