You know you have leaf miners if the leaves on your beans, chard, tomatoes, and other vegetables have tiny, tan squiggles on them. And, as you can imagine, its not a very big entity since the larvae is tunneling inside the very thin layers of the leaves.

Most likely, the leaf miners found in the vegetable are the larvae of a couple of particular flies: Liriomyza sativae Blanchard or the American serpentine leaf miner (Liriomyza trifolii). Both target a large range of vegetables, and while they dont necessarily kill the entire plant or ruin the crop, they can cause significant damage, as well as transmit viruses in weak plants.

The good news is leaf miners dont live very long, just 2 to 3 weeks. The bad news is a female can lay well over 200 eggs so a single fly can create a lot of annoying little miners. The female punctures and deposits the eggs in the underside of the leaf usually during the height of mid-summer growth. They dont like immature leaves.

Besides tiny holes from the females ovipositor, the major damage is from the larvae tunneling through the leaf as well as fecal deposits in the tunnels. That sounds yummy, doesnt it? While its not as much of an issue on beans or crops that produce fruit, the ones that are grown for their greens (such as chard) are inedible.

Removing Leaf Miners

Once you have them, theyre hard to eliminate. Thankfully, they probably wont affect production to a significant degree. The best way to deal with them is to remove the damaged leaves.

Parasitic wasps are your best friend when it comes to vegetable leaf miners, which is why using insecticides is actually more harmful than helpful. Some commercial operations actually release large numbers of these wasps to keep the population under control. You can use Bt (Bacillus thuringiensis) to help knock down the numbers if theres a significant number of miners present in your vegetables. It will reduce their numbers without killing the parasitoids.

Another approach starts earlier in the season. If you have a problem with leaf miners this year, deeply till the garden next spring. The flies will overwinter in the soil around their favorite varieties, but cant reach the surface effectively if theyre buried deeper. Keeping weeds under control is also important since the leaf miners use them as favorable targets, as well. Using row covers can keep the flies away from the leaves so they cant feed or lay their eggs if you want to be extra cautious.

Leaf miners dont tolerate cold temperatures without protection (which is why they can survive if theyre buried in the soil or under mulch or weeds), and many species travel into previously unaffected areas of the country through nursery stock. The leaf miners do fine in a greenhouse, and can travel anywhere the plants are sent.

While bothersome, overall leaf miners arent as damaging as some species of garden pests. Encourage the parasitic wasps by keeping the area free of general pesticides, and make the area less hospitable for next year if theyre a major nuisance.