The spinach leafminer attacks spinach, beet, sugar beet, Swiss chard and many weeds including lamb's-quarters, chickweed and nightshade. The insect's mines make the vegetables unsightly and unappetizing as well as destroying part of the crop. The maggots feed between the upper and lower leaf surfaces of the host plants mining out the tissue in between. As the maggot grows, the mines coalesce with others and blotches are seen on the affected leaves. It is not unusual for several larvae to be in the same leaf. The spinach leafminer is most injurious to spinach, chard and beet greens. Injury to the leaves of beets and sugar beets also decreases the size of the root crop.
The adult of the spinach leafminer is a fly about 1/4 inch long and gray in color. The adults may be seen flying near the ground between the plants. The egg is white and about 1 mm long. The maggot or larval stage is white to yellowish and legless. The puparium or resting stage is a hard, brown structure about 1/5 inch in length.
In April or May adult flies of the spinach leafminer appear in the garden. The females deposit eggs singly or in rows of two to five side by side on the underside of the leaves. In four to six days the tiny maggots hatch from the eggs and work their way into the leaf tissue. The larvae excavate the mine, which is at first thread-like, but as the maggot grows the mine becomes blotch-like. The larvae are full-grown in seven to 16 days at which time they drop to the ground and burrow a few inches into the soil. Here each larva changes to the puparium or resting stage. Two weeks to a month later the adult flies emerge and will soon lay eggs for another brood. In central New York there are three generations and a partial fourth each year. Winter is passed as a puparium in the soil.
Because this insect overwinters as a puparium in the soil near where the crop was infested the previous year, crop rotation should be practiced especially if one tries to mechanically protect plants from this insect. Cover plants with fine netting or cheesecloth or floating row covers to protect them from adult egg-laying flies. Netting will not keep out insects that are already in the soil. Be sure that the edges of the row cover are well anchored so insects cannot get under them. The protective covering should be placed over the crop at planting time and, with spinach, might be left on until ready to harvest. Hand pick and destroy infested mined leaves before the larvae drop to the soil will help control the leafminers. If leaves are just placed on a compost pile, fly larvae might continue to develop and emerge as adults to reinfest crops.
Control weeds. Destroying the insects' wild food plants should also be helpful in reducing the numbers of leafminers. This includes lamb's-quarters, chickweed and nightshade in and around the garden.
Commercial beets are not sprayed for leafminer; spray only if leaves intended for consumption are infested. Use malathion: spray when mines appear and a seven-day intervals as needed. Wait seven days to harvest.
Use malathion: spray when mines appear and at seven-day intervals as needed. Wait seven days to harvest.
Before using any pesticide, check the label. Both the crop you wish to treat and the pest you are treating for must be listed on the label. If not, do not use the pesticide. Also make sure to read the label to determine how long to wait before harvest. Harvesting treated vegetables too soon after a pesticide application may result in excessive residue being present when consumed.
4/78 Prepared by: Carolyn Klass, Senior Extension Associate, Department of Entomology, Cornell University
2/91 slightly revised
Pesticide and management recommendations obtained from: Part I Guide to Pest Management Around the Home, Cultural Methods and Part II -- Pest Management Around the Home, 2003-2004 Pesticide Guidelines, Miscellaneous Bulletins 199S74-I and 139S74II, Cornell Cooperative Extension Publications.
This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly and human errors are still possible. Some materials mentioned may no longer be available, and some uses may no longer be legal.