Every year, the leaf miners ruin my Swiss chard all summer. Maggots in the leaves, with their accompanying frass (bug doo) render it rather unappetizing. In past years, I have sprayed with summer oil every week, or as often as I can remember to do it, but the improvement has been only slight.
I have been reading about a new product, called Spinosad, and have finally tried it on my Swiss chard. About a week and a half after the first spraying, there seem to be fewer new injuries. I also sprayed spinach that was being damaged, with similar results. Tomorrow, I will clean up the plants and spray it again.
Spinosad (also sold as Bulls-Eye), is made by fermenting the bacterium Saccharopolyspora spinosa. Two of the metabolites of this bacterium (that is, products of its metabolism) are highly toxic to a number of insects. It is said to stop susceptible insects from feeding in one hour, and remain toxic to newly arriving insects for the next week or two. It is also said to spare lady beetles, lacewings, and minute pirate bugs, all creatures that eat pest insects in our gardens. It breaks down in sunlight and does not persist in soil. The amount of time to wait after spraying before you harvest varies, but for Swiss chard and other leafy greens, it is one day. It is OMRI, meaining approved for use by organic farmers.
At this point, I don't know every detail about this material, but if the leafminers desist from mining in my Swiss chard leaves, I will find out the ones I don't know yet and report them. It is also said to kill thrips, caterpillars, sawflies, and a number of other pests. (A critter called a rose slug is a sawfly larva. I see it on San Francisco roses often. Summer oil is relatively effective against it, but for serious infestations, Spinosad may be just the ticket.)
By the way, the leafminer in the chard and spinach is the larva of a fly (therefore it's a maggot) that eats out the inside of leaves, causing ugly blotches.The fly lays eggs on the leaves of chard and spinach; the larvae hatch out and enter the leaves. When they have eaten their fill, they drop to the ground to pupate. When they emerge from the pupae, the flies breed and lay eggs again. This goes on in the warmer part of the year here in San Francisco, from late March until mid-October. (Chard and spinach growing October to March are safe, but you have to start them earlier than mid-October for good winter production.)
The fact that the pupae live in the soil means that beneficial nematodes watered into the soil might help as well. I am thinking of getting some this year. These microscopic creatures that you can purchase are only effective when the soil is warm, so I will wait a few more weeks before adding them.