Have you noticed damage to some leaves in your garden that looks like squiggly lines or a road map? The damaged leaves are the signs that you plant is being attacked by a leaf miner. Leaf miner is a term used to describe the larvae of several insects that feed on a plant's leaf tissue. The two most common culprits of this damage are moths and flies.
Your first instinct might be to reach for a bug spray, but hosing down your plants with a harmful chemical will actually do you little good in the fight against leaf miners. You see, leaf miners have adapted to feed on leaf tissue from within the leaf to protect themselves from predators. If predators can't get to them neither can the spray you're applying. I guess you could apply a systemic insecticide, which absorbed by plant tissue, but if you have leaf miners attacking your tomatoes this may not be the best idea.
In my garden I employ a live and let live philosophy with bugs, be they good or bad. Unless they're biting me I don't really go out of my way to fight them. I don't spray for bugs which is good for the beneficial insects that feed on the bad bugs and I try to attract birds which eat bugs, sometimes they eat good bugs but mostly they eat bad bugs.
Since my garden consists mostly of ornamental plants-- the yearly appearance of leaf miners doesn't phase me too much. That their damage to plants in my garden is confined mostly to annuals helps keep things in perspective. By the time leaf miners are munching away inside a leaf I've already enjoyed the blossoms and got to take my photos. can't ask for more than that from annuals.
If you're the kind of gardener that is bothered by imperfections and don't want to use chemicals--consider planting companion plants or sacrificial plants. In my garden leaf miners are on Columbines and Nasturtiums like white on rice. Although, I have to admit that every once in a while when I see the beginnings of a leaf miner trail in a leaf I may pluck it and dispose of it in the trash. Just to make me feel like I've done some work in the garden.
The photo above shows leaf miner damage to a Columbine plant. That stuff that looks like grass is actually not grass, not that there's anything wrong with it, but the leaves of some of my crocus bulbs.