Leaf Miners tunnel between layers within a leaf, eating the plant tissue and often leaving a very distinctive trail through the leaf or patches of dead brown leaf tissue. These sometimes strange looking markings on leaves are caused by the larva of a variety of insects; from sawflies, to months, and other types of flies.
If you’ve spent much time in your garden or even just out in the forests or parks, you may have seen their damage to leaves and sometimes even fruit. After eggs are laid on the leaf, larvae hatch and eat their way through the leaf, growing larger as they tunnel. After their required amount of time, which varies by species, the larva or “worm” emerges and usually falls to the soil below to pupate and later emerge as an adult.
A very common Leaf Miner in Seattle gardens is the beet or spinach Leaf miner. It’s often seen on chard, beet and spinach leaves and can become so prevalent that the crop is ruined. Specific information on this pest can be found here.
Methods of control for all species of leaf miner vary from physical removal of infected leaves, to specific insecticidal sprays. Sprays are often not very effective though, because the larvae are protected within the layers of leaf tissue.