Leafminers are a large group of insects that have species in almost all the major insect orders, including Coleoptera (beetles), Diptera (flies), Hymenoptera (wasps and sawflies), and Lepidoptera (caterpillars). Below are the leafminers that may be encountered in Illinois:
Coleoptera: Locust leafminer, Odontota dorsalis
Diptera: Holly leafminer, Phytomyza ilicis
Hymenoptera: Birch leafminer, Fenusa pusilla; hawthorn leafminer, Profenusa canadensis; alder leafminer, Fenusa dohrnii
Lepidoptera: Arborvitae leafminer, Argyresthia thuiella; spruce needleminer, Endothenia albolineana
Damage is primarily aesthetic or visual, rarely killing plants. Leafminers cause plant damage primarily when in the larva stage. An adult female may also cause damage by puncturing leaves as she inserts her ovipositor (egg-laying device). This creates white specks on the surface of leaves. Leaf miners are generally host specific, which helps in identificaiton.
It is difficult to generalize the life cycle of leafminers, due to the wide diversity of species. So for the sake of brevity and page length, this article will present the life cycle of fly leafminers. The Diptera leafminer adults are 2- to 3.5-mm-long, shiny, black flies with yellow markings on the abdomen. Several species resemble fruit flies. Adult females lay individual translucent, white, oval eggs into leaf punctures created by the ovipositor during probing. Both the female and male feed on the sap that exudes from these wounds. Each female can lay an average of 60 eggs during the 2-to-3-week lifespan. The number of eggs may depend on the food source and temperature. Eggs hatch into bright yellow to white larvae, or maggots, that feed within the mesophyll layer of cells, creating mines in the leaf. The larvae puncture cell walls and withdraw the fluid contents into their mouth. Heavily infested trees may appear scorched, particularly for leafminers in the order Hymenoptera, such as birch and hawthorn leaf miners. Leafminer larvae can range in size from 1.0 to 7.0 mm in length, depending on the species.
Mines enlarge as the larva grows or molts to the next instar. The mine pattern, location, and plants attacked vary, based on the species. There are generally three to four larval instars that take about 5 to 8 days to develop before pupation. The last larval instar cuts a semicircular slit in the leaf and typically drops to the soil to pupate. Adults emerge from the pupa stage in about 10 days. The number of generations depends on leafminer species and type. For example, birch and hawthorn leafminer may have three or four generations, whereas arborvitae leafminer and holly leafminer have one generation per year in Illinois. Leafminers may overwinter as larvae in mines, becoming active in the spring.
Planting “resistant” or tolerant plant varieties may reduce problems with leafminers. For example, river birch (Betula nigra) and Dahurian birch (Betula davurica) are less susceptible to attack by birch leafminer. There are a number of hawthorn (Crataegus spp.) species that are tolerant of hawthorn leafminer.
Insecticides recommended for control of leafminers include abamectin (Avid), acephate (Orthene), imidacloprid (Merit), and spinosad (Conserve). These insecticides need to be applied when leaf mines first appear. These insecticides are effective against leafminers because they have translaminar properties, which means they penetrate the leaf surface. The active ingredient resides within the leaf tissues where the larvae feed. As a result, insecticides with translaminar properties last longer than typical contact insecticides, and their efficacy is not influenced by rainfall.
For more descriptive information on specific leafminers consult previous issues of the Home, Yard, and Garden Pest Newsletter or the publications listed below:
Cranshaw, W. 2004. Garden Insects of North America. Princeton University Press, New Jersey.
Johnson, W. T., and H. H. Lyon. 1988. Insects that Feed on Trees and Shrubs. Comstock Publishing Associates, Cornell University Press, Ithaca, NY.