Lately you have noticed weird squiggly lines on some of your plants leaves. They look like someone has taken a tool of sorts and drawn on the leaves, leaving only hollowed out trails with some black specks in them. This is leafminer damage. Although it can add an interesting effect, if left unchecked, it can be detrimental to plant health and appearance.
The leafminer (Lyriomyza sp.), is a small fly that resembles a bee. Adults are only about 1/10″ long, which makes it difficult to see them. They are black with a yellow stripe on their upper body close to where the wings are attached. They can be found throughout North America and they use many different plants as their host, but some more than others. Some of the leafminers favorites include columbine, lettuce, basil, turnips, blackberries and cabbage.
So basically, this is what happens. A female leafminer finds a plant to her liking, lands and deposits an egg on the surface of the leaf. Once the egg hatches, the larvae then proceeds to burrow into the leaf and feed. This is where all those lovely trails come from. Oh yeah, the black spots in the trails that I previously mentioned, that is their frass, or poop. Once they have eaten enough, they become adults and leave the sanctity of the leaf to start the process over again.
There are a few methods of control. I usually do nothing because the damage is never heavy enough to cause harm to the plants, but if you need to here are some suggestions. First, remove any leaves that have the trails through them. These should be destroyed in some way to kill the larva that may still be there. This can be done by burning them or composting, just bury them deep into the pile. The next thing you can do is to spray your plants with insecticidal soap. This is a contact killer and can work as a repellant also. These soaps are available at hardware stores and garden centers, but be sure to follow the instructions exactly as the label says. You could cause more harm than good if you do not follow the label. It is also a good idea to test a few leaves of the plant you intend to treat, just to be sure the spray will not burn the plant.
If you have leaf trails every year and can’t stand it, you could try to use the insecticidal soap right before it happens or at first sight of damage. The way to do this is to mark your calender when you first noticed the damage and then the following year you’ll be ready. Start your plant treatment for leafminers about a week before your mark on the calender and see what happens. This method can be used for any other pest as well. After a couple of years you will notice patterns and be able to control what goes on in your garden with little effort.
Stephens, James M., Vegetable Gardening In Florida, University Of Florida , Institute Of Food And Agricultural Sciences,1999. Pp.56-57
Carr, Anna. Rodale’s Color Handbook Of Garden Insects. Rodale Press, Inc. 1979. Pg. 162