More on the Chard Leafminer

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.

And here are the larvae, which, because the insect is a fly, are correctly called maggots. When I opened the leaf, they tried to crawl back betweeh the opened layers.

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.

Comments

[Kelly]

Thanks for this post. I just put in some swiss chard transplants, so I appreciate this advice.

[Jacoby]

I was horrified to find these guys on my Italian flat-leaf parsley as well!

[pam]

Hi Jacoby,

I don't think the critters on your flat-leaf parsley are the same insect as the one damaging the chard. I think these leafminers are either spinach or beet leafminers, which don't seem interested in plants that aren't in the beet family. They make blotches on the leaves by feeding between the upper and lower epidermis--inside the leaf.
There are a bunch of different leafminer species, and some have a wider range of host plants. One, in particular, that is very common is the serpentine leafminer. It makes a serpentine, or winding, trail in a leaf. It lives on cabbage, nasturtium, sweet pea, pepper, potato, etc. I don't think it is usually as damaging as this chard pest, and I don't see it often. None of the host plants listed are in the parsley family, either.
Parsley does get cabbage loopers (little green caterpillars that move like inchworms) and also a "leaftier" which is a caterpillar that rolls the leaf around itself with strands of silk and feeds in the roll. But the leaftier isn't actually between the leaf layers. I haven't seen a photo of that pest, but you could probably find one.
If the damage is only on a few parsley leaves, you can just pick them off and forget it. If damage gets really bad, though, it's a good idea to identify the pest correctly, since you will have different next steps depending on what it is.

[Jacoby]

I'm pretty sure they're just aphids. We've had lots of problems with leafminers on chard in the past, so that's what I feared at first, but upon further inspection - aphids. Lots of ladybugs crawlin around in there though, so that's good. We've defeated leafminers on our chard pretty effectively with floating row covers that go all the way to the ground, creating something of a seal.