After a series of posts on really pretty plants, I thought it was time to show something no less important but not as charming. I have written several times about Swiss chard leafminers and their management. These pests are starting to emerge from pupae and lay eggs on the undersides of my chard leaves now. The little brown pupae have been in the soil all winter, either in your garden or one nearby. The adult insects, which are small flies, have emerged from the pupae, flown away, found flies of the same species, but the opposite sex, and mated.
There are the eggs that the females lay on leaves. They are quite tiny and white, elongated, arranged in parallel rows.
Yeah, I know, they're kind of disgusting, but now you know. These are not, by the way, the maggots that live in garbage. This insect only lays eggs on Swiss chard, beets, and spinach. They are active between late March and about mid October here in San Francisco.
If you see their damage on your plants, your first defense is to brush off the eggs, or pick off any leaves on which you see the blotches that show they are feeding inside. If you are vigilant, you can prevent them from maturing to pupae and dropping to the soil. If they get ahead of you, they will repeat their life cycle several times a summer. Summer oil sprays (choose one based on an edible oil, such as canola or soy) or a bacterial extract spray that is sold under the brand names Bull's Eye or Spinosad, used according to the directions on the label, can help. However, in my college garden, the critters got ahead of us over the past few years, so my strategy this year is to remove all susceptible crops from the garden during the summer. I will replant chard only in late August or September, and protect it with a row cover (a white polyester sold for this use in gardens) until cool weather has killed any adult flies. I plan to start the plants inside about six weeks before I plant them out, to give them a little bit of a boost, and hope for a good harvest until the following March, when out they will come.
Curious to see the blotches on chard leaves? Use the legit search feature on the right side panel of this blog to search for "chard" and you can see other posts on this subject.