Leaf Miners

Leafminers are small 1.5cm whitish grubs with a black head that feed in between leaves making strange almost transparent patterns in leaves of hosts. 

Leaf Miner Grub (left)-Bradley Hydroponics, Leaf Miner Larvae (right)-Virginia Cooperative Extension

Description
The adult is a small fly (most having a yellow spot on their back) that lays it’s eggs on the leaves and when they hatch, grubs work inside leaves eating the centres out. There are several different species. Each species named for the plant they attack….ie: Lilac leafminers, azalea leafminers, spinach leafminers, boxwood leafminers etc.

Adult fly-P. Parrella, University of California

Habitat and Lifecycle
Found everywhere in North America. Flies occur in early summer and lay eggs in holes that they pierce in leaves. Eggs hatch within 4 days and begin feeding. When they have completed feeding they drop to the ground and pupate. They overwinter in this stage.

Damage (top left)-Virginia Cooperative, Damage and larvae (top right)-Virginia Cooperative, Damage (bottom)-USDA, John A. Byers

Diet and Damage
They feed on beets, beans, tomatoes, swiss chard, spinach, celery, lilacs, most flowers and boxwood varieties. They have been known to feed on other delicate leafy veggies and some ornamental like azaleas. They cause unsightly yellowish blisters on the leaves of host plants in a very erractic many (they look like trails). There can be as many as half a dozen present per leaf in serious infestations. Fortunately these guys rarely do much damage unless….it’s your spinach or chard. This year I am experiencing an influx of these guys and they seem to be eating everything so perhaps this is a bad year for them.

Control
1) Hand Pick
If you see the trails, you can crush them between thumb and forefinger. Or you can remove the affected leaves and drop them in soapy water or burn.
2) Row Covers
Protect you crops with row covers before the flies emerge
3) Till Around Susceptable Plants
If you till the areas where you have them you will uncover the larva and birds will greedily eat them or you can pick them up and drop in soapy water.
4) Parasitic Wasps
Hymenopterous (wasp) parasites are natural predators.
5) Plan Your Harvest
Harvest beets and other veggies earlier then you normally would to decrease loss of veggies.
6) Insecticidal Soap
Spray when you see the flies on leaves of plants. I’m not sure it works on the larvae inside the leaves but you never know.
7) Sticky Traps
Put up sticky traps near plants where the flies appear or plants that have been attacked.