The holly leafminer, Phytomyza ilicis, is one of at least seven species of leaf-mining flies that feed on holly (Ilex spp.). It feeds primarily on English/Euro-pean holly, Ilex aquifolium. Heavy infestations of holly leafminer may result in every leaf on a plant being mined. This may cause plants to drop many leaves, which reduces the plantís aesthetic appearance until new growth begins next spring. There is one generation per year.
Adult females are present shortly after new leaves develop on plants. They can emerge over a 6-week period in the spring. The female lays eggs into the leaves in early to late spring. The eggs are normally laid on leaf undersides and in the midvein of individual leaves. Females pierce the leaf epidermis with their sharp ovipositor and then insert an egg into the mesophyll layer of the leaf. Egg laying can create a noticeable green blister on the leaf. The eggs hatch into yellow larvae (maggots) that are 1.5 to 2.0 millimeters long when fully grown. The larvae create narrow, winding mines as they feed between the leaf surfaces. These mines are translucent or white at first but eventually turn brown. Larval feeding can create blisters or blotches that are very noticeable. As the larva continues to feed, the mine enlarges until it is ready to pupate in late fall to winter. The larvae overwinter in the leaf mine.
Plant injury not only results from larval feeding activities but also from adult feeding. This occurs when the adult female punctures the leaf with her sharp ovipositor. The wounds created by the ovipositor allow plant fluids to flow out, which are consumed by the female. These oviposition wounds leave small, round, deep fissures that are visible from both sides of the leaf. Pest-control materials that are recommended for managing the holly leafminer include acephate (Orthene), dimethoate (Cygon), and spinosad (Conserve). Make applications in late May or early June when leaf mines first appear. The reason these materials are effective against holly leafminer is that they are either systemic (move within the water-conducting tissues) or have translaminar properties. Translaminar is a term used to describe materials that penetrate the leaf surface; the active ingredient then resides within the leaf tissue where the leafminer larvae feeds. As a result, these materials last longer than typical contact insecticides, and their efficacy is not influenced by rainfall.