|The leaves of my columbine are covered with squiggly whitish lines. At first I thought it was slime trails left by slugs, but I don't think that's it. What is causing the lines and is it harming the plants?|
|It sounds like leafminers, which are a common pest of columbine. Leaf miners are the larvae of a small moth or fly that feed by removing green tissue from between the upper and lower surfaces of leaves. Since they're between the leaf surfaces, no amount of insecticide will reach them. You can pick and destroy infested leaves to kill the larvae and to reduce the population of future generations.
If you can encourage beneficial predators, such as lacewings and spiders, to take up residence in your garden, the beneficials will help keep the leafminer populations down. If you have a variety of plants in your landscape so that there's something blooming at all times, you'll provide the perfect environment for a multitude of beneficials.
In the meantime, just pick off the leaves of your columbines if they develop the tell-tale trails of leafminers. Although unslighly, they generally don't cause much harm to the plant. In addition to picking off infested leaves now, do a good thorough clean up around the plant in the late fall to remove any overwintering pests.