Golden Gate Garden

My Golden Gate Gardener column in the Home Section of the SF Chronicle this morning (accessible at www.sfgate.com) was about the leafminers that attack Swiss chard, beets, and spinach. I thought you’d like to see a photo to help identify this Swiss chard pest. Leafminers are larvae of a fly. They lay tiny white eggs on the leaf undersides. When the maggots hatch, they eat a hole in the leaf surface and start eating out the leaf–the part of the leaf between the upper and lower epidermises (epidermi?). Some leafminers, like the ones that infest columbine, eat winding trails in the leaf, but the one shown here usually creats big ugly blotches.

What to do? I have been fighting these critters for several years, removing leaves with damage, spraying with summer oils (purchased at nurseries) to kill the eggs. (The oil product I use is based on canola oil, with a sticker-spreader, so should be nontoxic to people.) Even tried Spinosad, a new product, containing an exudate of a bacterium, that is registered for use by organic farmers. Some neem oil products are also registered for use on this pest and crop.

But the problem is that the maggot isn’t damaged much by chemicals unless they penetrate into every cell, and products that do this would render the plants inedible. The catch 22.

What else to do? You can try to kill the pupae. They fall to the soil and later emerge as adult flies. The life cycle repeats about monthly from the end of March to mid October in San Francisco’s climate, with the winter generation overwintering as pupae. I plan to keep the chard I am growing until March (may as well, it won’t have the damage then). In March, I will take it all out and dig the soil to turn up any pupae it contains where they are likely to dry out or be damaged, or maybe (hopefully) eaten by birds. (In summer, when the soil is warmer, you could apply beneficial nematodes to the soil. They are a bit pricy, but should eat this and other pests.)

Then, no chard, beets, or spinach all summer. (There is plenty else to grow.)

In late summer, I shall start some chard seed indoors. When it is a month or two old, I will plant it in the garden. If it is ready to plant, but still before mid-October, I will cover it with row cover (a thin spun-polyester material that lets light and water through) making sure it is secured at the ground so flies can’t fly in. When the weather turns cool enough to kill the flies, off will come the row cover. Then I should have chard unmolested until the following March.

Has anyone had any better ideas for dealing with this annoying pest of chard, beets, and spinach?